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1. color bright pink normal NO Check for HV circuitry 1 it OK HV power supply works normally NO NO NO NO Check for forevacuum Check for oscil Power supplies are overloaded and high vacuum lator and rela components ted power supply NO NO YES NO Check the forevacuum Check the electric Check the circuits components for continuity NO YES YES If there is ne No or old target Change the target utron production check the circuit of the meter and high vacuum pumps cooling water supply YES Check for 10n HV power supply Check the voltage Is the D3 Check for 10n source components out of order divider chain of supply OK source components Pyrex bulb the acceleration tube NO NO Check the loads and related Accelerator tube circuitry l electrodes contaminated Check the D tank Are the vacuum i meters OK Pressure below NO 10 mbar ANNEX C TROUBLESHOOTING FLOW CHART FOR SEALED TUBE NEUTRON GENERATORS START YES YES YES YES YES YES YES FINISH Is target Neutron GOOD SYSTEM current OK production NO Check the No tntium arcuitry old tube Is the 10n source current OK Are all power supphes OK Is the pressure Extraction in the tube OK Is acceleration normal voltage 80 kV Power supplies overloaded Check for HV a
2. without excitation 77 control with excitation control Fig 111 Load characteristics of the Felici type HV generator 158 The Felici generator has an ideal low short circuit current and ideal CV CC Constant Voltage Constant Current output characteristics The fluctuation in voltage is less than 0 1 below the critical loading current The loading char acteristics of this type of HV generator are shown in Fig 111 with and without excitation charge control Fig 112 The half wave rectifier 13 2 AC DC CONVERSION HIGH VOLTAGE POWER SUPPLIES 13 2 1 The single phase half wave rectifier The single phase half wave rectifier with capacitive smoothing shown in Fig 112 is of basic interest Neglecting the reactance of the HV transformer and the internal impedance of the D diode during the conduction capacitor C is charged to the maximum voltage of the AC voltage V t of the transformer if the D diode conducts The D diode must be dimensioned to withstand a peak re verse voltage of 2V max This would also be the case if the HV transformer is grounded at the terminal b instead of the terminal a The output voltage V no longer remains constant if the circuit is loaded During one period T 1 f of the AC voltage a charge Q is transferred to the load R which is represented as o a VO ot m 18 38 a I is therefore the mean value of the DC output ir t and V t is the DC voltage which includes a ripple
3. 109 H Maekawa et al JAERI M 83 219 Japan Atomic Energy Res Inst 1983 110 M Buttig INDC GDR 40 6 ZfK 562 1985 p 56 111 Instruction Manual of KAMAN 711 Sealed Tube Neutron Generator Colorado Springs USA 112 T Sztaricskai B Yotsombat to be published 230 113 T Sztaricskai ATOMKI K zl 13 1971 p 117 114 G Pet Pneumatic Sample Transfer System for Neutron Generators to be published 115 T Sztaricskai et al ATOMKI K zl 27 1985 p 105 116 B V Anufrienko et al FEI 307 Fiziko Energ Inst Obninsk 1971 in Russian 117 N Chirapatpimol et al INDC TAI 004 GI IAEA Vienna 1986 118 T Vilaithong et al Proc Conf Nuclear Data for Science and Technology J lich Springer Verlag Berlin 1992 p 486 119 S Singkarat et al Proc Conf Nuclear Data for Science and Technology Myto Japan 1988 120 J C Robertson KJ Zieba Nucl Instr Meth 45 1960 p 179 121 C M Herbach et al Techn Univ Dresden Report 05 06 85 1985 122 Neutron physics in Hungarian Editors D Kiss and P Quittner Akad miai Kiad Budapest 1971 123 G F Knoll Radiation Detection and Measurements John Wiley and Sons New York 1981 124 S Nagy Thesis Debrecen 1976 125 T Sztaricskai Troubleshooting and Maintenance of Neutron Generators Proc IAEA Advisory Group Meeting on Small Accelerators 1 5 June 1992 Debrecen Hungary to be published 231 ANNEX A LIST OF
4. are usually perforated to form a lower vacuum resistance between the ion source and the vacuum pumps 132 pt OO 8 Steps in the rings Fig 89 The structure of a homogeneous field accelerator tube The structure of a homogeneous field acceleration tube is shown in Fig 89 69 This tube consists of conical acceleration electrodes and ceramic insulating rings The cone shaped electrodes protect the walls of the ceramic insulator rings against contamination The tube s lifetime is limited for the following reasons Secondary electrons produced by ions from the residual gas not only oil vapour in the acceleration tube accelerating and bombarding the electrodes and insulators Electrons bombarding the epoxy resin or PVA fillet at the bond between the insulators and the metal electrodes Poor vacuum in the tube Heavy ions or molecular ions formed from the ion or electron bombardment of residual gas and from electron bombardment causing sputtering of the metal accelerating electrodes X ray or ultraviolet photons causing photoelectric emission of electrons Field electron emission from the metal electrodes It is probable that all of these processes contribute to a greater or lesser extent to the operation of conventional acceleration tubes but it is clear that electrons whatever their source play the major role The lifetime of the accel eration tubes can be increased by the following means 133 Maintaining
5. sss 113 9 3 Pressure vacuum measurements ccccece cence ence eset essceteateteneeeanens 115 9 3 1 Thermal conductivity gauges c cece cece cence ence ee ene eeee ena eees 115 9 3 2 Ionization SAUCES oireeni cnc tondere k ond ade NEE uae y best vad Dad 116 a Thermionic ionization gauges cese 116 b Cold cathode or Penning ionization gauge ssss 117 9 4 Pressure monitoring and leak detection ssssessssssessseserererererrereersses 118 9 4 1 Leak rate measurement ssssseseeeee hh e e heme ese een 118 9 4 2 Pumping speed measurement sesessereeereesrrerersererererrererrrss 119 9 4 3 Leak detection eset RR ER NOE VERA EY NE 123 BEAM ACCELERATION AND BEAM TRANSPORT SYSTEMS 128 10 1 Electrostatic lens creierii nai asainne HH Hmmm rere rens 128 10 2 Unipotential or Einzel lens 5 nero e rete e n ry ee nenne 129 10 3 Troubleshooting of electrostatic focus lenses sse 130 10 4 The acceleration tube cccsssssesseee enne heresis eere 131 10 5 Troubleshooting of acceleration tubes sssessresererrerererrseeresressrerere 134 PRINCIPLES OF BEAM FILTERS seesesesee Hee e esae 135 11 1 Electrostatic and magnetic beam deflection sesesseseerrrerererereersrsees 135 11 2 Troubleshooting of electrostatic deflectors seeeeeee 137 11 3 Analyzing magnets of neutron generators sese 138 11
6. 14 1 BEAM STOPS A fast switch on and switch off of the neutron production at a neutron gen erator can be carried out by pulsing the ion source by deflection of the accel erated beam from the target or by a mechanical shutter closing the path of the accelerated beam to the target This mechanical shutter is called a beam stop The beam stop has a metal sheet usually water cooled which shuts off the beam in front of the target This shutter can be operated electromagnetically or pneu matically The response time of the beam stops is relatively fast they close and open the beam within almost one second The ion source and the extraction voltage pulsation or the electrostatic beam deflection are faster The principles of the two main types of beam stop are indicated in Figs 123 and 124 50 14 2 BEAM SCANNERS In general the scanners consist of two wires one rotating around the hor irizontal axis scanning the beam vertically while the other moves perpendicu larly to the first and gives the beam a horizontal profile The planes scanned by the two wires are perpendicular to the deuteron beam direction If the two wires move rotate synchronously the shape of the accelerated beam can be determined The principle of the single axis rotating beam scanner is shown in Fig 125 and of the two axis beam scanner in Fig 126 14 2 1 Determination of the beam profile Let us suppose a homogeneous beam of radius r along the x axis in the Carte
7. Target replacement 2 cee creo resa ERRORS Su es veru is ai eaa 185 14 5 2 Air cooled target holder 1 eoe heus 188 14 5 3 Replacement of the target at air cooled target holders 188 14 5 4 Rotating and wobbling target holders eeeeeee 191 15 CLOSED CIRCUIT COOLING SYSTEMS 4 iicet tarn rane pa ha E NER VS 192 15 1 The Kaman cooling system e desis espe RES YE FARA ORE DEP RATER MU see 192 15 1 1 MAIN ANCE ses rose eee Free ROCA nsa aan a cussed aaah deron Usus d uai te 193 15 2 Closed circuit cooling system with soil heat exchanger 195 16 PNEUMATIC SAMPLE TRANSFER SYSTEMS sese 197 17 NANOSECOND PULSED NEUTRON GENERATORS esee 200 17 1 Pre acceleration nanosecond bunched ion beam neutron generator 200 17 2 Post acceleration klystron bunching of a commercial neutron generator 204 18 THE ASSOCIATED PARTICLE METHOD 4 noter coe a Rer nh EERRUR 206 18 1 Self target formation by deuteron drive in sese 208 19 NEUTRON MONITORS ceecee ehe quER Eu EQUES DABATUR XT MEER a EENERE E EAS BRE 211 19 1 Monitoring by long counter iioi eer rre rer eh saves ERAS ERU ue rains 211 19 2 Fission chamber monitoring uie oper meh xor eara sa Ca na L0da Ea Meme a aes 212 20 SAFETY HAZARDS RELATED TO NEUTRON GENERATORS 215 20 by Radiation Bazar oo cao ete 6n on es Yo o Ro Uis PN aae genu e Eat 215 20 2
8. The door between the NGR and the control room must be airtight and protect the measuring laboratories from contamination by radioactive gases To increase the efficiency of the ventilation in the target storage glove box and target areas the door between the NGR and control room constructed of paraffin or polyethylene blocks should be covered by plastic foils A crane with a 20 25 t capacity is recommended for transporting large parts and heavy equipment e g source container lead spectrometer subcritical tank etc Three chimneys are needed for ventilation a One to refresh the air in the closed NGR about 5 times per hour to ensure that the concentration of tritium does not exceed the value of 5 x 10 uCi m b One to ventilate the glove box when it is in use this can be done by placing a small fan in the appropriate chimney c One a 10 cm diameter tube which can be connected to the exhaust of the forevacuum pump At the top of this chimney the tritium content should be controlled continuously Fresh air for ventilation can be supplied through the measuring room by a tube system e g using the channels for cables and pneumatic transfer In humid climates dehumidifiers are needed at least one in the NGR and one in the measuring room To avoid noise and contamination in the NGR the compressor and the ventila tors should be placed in a separate room The compressed air requirement varies between 4 and 10 m h depe
9. Remove screws 16 and sleeve socket 4 Pull out carefully spindle 5 together with spring 8 and K shape ring 14 from valve body 1 Undo the screw 7 and remove filter 6 b Cleaning the dismantled valve parts Wash needle 5 seating filter 6 and the hole in the valve body 1 with chloroethane or similar using a soft brush Carefully remove any deposits from the needle using a soft tool of wood or plastic Then rub the needle vigorously but carefully with a cloth drenched in solvent cleaner Make sure the needle sur face is clean and smooth IMPORTANT NOTE DO NOT BEND THE NEEDLE DURING THE CLEANING Rinse all parts in alcohol or acetone in an ultrasonic bath if possible and dry them Blow out the seating with clean compressed air or hair dryer Be sure that the seating surface is score free and smooth Wipe the holes and seal ing surfaces of sleeve 4 and valve body 1 with lint free cloth drenched in alcohol remove all dirt and dust Finally rinse O ring 15 and K ring in alco hol or acetone if possible in an ultrasonic bath c Reassembling the valve Slightly grease O ring 14 and K ring 15 with Flombin grease or silicon grease and slide them onto spindle 5 Be sure to protect the needle from contact with lubricant Carefully insert spindle needle 5 with washer under the spring into the valve body 1 until needle rests in the seating Slide spring 8 onto spindle 5 Fasten slee
10. gt 200 Beam current mA gt 1 gt 3 5 Ion source Penning 4 4 RF c Duoplasmatron Gas supply PD leak Electrolyzer Mechanical Sealed tube HV supply Electrostatic 4 Mains frequency Medium frequency 4 Terminal control by Ins transfor tot Ins rod 4 Fiber optics TOSHIBA Table 9 cont Type No 1 Homogeneous Inhomogeneous Diff pump Titan getter Turbomolecular Pneumatics Air Oil SF 6 Open Closed circuits 2 5 kVA gt 2 5 kVA gt 4 kVA gt 8 kVA lt 100 us gt 100 us Beam stop Easy Available Not easy Comfortable Not comfort 32 SAMES 2 3 4 HV terminal insulation Power consumption 7 8 Accel tube Vacuum system Cooling system Optional pulsing Spare parts 9 10 11 12 13 solid Table 10 Advantages and limitations of the commercial neutron generators ADVANTAGES insulation oil Small room for installation Reliable electropneumatic Easy beam line assembling Easy maintenance and repair Reliable electropneumatic Insulator shaft motor generator Small room for installation Safe operation in humid
11. 121 Lu uae ETT AFC n AE a o 3 PUMPING SPEED mJ h1 Fig 77 Pumping speed characteristics of a twin stage DP and a single stage UP TUNGSRAM rotary pump 65 TPE CRYSTAL 100 PUMPING SPEED t s 10 1078 10 7 w06 1075 0 5 03 097 PRESSURE mbar Fig 78 Pumping speed vs pressure characteristics of ALCATEL diffusion pumps 64 1200 9001 s B v 800 T0Us C 400 1076 17 v p mbar Fig 79 Pumping speed of a diffusion pump A without traps B with water cooled trap C with water and liquid N 2 cooled traps 122 ACCELERATOR TUBE Fig 80 Arrangement for the pumping speed measurement on a neutron generator less than is needed Normal oil level is 8 10 mm high measured from the bottom of the diffstack In rotary pumps the lower pumping speed stems mainly from problems with the oil level so this should be checked regularly In ion getter pumps the contaminated cathode surfaces mainly by oil de crease the electron emission which may cause a fairly large decrease in the pumping speed 9 4 3 Leak detection As mentioned in Section 9 4 1 if the measured intake exceeds a value of about 10 mbar s then the vacuum system is not tight enough and therefore a leak detection becomes necessary There are many technical methods available to detect a leak starting from single vacuum gauges to the most modern mass spec trometers 67 Howeve
12. 49 The gas flow rate the air admit tance of the EVN 010 H1 and EVN 010 0 H2 is shown in Fig 41 The conductance of a needle valve depends on the viscosity of the gas to be admitted into a vacuum chamber If gas other than air is used the original cali bration curve can be utilized only by taking into account the viscosity of the given gas related to air Table 13 shows the viscosity of the most frequently used gases 49 Table 13 Viscosity of the most frequently used gases Gas 5 Viscosity Poise K air 7 gas Air 180 1 Nitrogen 173 1 04 Carbon monoxide 172 1 05 Oxygen 200 0 9 Carbon dioxide 145 1 24 Hydrogen 87 2 07 Water vapor 93 1 97 Helium 194 0 93 Argon 220 0 82 Crypton 246 0 73 Xenon 225 0 8 For a laminar gas flow in the needle valve the following equation can be used for correction of the flow rate Q of the admitted gas Ogas Si leas Qir K x Qir mbar 1 s 17 69 Example 1 For a neutron generator where the ion source should be fed by deuterium gas nD nH the preset conductance of 10 mbar l s for air Sk scale mark of needle valve EVN 010 H1 is about 13 based on Fig 41 will be higher than for air owing to the K value of hydrogen 2 07 As Fig 41 shows the gas flow is higher than 0 001 mbar l s a flowrate below this value can be set reliably if a vacuum gauge is used The gas flow can then be calculated on the basis of Q Ap x S mbar 1 s 18 where the gas flowrate is Q
13. 52 T Sztaricskai Remote control of Pd leak electrolyzer unpublished 53 S Bederka J Kr l Nucl Instr Meth 145 1977 p 441 228 54 Instruction Manual of J 25 Neutron Generator SAMES Grenoble 55 Instruction Manual of T Neutron Generator SAMES Grenoble 56 J Pivarc Proc XIV th Int Symp Interaction of Fast Neutrons with Nuclei Nov 19 23 1984 Gaussig ZfK 652 p 16 57 P Eckstein et al Proc XIV th Int Symp Interaction of Fast Neutrons with Nuclei Nov 19 23 1984 Gaussig ZfK 652 p 24 58 M P thig P Eckstein personal communication 59 M S ndor D Boly n T Sztaricskai to be published 60 Product Catalogue BALZERS AG Lichtenstein 61 Vacuum Technology its Foundation Formulae and Tables Leybold Hereus GmbH Germany 62 High Vacuum Products EDWARDS Crawley England 1984 63 N S Harris Vacuum Technology EDWARDS High Vacuum Crawley Sussex England 64 High Vacuum Technology Vacuum Components ALCATEL France 1980 65 High Vacuum Components TUNGSRAM Budapest Hungary 66 S Dushman Scientific Foundation of Vacuum Technique John Wiley and Sons New York 1968 67 A Chambers R K Fitch B S Halliday Basic Vacuum Technology Adam Hilger Bristol and New York 1980 68 J F O Hanlon A Users Guide to Vacuum Technology Wiley Interscience New York 1975 69 M von Ardenne Tabellen der Elektronenphysik Ionenphysik und Uebermik roskopie VEB Deutscher Verl
14. HI Check the oil level in the compressor of the cooling unit NOTE During all these maintenance procedures the main power cable is dis connected from the junction box as a safety precaution to prevent inadvertent energizing of the system while the personnel are working on the equipment The maintenance operator should hang a DO NOT SWITCH ON board on the control conso le of the generator There is no possibility of neutrons being generated when the main power cable to the central control console is disconnected The test of the cooling unit should be carried out after maintenance with the service jumper cable without switching on the whole neutron generator The maintenance procedure should follow the instruction in the Manual of the given neutron generator The normal flow rates of the coolants of the cooling system they should be measured periodically are FREON 113 2 gallons min 8 9 l min WATER 7 5 gallons min 28 l min The temperature of the water in the water sump after 40 60 minutes of operation is 5 C or 40 F 194 Isotated puc ee me Flow indicator pipes Neutron generator Ne building D EM N Isolated Sg water E tank 4 15m3j Ng A 5 o m poy Neutron vi S Sprayed water generator e oa D S Ground 10cm Fig 144 Closed circuit water cooling system with buried soil cooled pipes 15 2 CLOSED CIRCUIT COOLING SYSTEM WITH SOIL HEAT EXCHANGER
15. Holds sometimes the voltage divider resistor chain and the ion source The electrodes should screen the insulator walls of the tube to protect them from oil vapour or carbon layer deposit due to the ion beam scattering on the oil vapour in the vacuum The manufacture of an acceleration tube requires a well equipped workshop with special tools for glueing and bonding the components of the tube as well as for its alignment The bonding is usually carried out under pressure The use of PVA requires a polymerization process under high temperature in a special oven The resin and glue are squeezed out to form a fillet both on the inside and the outside of the metal insulator joint This fillet can cause problems because it is situated at the point of the highest electric tension and because it outgasses into the vacuum during the operation of the tube In some tubes the accelerator electrodes are not flat diaphragms but are strongly dished or are cone shaped to shield the fillets and insulators against the electric charges scattered oil and other vacuum dirt The electrodes of an accelerator tube especially the multi electrode homogeneous field accelerator tubes will increase the vacuum resis tance of the accelerator tube as a vacuum component As the ion source the gas inlet is situated at one end of an acceleration tube and the vacuum pumps are situated at the other at the ground potential the accelerating electrodes of an acceleration tube
16. The grid and the cathode resistors 7 5 kQ and 220 Q respectively are high power ca 20 30 W 52 Ua 1400V max Fig 28 Circuit diagram of a 27 MHz 100 W ion source oscillator V OT 100 or GL8005 L 7 50 mm dia 2 x 5 windings 4 mm dia CuAg Lj 18 mm dia 3 x 12 windings 0 8 mm dia Cu L3 18 mm dia 40 windings 1 0 mm dia Cu wire wound resistors The 60 pF resonant circuit capacitor is ceramic high volt age tube type the 1 nF and 5 nF capacitors are inductance free disk type The shape of the high frequency may be observed by an oscilloscope probe placed in the vicinity of the oscillator The bandwidth of the oscilloscope should be at least 30 MHz To pick up the signal from the oscillator a simple wire as aerial or small coil on the end of the coaxial cable input may be used Use always the 1 10 probe at the AC input of the oscilloscope to avoid accidental damage to the ver tical input If the shape of the oscillation is not sinusoidal and shows some saturation this is an indication of the abnormal operation point of the oscil lator valve the tube or the grid resistor should be changed A push pull type oscillator of 100 MHz frequency using two triodes is shown in Fig 29 The output power is about 200 250 W and the circuit forms a grounded grid symmetrical oscillator The metal ceramic plane valves are vented by forced air by a normal fan like fans in the 19 cabinets and they are placed in two metal tube
17. TiTe thick target vs deuteron energy e Meas by Zr Nb Calc Calc s in NEUTRON ENERGY MeV AT 7 230 keV 99 OF Nip may 7 ue z e a r 2 ua z z e wv 2 c 1 ae 0 30 60 90 120 150 180 EMISSION ANGLE Fig 2 Thick target neutron energy vs emission angle at different bombarding deuteron energies The angular dependence of the neutron energy spread relates to E a 150 keV The energy broadening of D T plasma neutrons at T 10 keV 6 is also shown energy However monoenergetic neutrons can be produced only at E d 4 45 MeV below the deuteron break up threshold The energy of the emitted neutrons in reactions 1 and 2 depends on the bombarding deuteron energy and the emission angle of neutrons to the direction of the deuteron beam The thin target data recommended 4 for the angular Y and energy E distributions of neutrons emitted in the D D and D T reactions can be well ap proximated by the following expressions 1 6 in laboratory system Y E40 Y Ep ab YiEgeoso 3 n E E 0 E Eg E EjEgxose 4 In eqs 3 and 4 n 5 and 3 for D D while n 3 and 2 for D T reac tions The evaluated data 5 for thick targets can also be described by eqs 3 and 4 The values of the Y o Yi E and E coefficients from a least squares fit are given in Tables 1 5 for the 20 500 keV deuteron energy range 1 6 In 12 the case of the D T reaction eqs 3 and 4 have been check
18. case the operator of the system should close the V5 valve in the return leg and open the emergency water outlet periodically by the valve V3 see Fig 144 De pending on the water consumption of the diffusion pump the water should be re moved from the diffusion pump every 2 3 minutes The cooling time of the diffu sion pump is about 30 min This means that the V3 valve should be opened and closed 10 times for about 20 30 seconds Using a fast cooling loop around the diffusion pump in case of emergency the cooling down of the diffusion pump will be faster and water consumption from the tank will be less 196 16 PNEUMATIC SAMPLE TRANSFER SYSTEMS A pneumatic transfer system is required to transport samples between the ir radiation and measuring sites Such systems are used for activation analysis by reactors or by intense radioactive neutron sources Cf AmBe PuBe etc In a re actor the neutron field is homogeneous while for neutron sources or neutron generators in a nonmoderated arrangement the irradiation is carried out by point sources These two cases are demonstrated in Fig 146 a b Neutrons Sample Sample Neutron PO eae re DE Source Fig 146 Irradiation of samples in isotropic and nonisotropic neutron fields a Reactor irradiation of thin nonabsorbing sample in an isotropic neutron field b Irradiation of a scattering free sample in a neutron field produced by a point source For a reactor the average activating flu
19. every possible high energy discharge e g buffer condenser of a mains frequency Cockcroft Walton circuit can induce enough power in the circuit of an electronic multimeter to kill the active components In troubleshooting and repair of high voltage units maximum precautions should be taken during the work The voltage and energy of the HV power supplies of a neutron generator are high enough to kill the service personnel therefore troubleshooting and repair should be carried out with great care and never alone For work on high voltage power supplies insulated handled tools test pins and other proper instruments should be used The recommended HV meter for measurement of the output voltage is the usual TV service voltmeter This is a cheap commercially available meter measuring voltages up to 30 kV with a pro perly insulated handle For AC voltage measurements the similarly constructed HV voltage dividers HV test probe are recommended for use with analog multime ters For HV measurements the grounding of the ground contact of the HV meter or HV probe should be tested carefully before switch on of the power supply A second important instrument for HV power supply testing is an ohmmeter working with a few hundred volts for the measurement of high resistances over 10 MQ and for testing rectifying diodes as well as HV condensers or insulations As every neutron generator laboratory has nuclear electronic instruments recti fiers a
20. 37 dn He reaction are detected in a well defined solid angle around a given emission angle to the incident deuteron beam the error in the neutron yield being minimized for an alpha detector placed at 90 to the deuteron beam In this geometry the spread in the neutron energy is the lowest Generally a surface barrier Si detector is used to detect the alpha par ticles In the case of a fast charged particle detector usually thin plastic scin tillators the timing feature is excellent to give the start signal in TOF mea surements and the solid angle of the alpha detector due to the reaction kine matics will determine a coincidence cone of the fast neutrons from the target These fast neutrons will be utilized later as the primary neutrons for the study of neutron induced reactions A typical APM target head utilized at a commercial neutron generator is shown in Fig 153 The deuteron beam is collimated and focused on the target and the alphas are detected by a scintillation detector The scintillation detec tor foil 0 05 mm thick NE 102A is mounted inside the target chamber while the photomultiplier is optically connected by a light guide Perspex cone The solid angle in which the coincidence 14 MeV neutrons can be detected is also indicated 119 The surface barrier SB detectors have excellent energy resolution com pared to the scintillators which allows the measurement of the deuteron or he lium buildup within the tr
21. 4 Vacuum chambers of deflecting magnets sees 144 11 5 Problems with analyzing magnets s ssesseserereererererrerererereersrsreseeo 145 QUADRUPOLE LENSES eteteteesonta eut eseve ene d qe RM Ra I CIA UE d rx Cae 146 12 1 The biased quadrupole lens 4 go bene tre xu n hera eR ae dee ee xe ead 148 12 2 The biased magnetic quadrupole doublet 0 00 cece eee ecee ene eaeeenees 150 12 3 Troubleshooting of a magnetic quadrupole lens ssseessssse 154 HIGH VOLTAGE POWER SUPPLIES seeseseeeeeee eR mash ia 155 13 1 Electrostatic Felici high voltage generator sese 155 13 2 AC DC conversion high voltage power supplies ssssesesess 159 13 2 1 The single phase half wave rectifier ssssseseeesessese 159 13 2 2 Cascade generators oco en ere yap d ona o av RR A RE ARA raa 161 13 2 3 Improved cascade circuits 1 1 eei eee rennen ee 164 13 3 Troublehsooting of high voltage power supplies ccccceseseeeeeeneees 168 BEAM LINE COMPONENTS ccccccccccccccecuccececesevevseueeuvetuuvnvenereres 173 14l Beam SIOPS M S ep 173 I4 2 Beam Scanners uses eter e rd tede bosch e edes E Wee Cip 173 14 4 Ihe Faraday Cup leoi ioo ken om EVRWEX ERR CESEPERI ERE UL TURPE S TEE APPEAR 179 14 4 1 Beam current integration csssseeem nn 182 14 5 Target assemblies aseo ep zia eux eda C TERR IERI US QUERER ER ERE Ye alg 182 14 521
22. A simple closed circuit water cooling system can be constructed in tropical and subtropical regions with a heat exchanger consisting of two water pipes buried in the soil taking advantage of the cooling down of the soil due to the evaporation of the water from the soil Such a system has been built tested and utilized at the Physics Department of Chiang Mai University Thailand 112 The schematic diagram of the system is shown in Fig 144 The water tank of 1500 litres volume is thermally isolated and placed on the roof of next building The circulation pump is also placed there and connected to the bottom of the tank The water pipes are thermally isolated in the air and buried without any isolation in the soil between the neutron generator building and the building housing the tank The two 1 2 dia pipes were buried 10 cm deep in the normal soil There was no grass or other plants over the tubes The temperature of the soil remained always around 20 C even during the hottest days of the dry season when the temperature of the air rose to 28 C during the night and 39 C during the day The low temperature of the soil was achieved by spraying the soil with water two or three times a day Evaporation of water consumes energy so the drying of the soil cools it down This heat exchanger worked satisfactorily when sprayed with water a couple of times a day The system also worked satisfactorily during the long working days without any decrease in pu
23. MeV neutron burst in the ws range while the nanosecond pulsing can be made before and after acceleration using special bunching units The acceleration tube is generally a homogeneous field type allowing a high pumping speed for the vacuum pumps situated at the ground potential The intense neutron generators beam current over 10 mA utilize strong focusing single gap or two and three gap tubes by which the space charge effects are taken into account The high voltage power supplies are usually Cockcroft Walton voltage multipliers Felici type electrostatic machines medium frequency Allibone type voltage multipliers insulated core transformer HV supplies or parallel powered Dynamitron voltage multipliers In a number of commercial neutron generators separate insulating transform ers are used to supply and control the HV terminal These types are as follows SAMES J 15 and J 25 TMC A 111 and KAMAN 1254 models Most of the non commercial neutron generators use a single insulating transformer or an insulated motor driven generator see Fig 11 to ensure the power for the units placed on the HV terminal The control of these power supplies and other components like needle valves can be performed by insulating electromechanically driven rods MULTIVOLT KFKI and TOSHIBA neutron generators or by insulated optical fibers SAMES series T neutron generators INGE 1 in Dresden RTNS II at LLL OCTAVIAN and FNS in Ja pan The optical cables bet
24. Radioactive material storage and waste disposal hazard 219 20 3 High voltage hazard 30 ree Cien EORR EX AERE re E PR i R RP Eee Rp 219 20 4 Implosion Hazard esso sii rere akt oe maa Feo da Ud Re o4 URN d era a E 220 20 5 Pressut hazard ioci ceo eee ae FUE SOR EERUXE VERE HAY coed Faq cu terea dU outs 220 20 6 Fire hazard oec sien Seng NE SEIAS C OV Two ap Mae AsSa e tb m ME ad 220 21 CONSTRUCTION OF A NEUTRON GENERATOR LABORATORY 221 21 1 Construction details 15 1 dorus oaa rel iiaiai n T dee ERR TUS 221 21 2 DU cin PEN 223 21 3 Lbaboratory log Dook 2s ez e td peel pAeu egere bre gp epa e s 224 REFERENCES a tees erts eas aves uqdids deu ea cedes EEEE OE ied EAE 227 ANNEX A LIST OF MANUFACTURERS AND COMPONENT DEALERS 233 ANNEX B TROUBLESHOOTING FLOW CHART FOR NEUTRON GENERATORS WITH RF ION SOURCE sss 241 ANNEX C TROUBLESHOOTING FLOW CHART FOR SEALED TUBE NEUTRON GENERATORS setet inae yet esne seo eterna rane 243 ANNEX D TROUBLESHOOTING FLOW CHART FOR NEUTRON GENERATOR VACUUM SYSTEM eseseeee eene 245 CONTRIBUTORS TO DRAFTING AND REVIEW ccceccceece sere He 247 1 INTRODUCTION Neutron generators are small accelerators consisting of vacuum magnetic electrical and mechanical components radiation sources cooling circuits and pneumatic transfer systems There are various types of ion sources beam acceler ating and transport systems targets high v
25. Tel 44 494 543 488 Fax 44 543 242 Telex 83 141 Products tritium and deuterium targets for neutron generators radioisotopes Contact person Mr Keith L Fletcher subsidiaries in North and South America Asia and Europe NUKEM Industriestrasse 13 D 8755 Alzenau P O Box 1313 GER MANY Tel 49 6023 500 0 Fax 49 6023 500 222 Telex 418 4123 Products tritium technology tritium targets glove boxes MULTIVOLT 26 Loppets Rd Crawley Sussex RH10 SDW ENGLAND Tel 44 293 22630 Fax 44 273 747 100 Products neutron generators components SAMES neutron generator components Contact person Mr D Cossutta 236 KAMAN NUCLEAR 1500 Garden of Goods Rd Colorado Springs Colorado 80933 P O Box 7463 Tel 303 599 1500 Telex 452 412 USA Products neutron generators sealed tube neutron generators neutron generator components related equipment Contact person Mr Frey MF PHYSIC 4720 Forge Rd Suite 112 Colorado Springs Colorado 80907 3549 USA Tel 719 598 9549 Fax 719 598 2599 Products KAMAN components equipments sealed tube neutron generators SGN Societe generale pour les techniques nouvelles 1 rue des Herons Montigny le Bretonneux F 78182 Saint Quentin en Yvelex Cedex RANCE Tel 33 1 3058 6814 Fax 33 1 3058 6852 Telex 698316 Products neutron generators neutron detectors Contact person Mr B Vigreux director INTERATOM Friedrich Ebert Strasse D 5060 Bergisch
26. The discharging resistor should be long enough 171 corresponding to the voltage conditions of the HV power supply and have high enough wattage to prevent the power from dissipating during the condenser dis charge e Testing the resistors Testing the resistor like any other test should start with visual inspec tion The brownish blackish colour of the resistors indicates an overload or burn off of the resistors In the case of a high voltage drop on the serial resistor the interruption in the resistors leads to sparks these can be observed on the surfaces of the painted resistors High voltage resistors covered with epoxy resin layers also show some coloured regions in the case of malfunction Lower value resistances in the range of 1 20 MQ can be tested by the usual multi meters but the higher 10 100 MQ resistors requires megaohmmeters or a few hundred volts power supply and a suitable micro nanoammeter The usual 3 digit hand held multimeters can be used for this purpose The ammeter should always be connected between the ground terminal of the power supply and the output voltage should be connected to the resistor The voltage across the resistor and the measured current determines the resistance of the resistor The resistance of the insulators connectors and other insulating components can be measured by the same method utilizing a nuclear detector HV power supply and suitable nanoammeters 172 14 BEAM LINE COMPONENTS
27. V max 7 100 kV f 500 Hz and I 500 mA n opt 10 It is however not desirable to use the optimum number of stages as then Noda is reduced to 2 3 of its maximum value 2nV and the voltage variations for different loads will increase too much The DC voltages produced with a cascade circuit may range from some 10 kV up to more than 2 MV with current ratings from some 10 A up to some 100 mA The supply frequencies of 50 60 Hz heavily limit the efficiency therefore higher frequencies up to some 10 kHz are dominating A 50 Hz transformer circuit needs a much larger capacitor than 10 kHz so the breakdown along the smoothing column much more likely to damage the acceleration tube or other ion optical components than the low value capacitor stack of a medium frequency cascade circuit 13 2 3 Improved cascade circuits A number of improved HV power supplies have been developed e g the insulat ed core transformer ICT the Allibone voltage multiplier the cascade circuit with cascaded transformers the Deltatron and the parallel capacitively powered Dynamitron Fig 117 Insulated core transformer ICT high voltage power supply 164 a Insulated core transformer type HV power supply The insulated core transformer based power supply Fig 117 is a series con nection of single or full wave rectifiers on different well insulated from the transformer core and from each other secondary coils of a transformer Each recti
28. a channel for water pipelines and cables with a removable cover plate below the floor from the control room to the NGR 15 cm deep and 20 cm wide 21 A combined ventilation and cooling system is recommended for the NGR 22 Further improvement of a neutron generator would make possible to use a TOF spectrometer therefore a channel of about 10 cm dia between the NGR and the control room should be constructed 21 2 WORKSHOPS It is recommended to complete the neutron generator laboratory with a mechan ical workshop containing the usual locksmith s tools table top drill lathe 500 1000 mm and milling machines Arc plasma or acetylene welding is some times useful but these facilities are usually available at other units at the site of the laboratory It is recommended that frequently needed materials be stored in the workshop These are as follows steel aluminium brass plastic and Perspex rods 5 to 50 mm dia screws nuts steel aluminium and bakelite or Perspex sheets up to a thickness of 10 mm Standard profiles for stands and holders are also recommen ded The usual tools and instruments in the electronics workshop are pliers sol dering irons multimeters oscilloscopes It is also advisable to complete the list with some high voltage meters as used for TV repair and some home made high frequency test instruments and tools e g resonant circuit with incandes cent lamp etc The short lived components of the neutron gene
29. and the target lifetime will be increased by a factor of 5 10 The manufac ture of wobbling target holders requires a well equipped mechanical workshop and well trained staff 191 15 CLOSED CIRCUIT COOLING SYSTEMS The cooling water consumption of the usual neutron generator is about 0 5 to 1 5 m h which can supply the target and the high vacuum diffusion or turbomo lecular pumps As the public water supply in many developing countries is un reliable closed circuit cooling systems are recommended In tropical or subtrop ical regions the circulated water should be cooled and chilled However commer cialal water chillers consume a lot of energy so other types of heat exchanger may sometimes be preferable 15 1 THE KAMAN COOLING SYSTEM The A 711 sealed tube generator and the A 1254 pumped neutron generator have the same combined cooling system The Penning type ion sources of these genera tors are located on high voltage terminal and need external cooling The insula tion problems related to high voltage are solved by circulating the electri cally insulating FREON 113 coolant For other neutron generators the ion source or high voltage terminal cooling uses petroleum or transformer oil coolant 110 The cooling system of the KAMAN neutron generator is a compact unit it has two closed circuit coolant loops with circulating pumps and a refrigerator The heat exchanger of the cooling unit chills both the circulated target cooling
30. as close as possible to the vacuum vessel to be checked for leaks short interconnection Vacuum gauges mounted on a vacuum stand can give even before the halogen using in situ leak detection process rough preliminary information on the expected leak if it exists A needle valve can be used to simulate a leak and therefore to check the leak detector Aluminium holder 2221 Ze ring Fig 82 Typical O ring seal for interconnection The vacuum vessel to be tested should of course be well fitted to the vac uum system for the leak test For this it is advisable to use a well fitting O ring for example one like that shown in Fig 82 The diameter of the ring is 125 determined by the diameter of the vessel and by the opening at the top of the vacuum test stand High vacuum plastics like Viton available from a couple of manufacturers can be used as well It should be noted that the halogen leak detector can also be connected to the vacuum system of a neutron generator The most advantageous place for the gauge is near the forevacuum pump In this case the test gas will probably reach the detector The most sensitive leak detection can be performed by a mass spectrometer leak detector which is an essential piece of equipment in almost any laboratory involving vacuum equipment and its associated technologies This may be an existing residual gas analyzer or He leak detector according to whether it is a magnetic sector
31. as shown in Fig 113 If we introduce the ripple factor oV we may easily see that V t now varies between V un and Vinin 8nd V min Vmax 7V The charge Q is also supplied from the transformer within a 159 Fig 113 The output voltage and charging current of the half wave rectifier with buffer condenser C and load R L short conduction time t em T of the diode D during each cycle Therefore Q also equals Q l i t dt ip t dt 39 T T As 1 the transformer current i t is pulsed as shown for an idealized form in Fig 113 The ripple oV is given by IT 1 Q 20VC IT oV 40 2C 2fC This relation shows the interaction between the ripple the load current and the circuit parameter design values f and C As according to the ripple the mean output voltage will also be influenced by oV even with a constant AC voltage V t and a loss free rectifier D no load independent output voltage can be reached The product of fC is therefore an important design factor For neutron generator circuits oscillator focus etc power supplies a sudden voltage breakdown at the load Ry gt 0 must always be taken into ac count The disadvantage of the single phase half wave rectifier concerns the pos sible saturation of the HV transformer if the amplitude of the direct current is comparable to the nominal alternating current of the transformer The biphase half wave rectifier shown in Fig 114 overcomes this d
32. beam is narrow The beam profile observed by quartz or other visual detectors depends on the energy spread of the beam and the ripple of the magnet current The high ripple gt 1 in the magnet current may come from the malfunction of the current regulating transis tors see e g Fig 95 the breakdown between collector to emitter will shorten the regulating circuit and the original ripple of the full wave rectifier will cause a spread in the deflection The punchthrough is a typical problem with pow er transistors and it can be detected by testing the diode base to emitter and base to collector Both junctions wil indicate correct operation while the emitter to collector shows total short circuit The extraordinary warming up of the coils shows the short circuit within the coil The cold and warm resistance of the magnet coil should be tested and checked and regularly recorded in the log book of the neutron generator during maintenance work 145 12 QUADRUPOLE LENSES Magnetic or electrostatic quadrupole lenses are commonly used as post acceleration ion beam lenses at almost all accelerators For neutron generators such as low energy accelerators the magnetic quadrupole is the most frequently used lens Electrostatic quadrupole lenses are more simple but they need power supplies and high voltage feedthrough into the vacuum system Furthermore a high vacuum is required in the system to avoid corona discharges The relatively low energy
33. by a skilled glass blower or made of commercial laboratory glassware Before starting the heavy water electrolysis the electrolyzer and the deuteri um vessel should be evacuated A simple mechanical pump is used after opening the valves V V and Va The mercury level in the long 800 mm vertical glass tube indicates when the whole system is evacuated Closing the valve V3 the mercury level as in a manometer shows the tightness of the system The mercury level should not sink if the vacuum tight system is properly sealed If the Hg level does not sink within about 10 minutes the heavy water electrolyzer can be put into operation To get clean deuterium gas heavy water and deuterized alkalis D 0 NaOD KOD LiOD as electrolyte can be used Since the amount of hydrogen 78 Liquid nitrogen trap Mercury V traps 2 lj Le i j gef Wu D2 vessel us p V3 for ion Dewar a sources vessel ca 1l d P Mechanical pump Power supply 800 mm Water cooled jacket Pt electrodes RR Fig 46 Electrolyzer for filling deuterium tanks of neutron generator ion sources in the solution of 1 2 g NaOH or KOH is negligible compared to the 100 200 ml DO the utilization of normal alkalis to set the conductivity is not usually a problem Depending on the thickness of the Pt tube electrodes and the Pt wires the power supply is a 20 50 V 2 3 A unit A full wave rectifier with variable output volt
34. by a regulator valve The thin wall aluminium box is simply sealed by self adhesive tape The gas inlet and outlet tubes are thin polyethylene tubes The detection of counting gas flow and over pressure is carried out by a small silicon oil bubbling glass vessel The height of the silicon oil in the vessel is about 2 4 cm With methane the exhaust of the fission chamber should be led outside to prevent the formation of an explosive methane air mixture The collector is a well polished thin metal disc held by a metal rod soldered to a high voltage BNC socket The usual voltage applied to the fission chamber through the simple preamplifier is about 600 V 212 Pressure regulator Self adhesive tape Thin Al box argon gas bottle 238 BNC socket UF layer to preamplifier Mae nij Exhaust Silicon oil 300 i 200 S 8 100 0 100 200 300 400 CHANNEL Fig 161 Schematics of a fission chamber and typical pulse height distribution of the fragments 213 The insulating feedthrough is made of Teflon PTFE or similar good insulator The whole monitor counter consists of the fission chamber preamplifier high voltage power supply main spectroscopy amplifier multichannel analyzer and single channel analyzer counter For longer irradiations a monitor counter with time programmed printer or MCA in multiscaling mode is recommended A typical pulse height distribution of the fragments is shown in Fig 161 If the shape of the fiss
35. condensation takes place The vapour condensate reflows to the bottom of the boiler This process can be maintained continuously by permanent heating Air molecules in the pump and in the connected vessel can be mixed by Brownian movement diffusion between the oil molecules while moving towards the wall thus in all probability air molecules can get by collisions a velo city component directed downwards because their mass number is much smaller than that of the oil molecules Gases compressed in this way in the lower part of the pump can be removed by the backing pump The condition of the operation is that the mean free path of the oil molecules should be greater than the distance from the nozzle to the wall This condition can be reached in such a way that before switching on the diffusion pump the whole vacuum system should be evecuated to a forevacuum 101 10 mbar by apply ing a forepump usually rotary type as backing pump to the diffusion pump In addition the rotary pump must be kept in operation as long as the diffusion pump works in order to prevent the disintegration of the lowest vapour jet stream sys tem because of the accumulation of gases from the diffusion pump at the exhaust passage towards the backing On disintegration the velocity of the air molecules in the upward direction is the same as that in the downward direction This is interesting because a jet s disintegration leads to a gradual breaking down of th
36. counter for recording the alpha particles and protons from the 3H d n He and H dp H reactions respectively An electronic system required for producing the start signal at TOF measure ments is presented in Fig 158 This system can also be used for monitoring 210 19 NEUTRON MONITORS Monitoring the neutron yield of neutron generators during the irradiation of samples is important for determination of nuclear cross sections or in activation analysis The neutron field can be monitored by a Alpha particle detection using the associated particle method b Proton recoil detector e g hydrogen filled proportional counters organic scintillators etc c Long counter d Fission chambers Monitoring neutron production by alpha particles has been treated in previous section The use of a proton recoil detector is not discussed Manual but it is treated in Refs 1 2 123 19 1 MONITORING BY LONG COUNTER The long counter is the simplest neutron monitor with a flat shaped efficiency curve in a wide energy range These broad flat energy the pre in this energy efficiency curves allow easy absolute calibration by a standard neutron source As the long S Ville NY MLL LLLLL NN dom SS AA MARTE GCUATESE NN ERE SS y SS I SS SS 7 ri 7 AN 1 2 BFS counter ROO d a W 9 ca 400 metal tube paraffin Y 7 Fig 159 Schematic diagram of a l
37. cross sections of shielding materials for fission and 14 MeV neutrons Fission neutrons E ve 14 5 MeV Material SS ee b 2 cm em cm em Water 0 103 9 7 0 079 12 7 Iron 0 158 6 3 0 112 8 9 Concrete 2 4 g cm 0 077 13 0 Concrete 3 5 g cm 0 094 10 6 0 08 12 5 Barytes concrete 0 095 10 6 0 083 12 1 3 5 g cm Ironshot concrete 0 071 14 3 Gravel 1 83 g cm 1 0 052 19 2 Sand 1 6 g cm 0 047 21 3 Brick 1 83 g cm i 0 048 208 Graphite 1 83 g cm 0 079 12 7 0 058 17 2 Paraffin 0 071 14 1 Aluminium 0 063 15 9 Lead 0 088 11 4 216 1 A where A is the relaxation length of fast neutrons in the shielding mate rial in Table 21 the removal cross sections are given for some shielding mate rials The dose level obtained for a source intensity of 2 5 x 10 n s indi cates that at a distance of at least 2 m from the target an additional 1 5 m wall of concrete p 23 g cm is needed On the basis of the removal cross sections summarized in Table 21 the doses can be estimated both for D D and D T neutrons The macroscopic removal cross section is given by 0 602 Z Do emh 49 where 0 is the microscopic removal cross section in barns p is the density and A is the atomic weight The macroscopic removal cross section for a material com perising several elements is obtained by simple summation over its constituents z z X compound Gar
38. deflected beam results in a more clear emitted neutron spectrum The beam filters of neutron generators utilize electrostatic or electromagnet ic separators or both in the straight line Wien filter It is well known that in an E electrostatic field perpendicular to the direction of the ion beam as shown in Fig 90 the deflection y e along the plates of length is given by 2 E 1 ie on H 33 where E is the electric field strength E U d For small angles the deflection angle is eE1 UI PT ES where U 5 is the accelerator voltage Electrodes Fig 90 Electrostatic deflection of charged particle beams 135 Results show that by the small angle electrostatic deflection the beam com ponents cannot be separated for e m However the neutral beam components oil vapour residual gas will be separated from the ions Electrostatic beam de flection is normally used to produce a pulsed beam at the neutron generators The magnetically deflected beam has a circular trajectory Fig 91 and the dn l p deflection angle is given by 1B e 2mU 35 where B is the the magnetic field strength Equation 35 shows that the deflec tion angle of an accelerated ion beam depends not only on the U acceleration voltage B magnetic field strength but also on the specific charge e m of the given ion The Wien filter utilizes perpendicular E and B fields The Lm magnetic deflec tion of a given ion can be compensated by the a ele
39. determined by the secondary electron emission coefficient of the discharge at the wall In high frequency ion sources the walls of the discharge tubes are important because in a real case the secondary electrons play a role owing to the impurities on the wall which are usually caused by 2 mbar is electrode evaporation The impurities will reduce the extracted ion current and thus the efficiency of the ion source As can be seen in Fig 18 the ions are extracted through a 1 4 mm diameter and 5 20 mm long channel in the hard aluminium extracting electrode using a po 44 XY Al NIZR Fig 19 A version of a high frequency ion source 44 0 10 20 30 40 50 MHz Fig 20 Ignition voltage vs frequency 1000 p715yHg 30yHg 60 yHg 120 yHg 0 50 100 450 MHz Fig 21 Ignition voltage vs gas pressure Inhomogeneous magnetic field Homogeneous magnetic field 407 10 0 200 400 600 800 0e Fig 22 Extracted ion beam density in HF discharges vs static magnetic field tential difference variable from 2 to 10 kV To decrease the power and gas con sumption a permanent magnetic field is applied to the discharge volume either transverse or axial to the axis of the tube By an axial magnetic field the ion concentration at the extracting electrode can be increased because the electrons will circulate on a helical orbit at a Larmor frequency _ eB fL 4m 9 46 P gw 200 p 7
40. etc accidentally bypassing the magnetic circuits can be observed visually while the magnetic short circuits inside the ion source can be found only after dismantling the source If the source is opened carefully clean the ceramic HV leadthrough the cathode and the anode surfaces The oil deposit on the cathode can be removed only by sandpaper and polishing paper After drying the surfaces clean them with organic solvents When the components are dry assemble the ion source If the extraction slit becomes bored out by backstreaming secondary electrons re place the plasma extraction cup with a new orifice insert if it was used in the original ion source Test the mechanical position of the extraction electrode The electric test of the power supply and the ion source requires a high voltage voltmeter up to 30 kV and a MQ meter using a couple of hundred volts Test the HV transformer the rectifiers and the condensers The components can be checked with a normal everyday multimeter through an HV probe The resis tors Ry and Ry can be tested similarly with the same multimeters Test the HV cable and the connectors under working conditions as well The operation the insulation of the HVPS and the source can be tested at atmospheric pressure If the system seems to be working electrically normally the fault should be searched for elsewhere for example in the gas supply or in the vacuum system 62 6 0 DEUTERIUM LEAKS The ratio of at
41. from the first stage is internally connected to the inlet of the second stage Fig 61 102 Inlet Exhaust Fluid seals Fig 61 Cross section of a two stage rotary vane pump Gas molecules Inlet flange Vapour jet Cooling coils Condensed vapour Exhaust flange Backing line Hot vapour Fig 63 Schematic cross section of a diffusion pump 103 This solution improves the ultimate pressure of the pump by reducing the back leakage where the rotor and stator are fluid sealed To reduce the condensation of vapours during the compression cycle gas ballasting can be used where a controlled quantity of a suitable noncondensable gas usually air is admitted during compression Fig 62 In Fig 62 vane A is about to close the crescent shaped chamber B containing the gases and condensable vapours being pumped As this volume is sealed from the inlet a controlled volume of air at atmospheric pressure is admitted at point C The air intake raises the pressure in B and prevents condensation of vapour by opening the exhaust valve before the conditions of condensation are reached b The diffusion pump The operation of the diffusion pump can be seen in Fig 63 The pump fluid usually oils with large molecules is heated electrically and the oil vapour streams through the chimneys Then the vapour molecules emerging from the ring shaped nozzles with supersonic speed are directed toward the cooled pump wall where
42. in gj 26 3 1 h Pwo Maximum tolerable water vapour inlet pressure in mbar S Volume flow rate in m at inlet pressure Pwo T Thermodynamic temperature of the water vapour pumped in K 96 Table 15A Multiples and units Power of Prex Desig 10 nation 10 giga G 10 mega M 10 kilo k 10 deci d 10 centi c 10 milli m 10 micro u 10 nano n 10 pico p Table 15B Vacuum technological quantities 62 Quantity Pressure Total pressure Partial pressure of gas aw constant i Saturation vapour pressure Vapour pressure Residual total pressure Residual gas pressure Residual vapour pressure Ultimate pressure Mean free path Collision rate Collision rate related to area Particle density Throughput Leak rate Pumping speed Conductance Impedance Conductance Symbol SI units p N m Pa P P P Pq P P Pa Pona r A m Z 1 s z 1 s m n 1 m qv N m s q N m s S m s C m s R s m L l s Recommended units bar mbar mbar mbar e g PH PN mbar mbar mbar mbar mbar mbar m cm E Qi m Qi cm mbar l s mbar 1 s l s m gt h m s l s s m s l l s 97 Table 16 Conversion tables a Temperature K 9c OF Kelvin 1 C 273 15 5 9 F 459 67 Celsius K 273 15 1 5 9 F 32 Fahrenheit 9 5 K 459 67 9 5 C 32 1 b Pressure Pa mbar bar torr atm 1 N m 1 Pascal 1 0 01 10 7 5x10 1 mbar 100 1 10 0 75 1 bar 10 1000 1 750
43. in the wall For electric cables about four 5S cm dia channels should be constructed of steel tubes in the wall close to the ceiling of the control room The unused channels can be closed with iron bars For the pneumatic transfer system 5x3 cm and 8x4 cm channels should be con structed by steel profile tubes in the wall The unused channels can be clo sed with an iron plug Although the pneumatic tubes could run from the gene rator to the measuring rooms along the wall it is nevertheless advisable to construct the proposed channels It is necessary to construct lead boxes in the wall of the NGR for storage of radioactive sources and tritium targets The boxes should measure 30 cm x 30 cm x 30 cm and be completely surrounded by a 5 cm thick layer of lead Warning signs and light beacons should be instaled in the control room to warn of high voltage hazards The generation of neutrons should be indicated by a warning light or rotat ing beacon connected to the HV on switch and to the neutron flux monitor An HV interlock system should be installed at the NGR door Water safe lamps and plugs must be used for the lighting and electrical con nections in the NGR The floor of the NGR must be able to carry about 20 t m and have an out let tube to a water sink In tropical climates an air conditioning system is required in all rooms of the NGL A water tank with a pump is needed for closed circuit cooling 20 It is advisable to construct
44. instant is about U Q C above the ground where Q is the charge collected and C is the capaci out tance of the HV electrode to the ground The potential of the HV electrode ie the voltage of the HV terminal of the neutron generator rises at a rate given by the expression dU dt I C where I is the net charging current to the terminal I sbv where s is the charge density on the the surface of the cylinder in Cou lomb cm b is the height of the cylinder and v is the tangential velocity of the cylinder in m and m s respectively As the loading behaviour of the Felici generator depends on the dU dt charging capability of the construction the larger size of the insulating cylin der ensures a larger loading possibility of the HV generator as well The output high voltage will be controlled by the regulation of the U charging voltage using a feedback loop consisting of an Ri R voltage di exc vider and an operational amplifier 157 The maximum charge density on the rotor surface is about 0 01 0 02 uC em the tangential field on the surface of the rotor is maximum 15 kV cm The output current of the HV output varies between 100 A and 15 mA depending on the speed of the rotation the surface size of the rotor and the number of charge collect ing inductor ionizer pairs poles Figure 110 shows a two pole machine The max imum achievable output current of the Felici generator depends on the number of poles its maximum output
45. ion beams in neutron generators need simple low voltage magnet power sup plies and the lack of vacuum feedthrough makes the magnetic lenses much simpler to use The only drawback of the magnetic quadrupole lenses is the relatively heavy weight of the coils and the iron cores of the lenses Fig 99 Electrostatic quadrupole lens focusing in the horizontal or vertical plane Fig 100 Magnetic quadrupole lens focusing in the horizontal or vertical plane 146 Fig 101 Approximation of the hyperbolic electrode by circles cylinders electrostatic quadrupoles Fig 103 Focusing particle trajectories of a quadrupole doublet 147 The quadrupole lens consists of four hyperbolically shaped pole faces or electrodes The quadrupole lense focuses in one plane and defocuses in the per pendicular plane Thus several such lenses must normally be combined to make a useful lens system Usually quadrupole doublets and triplets are in use at neu tron generators Since the hyperbolic pole faces or electrodes are difficult to fabricate the shape is approximated by circles or steps If the focus plane of the quadrupole lens is in the vertical or horizontal plane the beam directions z axis of the electrostatic and the magnetic quadru pole lenses will have the shape shown in Fig 99 and Fig 100 45 rotation respectively In practice the hyperbolic pole shapes are approximated by cylinders elec trostatic quadruplets and by steps magnetic
46. is in the order of 1 per day Tritium in gaseous form is absorbed by the lungs at a rate of 0 1 The maximum permis sible air concentration for tritium is 102 Bq cm A few hours after exposure the body fluids contain the same concentration of HTO and T5O therefore urine analysis indicates the level of incorporation A tritium concentration of 1 Bg l in urine represents the maximum permissible dose All components of the ion source accelerating tube beam transport system target system vacuum equipment exhaust lines and target cooling water system become tritium contaminated A small amount about 5 96 of the total tritium re leased from the target is deposited on the static components of the generator The bulk of the tritium is accumulated in the pump oil and the elements of ion pump or vented to the atmosphere by the roughing pump In the forepump oil the 218 contamination was found to be 10 to 20 times higher than in the oil of the diffu sion pump During repair and maintenance contamination must be kept as low as possible For example components in the vacuum system of RTNS II near the target have 10 to 107 dpm cm of surface contamination which is detectable by wiping Routine target changes can cause tritium incorporation by personnel at a level which is observable in urine The handling and replacement of tritium targets re quire the use of a well ventilated glove box to reduce the hazards from inhala tion of triti
47. mbar l s the pressure change in the vacuum chamber is Ap mbar and the pumping speed of the vacuum pump is S l s Example 2 If the pressure in the vacuum system is increased from a value of 2x10 to 3x10 6 mbar and the pumping speed is 100 l s the value of Q will be 6 x 100 10 mbar 1 s 19 Q 10 Gas flows below 107 mbar l s can be set relatively easily by means of a vacuum gauge Experience has shown that a lower limit of ca 106 mbar l s can be reached but that the long term stability of the valve begins to decline at gas flowrates below 10 mbar l s 6 2 4 Maintenance of needle valves Needle valves do not require any maintenance in general In the course of time however some contamination effects will become apparent Very fine dust particles can still pass through the filter of the inlet port of the needle valve and they can settle in the gap between the needle and the needle seating Adjustment of the low gas flows becomes difficult and the leak rate of the closed valve deteriorates These symptoms indicate that the valve requires clean ing The dismantling and cleaning procedure of needle valves is described for BALZERS EVN 010 H1 type valve in the manufacturer s Manual 49 70 a Dismantling the valve see Fig 40 Take off the cap 9 from the actuating knob 2 Remove locking washer 12 from spindle needle 5 Unscrew knob 2 with scale drum 3 counterclockwise from sleeve socket 4
48. or quadrupole type but it does not need a very high resolution because it is only necessary to separate the two most commonly used gases hydro gen and helium at mass numbers 2 and 4 respectively The conventional mass spectrometer leak detector is normally a portable unit having its own vacuum system including a rotary and a turbomolecular pump liquid nitrogen trap gauges and electronics as shown schematically in Fig 83 Mass spectrometer Neutron generator Penning gauge Speed control Backing Fig 83 Utilization of mass spectrometer as leak detector Gas em CN oN intet M A oov INA C esser T3 s Electron otectiag loniser T Quadrupole multiplier aids Focusing electrodes Fig 84 Schematic diagram of a quadrupole mass spectrometer 126 The most specialized mass spectrometer leak detector is usually quadrupole in view of its small flange mounted gauge head and easy operation The ion beam produced by electron collision in the ion source is accelerated and injected into a quadrupole separation system having four electrodes of hyperbolic cross section see Fig 84 69 A constant high frequency voltage U f 2 5 MHz and a superimposed DC voltage V are applied to the electrodes As the value of U and V are increased simultaneously the mass will be scanned from an initial value up to a maximum whereby the ratio U V must be kept precisely constant The mass number of the ions emerging through the
49. oscillation 55 Brighter light a Ce i Fig 32 The use of tube light for testing an HF oscillator Perspex rod Incandescent lamp Fig 33 Qualitative measurement of the output power by incandescent lamp The incandescent lamp also gives a qualitative indication of the output pow er of the high frequency oscillator for higher power the light is brighter Fig 33 If the tube light or neon lamp lights up in the vicinity of the oscillator but there is no light in the discharge tube this indicates that vacuum is bad high pressure in the whole system In normal operating conditions the light emitted from the balloon is vivid reddish violet or bright pink A light blue colour indicates bad vacuum in the discharge balloon or that the D gas is dirty containing air For testing the neutron generators the use of H gas instead of deuterium is recommended The difference is only the lack of neutron production on the blind target or constructional parts which is an advantage when testing and repairing 56 neutron generators The bremsstrahlung still exsists If the discharge is grey ish pink or light blue and the extraction current shows a weak plasma it can happen that there is no more D gas in the bottle or that there are problems with the pressure regulator valve palladium leak thermomechanical leak or needle valve If the D bottle has a built in manometer check the pressure in the bottle ATTENTION All ma
50. quadrupoles These approximations are shown in Fig 101 and Fig 102 As the quadrupole singlet focuses in one plane and defocuses in the perpen dicular direction minimum doublets are therefore always used The particle tra jectories started at the center of the object x 0 and y 0 ina double fo cusing doublet as shown in Fig 103 The doublet is a double focusing system The calculation of the ion trajectories along the quadrupole lenses can be found in Ref 91 The exact calculation and lens design is beyond the scope of this Manual A magnetic quadrupole doublet is described in Section 12 1 12 1 THE BIASED QUADRUPOLE LENS The biased quadrupole lens is powered asymmetrically so that the particle beams are focused and steered The repeated and hopefully always convergent alignment of the beam line element by the steering effects of the quadrupoles can be eliminated Instead of mechanical adjustment it is quite practical to make an electri cal alteration of the effective position of the quadrupole by unbalancing the voltages of the segments or the current through the coils The circuits of the quadrupole biasing network are very simple 92 The magnetic quadrupole lens has four equivalent coils and in the simplest case the current flows in series through the coils When the symmetrical supply of the coils is altered the different pole pieces will be magnetized differently The balanced supply of the quadrupole can be changed b
51. separation system and being detected must satisfy the following conditions V M 32 2 2 7 2 f to where r is half the distance between two opposite electrodes As a result of the linear dependence of the mass M and the voltage V a lin ear mass scale may be obtained by a linear scan of V and U The filament of the ion source in the spectrometer is switched on when the pressure in its own vacuum system is less than 5 x 104 mbar and the vacuum system of the neutron generator under test is to be tested with a helium gas jet The leak is detected by the audio indication and or by the analog meter in the readout The sensitivity of this detector can be improved by increasing the gain of the electrometer amplifier and or reducing the effective pumping speed of the high vacuum pump by adjusting the speed control valve but still maintaining the maximum pressure at which the filament operates 127 10 BEAM ACCELERATION AND BEAM TRANSPORT SYSTEMS 10 1 ELECTROSTATIC LENS The electrostatic lens is the most usual focus device in low energy acceler ators and neutron generators The energy of the ion beam extracted from the source is a few tens of keV This beam can be focused easily by an electrostatic lens The magnetic lens is very rarely used in neutron generators The preaccel eration lens is usually an electrostatic diaphragm immersion or unipotential lens The unipotential lenses is also known as the Einzel lens
52. significantly by the use of Pd Ag alloy The rate of permeation of hydrogen through palladium is higher than that of deuterium the ratio Pgy Pp ranging from about 1 2 to 2 5 depending on temperature and the condition of the Pd metal 47 48 The usual setup of a palladium leak consists of a palladium tube closed at one end which is heated either indirectly by a separate heater spiral or directly by resistance heating The palladium metal has special features about one litre of hydrogen gas can be absorbed by one gram of palladium The two usual construc tions of Pd leaks are shown in Fig 37 The indirectly heated palladium leak manufactured of glass is usually con nected to heavy water electrolyzers The directly heated Pd tube is placed inside a high pressure D tank A common disavantage of palladium leaks is ageing After about 150 200 h o perating time the transparency of the palladium will decrease at a given tempera ture and the leak gives a lower D flowrate If this takes place the heating of 63 D to the ion source pressure 10 16 mbar Pd tube Thin glass insulator Heater spiral Up 6 10V AC DC Up 1 24A Outlet D to the ion source e r1 102 F F p 10 10 mbar Manometer Atmospheric pressure Filling inlet valve Insulated feed through Ip 10 124 D tank pressure bar Fig 37 Directly and indirectly heated Pd leaks the Pd tube should be increased As the heating in b
53. single two etc gap acceleration tube consists of cylindrical or conical immersion lenses see Fig 85 This tube is common in fixed acceleration voltage machines such as the sealed tube neutron generators Two gap acceleration tubes are used 131 in TMC A 111 and KAMAN A 1254 neutron generators The first gap is for focusing and the second for acceleration Recently accelerating tubes with either a single gap 74 78 80 or special multigap arrangements 81 84 are used in improved neutron generators The sin gle gap high gradient system requires particular attention to the purity of vac uum and the manufacturing of the electrode During operation a pressure higher than 10 Pa must be assured in the cavity free from hydrocarbon molecules A carefully polished and cleaned titanium electrode is needed to maintain a field strength of 200 kV cm Secondary electrons ejected by positive ions from the neutral atoms in the beam and on the electrode surface are accelerated towards the high voltage terminal The intensities maximum kinetic energies and mean energy values of electrons can be deduced from the measurements of the brems strahlung These data can give information on the optical behaviour of accelera tion tubes with different field structures 85 86 The acceleration tube in a neutron generator also Accelerates the extracted D ions Keeps together the D ion beam or focuses it Holds the vacuum in the neutron generator
54. the DAC is also demultiplexed The transmission techniques and the electronic equipment in the vicinity of the neutron generator were designed and developed taking into consideration the noise immunity and protection from corona discharges and sparking along the sys tem The analog and digital binary signals are transmitted and received by 91 12V Fig 58 Optical receiver of the signals from the HV terminal analog meter driving frequency to voltage converter in the control desk glass fiber optics The electronic components used in the terminal blocks are radiation hardened The optical transmission system utilized for the transmission of the mea sured and control voltages is shown in Figs 55 and 56 Fig 55 shows the transmit ter receiver configuration related to the operator console ground potential while Figs 57 and 58 show the HV terminal to ground optical insulation circuits The circuit in Fig 55 is a voltage to frequency converter with infrared LED out put This circuit is utilized both on the ground potential and at the HV termi nal The optical receiver frequency to voltage converter is shown in Fig 56 This circuit also has a comparator actuating a limit switch This limit switch is utilized as a mains switch on device at the HV terminal The analog output of the LM 331 integrated circuit utilized in frequency to voltage mode is led to the re mote control input of the power supplies on the terminal 59 92 9 V
55. the target exchange is finished collect the plastic foil from the floor and put it into the plastic bag which had previously covered the target tube Place the plastic bag and other probably tritium contaminated litter into the radioactive litter bin Put back the used target with its container into the vented target storage glove box in the place for used targets Put back the tools and gloves used for the target exchange into the same vented glove box that stores the tritium targets Wash your hands and test with a tritium monitor whether they were properly washed Do the required administration related to the targets Attention Tritium targets should be handled carefully The biological half life of tritium gas in human organs is about 11 days but the small chips and powder from the titanium layer absorbing the tritium has a much longer biological half life in the human body and they will act as hot spot intense beta ra diation sources To SAMES beam tube max 1mm Al backing Ti tayer A1 M31 969 PTFE RING Cin mm Fig 137 Thin wall tube air cooled target holder for SAMES neutron generators 187 14 5 2 Air cooled target holder The air cooled target holder shown in Fig 137 can be connected to the beam line of a SAMES neutron generator The target holder is made of AlMgSi alloy hav ing a relatively low inelastic scattering cross section The target is held by the thin wall tube and is isolated ele
56. voltage is approximately proportional to the distance between the poles For a given diameter of the rotor the output voltage is in versely proportional to the number of poles 95 The motor driving the HV generator rotor rotates at a speed below 3000 rpm This usually corresponds to a velocity of 15 25 m s of the surface of the rotor The efficiency of the HV generator is between 80 90 This is a remarkably high value compared with the efficiency of the usual voltage multipliers The utilization of pure dry hydrogen 10 25 bar pressure gives an excel lent insulation ion transfer onto the surface of the rotor and good thermal conductance ie cooling The tank of the Felici generator does not need any extra cooling facility in some Felici generators utilized at neutron generators the HV tank is sometimes cooled by the same cooling water as the vacuum pumps and the target The rotor the ionizers the stator the driving motor and the precision re sistor chain Ry are enclosed in the air tight tank called the hermetically sealed unit The unit does not require any maintenance and it can be repaired on ly at the manufacturer s In case of any difficulties related to the hermetical ly sealed unit the whole unit should be replaced The only duty related to the hermetically sealed unit is the regular say monthly checking of the hydrogen pressure in the tank The pressure gauge can be found usually at the bottom of the pressure tank Uout
57. wa ter and the ion source cooling FREON 113 The operation of the cooling system is controlled by the temperature of the circulated water and by the coolant flow detectors A schematic representation of the KAMAN cooling unit is shown in Fig 142 The two coolant flow switches detect the loss of the coolants and inter lock the operation of the neutron generator HEAT EXCHANGER COOLING MACHINE E I mE FREON 11 FLOW SWITCH LOOP l PUMP FAN FREON 12 Loop dr py RC ee DOPO NEN J Fig 142 Schematic diagram of the KAMAN cooling unit 192 water pump water filter water pressure gauge water bypass valve water sump FREON 113 pump FREON 113 filter FREON 113 bypass valve FREON 113 sump temperature control RAMCO liquid eye sight glass rnTP ETEZOmTUOgOU Fig 143 Location of the main components of the KAMAN cooling unit 15 1 1 Maintenance The smooth operation of the cooling system and the neutron generator re quires regular daily and monthly inspection of the coolant levels in the FREON 113 and the heat exchanger water sumps The lost of FREON 12 in the cooling ma chine can be observed through the liquid eye sight glass of the compressor Regu lar inspection of the coolant tubing and joints is recommended The built in fil ters and flow switches must be cleaned at monthly intervals depending on working hours and conditions If a coolant
58. 06 0 98 1 torr 1mm mercury 133 32 1 33 1 1 atm 101325 1013 1 013 760 1 1 at 1 kp cm 98066 5 981 0 981 735 6 0 97 1 m of water 9806 65 98 1 0 098 0 097 0 1 1 lb ft 1 psf 4788 O47 0 36 f 1 lb in 1 psi 68948 6895 5171 0 068 at kp cm mW s psf psi 1 N m 1 Pascal 1 mbar 0 0102 2 09 0 014 1 bar 1 82 10 19 2089 14 50 1 torr 1mm mercury 2 78 0 019 1 atm 1 033 10 33 21162 14 69 1at 1 kp cm 1 10 20482 1422 1 m of water 0 1 1 204 82 1 42 1 lb ft 1 psf 1 1 Ib in 1 psi 144 1 98 Table 16 Cont c Volume flow rate Conductance m s I s m h ft min 1 m s 1 1000 3600 2118 88 1 l s 10 1 3 6 2 119 m h 2 78x10f 0278 1 0 59 1 ft min 472x10 047 1 69 1 d pV throughput Leak rate W mbar l s torr l s cm m NTP 31 Pa m s 1W 1 10 75 592 1 mbar l s 0 1 1 0 75 59 2 1 torr 1 ls 0 133 1 33 1 78 9 3 1 cm m NTP 1 69x10 0 017 0 013 1 e Ion source gas consumption pV throughput unit 1 ml atm hour 2 8x10 mbar l s at Normal Temperature and Pressure NTP 9 1 2 Units 61 The basic units were established in October 1954 at the First General Con ference for Mass and Weight in Paris This uniform system was given the interna tionally binding initials SI from the French Systeme International d unites The recommended units are mostly multiples of fractions of the SI units Table 15A As exceptions to this some units which have become customary in ce
59. 1 5610 mmHg f 30 MHz 460 Uanode 800 V 120 80 Usnode S00 V 0 0123 4 5 6 7 8 wy w Fig 23 Dependence of the power consumption vs magnetic field and extracted ion density vs high frequency power 44 and consequently the ionization probability will increase Furthermore the plas ma with diamagnetic behaviour is compressed in the presence of a magnetic field The magnetic field parallel to the electric field can be either homogeneous or inhomogeneous It was found that the ion current density in the linear high frequency discharge is substantially higher in the presence of an inhomogeneous magnetic field see Fig 22 The power consumption of the discharge and the luminosity of the plasma show a resonance type increasing in a given interval of the static magnetic field see Fig23 This resonance phenomenon can be observed at a pressure of p 1x10 to 5x10 mbar The resonance in high frequency ring discharges in ductively coupled HF ion sources has been observed in transversal magnetic fields at frequencies wy 7150 to 30 10 while in longitudinal fields at frequencies Ou 30 to 60 11 47 5 2 EXTRACTION OF IONS FROM ION SOURCES Well collimated ion beams with small diameter which are mainly required in neutron generators can be obtained only from sources equipped with an ion ex tracting system The probe type and the diaphragm type extraction systems are generally utilized in the neutron generators In ion so
60. 151 Typical post acceleration klystron bunched neutron generator based on a commercial neutron generator 17 2 POST ACCELERATION KLYSTRON BUNCHING OF A COMMERCIAL NEUTRON GENERATOR A post acceleration klystron bunching nanosecond neutron generator was con structed in Chiang Mai Thailand based on a commercial SAMES J 25 neutron generator The extracted and accelerated beam is chopped by a 2 MHz electrostatic deflector and bunched by a 4 MHz two gap klystron The schematic representation of the post acceleration nanosecond pulsing is shown in Fig 151 The operation of the system is shown by the corresponding waveforms 117 The 150 keV energy previously selected deuteron beam enters the bunching section of the generator through the first water cooled slit The horizontal de 204 155 SUIT DEFLECTOR and SLiT ue n5 BUNCHER x pa PICK OFF Y Jv M3 TARGET Rim MOVABLE I 4 SLIT l FAST a Hoov oc OUTPUT TANK imm 400 V DC CIRCUIT RF DOUBLE AMPLIFIER STEERER CONTROL PANEL CHOPPER AND BUNCHER CONTROL PANEL CONTROL ROOM ss BEAM CURRENT MONITOR BOARD Fig 152 Block diagram of klystron bunched J 25 neutron generator flector plates chop the deuteron beam by 2 MHz sinusoidal voltage a in Fig 151 The selection of the pulses by changing the repetition frequency is carried out in front of the klystron buncher by the second vertical deflec
61. 250 3 139 2 511 2 009 0 300 3 220 2 524 1 978 He 0 050 4 041 3 531 3 086 from D T 0 100 4 261 3 520 2 910 reaction 0 150 4 436 3 551 2 779 0 200 4 587 3 501 2 672 0 250 4 723 3 491 2 581 0 300 4 848 3 481 2 499 3 He 0 050 1 093 0 807 0 596 from D D 0 100 1 223 0 795 0 516 0 150 1 329 0 782 0 460 0 200 1 423 0 769 0 416 0 250 1 510 0 757 0 380 0 300 1 591 0 745 0 349 Table 7 cont Residual Bombarding particles deuteron Energy of residual particles MeV energy MeV o 90 180 p 0 050 3 324 3 036 2 772 from D D 0 100 3 465 3 048 2 657 reaction 0 150 3 579 3 061 2 617 0 200 3 681 3 073 2 566 0 250 3 773 3 085 2 523 0 300 3 860 3 098 2 486 5H 0 050 1311 0 997 0 758 from D D 0 100 1 451 0 984 0 668 reaction 0 150 1 565 0 971 0 603 0 200 1 666 0 959 0 552 0 250 1 758 0 946 0 509 0 300 1 844 0 933 0 472 The Zn n p Cu 5Ni np Co and the P np si reactions are re commended as energy monitors for D D neutrons in the 2 3 MeV range The latter is a pure beta emitter Cross sections in the 2 and 3 MeV neutron energy range are summarized in Table 6 The angular yield of D D neutrons is measured by the HSIn n w In reac tion at 200 keV 15 bombarding deuteron energies As can be seen in Fig 7 the measured and calculated thick target yields are in good agreement with each other proving the flat shape of the cross section curve of IPIn n n P In re action in the 2 1 2 9 MeV range A value of a 325 5 mb is recomme
62. ACUUM SYSTEMS OF NEUTRON GENERATORS 9 1 IMPORTANT TERMS AND UNITS IN VACUUM TECHNOLOGY 60 9 1 1 Terms The terms used in descriptions and technical data for the pumps and compo nents in catalogues are standardized The more important terms given below have been chosen to assist those less experienced in vacuum technology in the use of the catalogues and to extend their utilization An absolute pressure gauge is a pressure gauge used to determine the pres sure from the normal force exerted on a surface divided by its area An abso lute pressure gauge is independent of the gas type used Absorption is a type of sorption in which the gas absorbed diffuses into the bulk of the solid or liquid absorbent Adsorption is a type of sorption in which the gas adsorbed is retained at the surface of the solid or liquid adsorbent Backing pressure is the pressure at the outlet of a pump which discharges to a pressure below atmospheric only Compression ratio is the ratio between the outlet pressure and the inlet pressure of a pump for specific gas Concentration of molecules is the number of molecules contained in an ade quately chosen volume divided by that volume Intrinsic conductance is conductance in the special case where the orifice or duct connects two vessels in which Maxwellian velocity distribution prevails In the case of molecular flow intrinsic conductance is the product of the con ductance of the inlet port of the conductance o
63. DP 31 42 44 A comprehensive monograph on atom and ion sources has been published by V lyi 44 discussing in detail both theoretical and practical aspects The advantage of KRF sources is their high monatomic ratio 90 906 while PIG and DP have high currents The high frequency ion source proposed by Thonemann and co workers 45 and improved at Oak Ridge 37 is applied in different versions in many laboratories The high frequency discharge in HF ion sources can be generated by an in ductively or a capacitively coupled oscillator of 15 to 100 MHz frequency with a power consumption of 100 to 400 W The discharge generated by the high frequency electric field applied to two electrodes placed outside or inside the discharge tube is called either capacitively coupled or linear high frequency discharge This type of ion source is used in the SAMES called later AID MULTIVOLT etc neutron generators see Fig 18 37 The discharge generated by the high frequency magnetic field in the dis charge tube placed inside the solenoid of the high frequency oscillator is called inductively coupled or ring high frequency discharge This type of HF ion source used at KFKI Toshiba etc neutron generators is shown in Fig 19 39 In both cases the discharge chamber of about 30 to 50 mm diameter and 100 to 200 mm long is made of Pyrex or quartz glasses The free electrons are accelerated by the induced alternating electric field E and will oscilla
64. Fig 136 is a spring The proper connection be tween the target backing and the connector the figure shows a female BNC connec tor can be ensured by the proper contact After reassembling the target holder the target connector should be tested for conductivity between the target connec tor and the target housing A couple of MQ resistance especially with cooling water does not influence the accuracy of the target current measurement if the target current meter circuit has an input resistance in the range of kQ The cooling water usually gives a specific resistance of MQ cm between the target and the target housing if the resistance is measured by the usual multimeter which uses a couple of volts at resistance measurements This water resistance between the target and its housing should be taken into account when a target current integrator is used The lower input impedance of the integrator can ensure the higher accuracy of the target current measurements 186 When the target exchange has been completed and the seals and contact have been tested and found to be normal close the valve of the forevacuum pump suck ing down the target tube and open the gate valve isolating the high vacuum part of the neutron generator from the target tube The reading of the high vacuum me ter should reach the normal value in about ten minutes If the reading of the high vacuum does not reach the normal value it is an indication of some leaking When
65. GLE 6 0 50 100 150 Ey 200 keV 500 1 00 R 6 0 6 90 at Ej 200 keV C mb 400 0 95 300 pu a 1 0 2 0 3 0 4 0 NEUTRON ENERGY MeV Fig 6 Absolute and relative cross section curves for the 238r n f reaction between the threshold and 14 MeV neutron energy 18 D d n He E 200keV O measured eye guide calculated Relative yield 0 50 100 150 E Emission angle Fig 7 Measured and calculated angular yields of D D neutrons Table 6 Recommended cross sections of energy monitors for D D neutrons Neutron o mb c mb o mb energy Zn n p 5 Ni n p 3IP n p 1 9 2 0 6 095 36 8 8 353 2 0 2 1 8 635 46 28 9 338 2 1 22 11 65 60 12 14 57 22 23 15 82 73 95 24 04 2 3 2 4 20 14 87 79 31 86 2 4 2 5 26 77 101 6 38 02 2 5 2 6 34 36 117 1 46 50 2 6 2 7 43 10 134 3 57 30 2 7 2 8 54 70 151 6 62 52 2 8 2 9 66 45 168 8 62 17 29 3 0 78 80 186 0 61 49 Table 7 Energies of residual particles emitted in T d n He D d n He and D d p H reactions Residual Bombarding particles deuteron Energy of residual particles MeV energy MeV 0 90 180 n 0 050 14 554 14 068 13 599 from D T 0 100 14 783 14 088 13 432 reaction 0 150 14 960 14 108 13 304 0 200 15 117 14 128 13 203 0 250 15 259 14 148 13 117 0 300 15 390 14 167 13 042 n 0 050 2 723 2 462 2 225 from D D 0 100 2 852 2 474 2 146 reaction 0 150 2 958 2 486 2 090 0 200 3 052 2 498 2 045 0
66. Gladbach GERMANY Tel 49 2204 840 Fax 49 22004 843 045 Telex 887 857 Pruducts accelerators neutron generators Contact person Mr K H Weyers IMAGING AND SENSING TECHNOLOGY Westinghouse Circle Horsehead NY 14845 USA Tel 1 607 796 3400 Fax 1 607 796 3279 Telex 490 998 9073 Products neutron generators neutron detectors Contact person Mr Wiliem Todt 237 EFREMOV Scientific Research Institute of Electrophysical Apparatus 189631 St Peterburg Russia Tel 7 812 265 7915 or 265 5658 Fax 7 812 265 7974 or 463 9812 E mail ROZHKOV NIJEFA SPB SU Products Neutron generators accelerators and their components upgrading components Contact person Nicolay Tolstun SODERN Societe anonyme d etudes et realisations nucleares 1 avenue Descartes 9445 Limeil Brevannes Cedex FRANCE Tel 33 1 45 69 96 00 Telex 270 322 Fax 33 1 45 69 14 02 Products sealed tube neutron generators for borehole logging related equipment Contact person Mr Serge Chezeau PHILIPS Eindhoven P O Box 5600 90050 NETHERLAND Tel 40 783 749 Products sealed tube neutron generators GLASSMAN Route EAS Salem Industrial Park P O Box 555 Whitehouse Station N J 08889 USA Tel 201 534 9007 Telex 710 480 2839 Products High voltage power supplies medium frequency 3 400kV 1 250 mA HAEFELY Lehenmattstrase 353 Basel CH 4028 SWITZERLAND Tel 41 61 535 111 Telex 62469 Products hi
67. IAEA TECDOC 913 Manual for troubleshooting and upgrading of neutron generators INTERNATIONAL ATOMIC ENERGY AGENCY A lE The IAEA does not normally maintain stocks of reports in this series However microfiche copies of these reports can be obtained from INIS Clearinghouse international Atomic Energy Agency Wagramerstrasse 5 P O Box 100 A 1400 Vienna Austria Orders should be accompanied by prepayment of Austrian Schillings 100 in the form of a cheque or in the form of IAEA microfiche service coupons which may be ordered separately from the INIS Clearinghouse The originating Section of this publication in the IAEA was Physics Section international Atomic Energy Agency Wagramerstrasse 5 PO Box 100 A 1400 Vienna Austria MANUAL FOR TROUBLESHOOTING AND UPGRADING OF NEUTRON GENERATORS IAEA VIENNA 1996 IAEA TECDOC 913 ISSN 1011 4289 IAEA 1996 Printed by the IAEA in Austria November 1996 FOREWORD During the past 20 25 years the IAEA has provided many new laboratories in the developing world with simple low voltage accelerators for the production of neutrons via the well known H d n He and H d n He reactions These neutron generators were originally supplied mainly for purposes of neutron activation analysis However the operation of these machines can often be halted or compromised by a lack of special or short lifetime components Serious problems can also arise when the original well trained tech
68. MANUFACTURERS AND COMPONENT DEALERS The authors are not responsible for the correctness of this list Telephone Fax Telex and postal code numbers may have changed and small firms may have closed Readers of this Manual are advised to contact local representatives of multinational manufacturers for current addresses and up to date information on the available products The with an asterisk LEYBOLD Bonnerstrasse 489 D 5000 Koln 51 P O Box 510760 GERMANY Tel 0221 347 0 Telex 888 481 20 Fax 0221 3471250 multinational manufacturers are marked in the list Products vacuum components pumps materials systems technologies EDWARDS High Vacuum Crawley West Sussex RH10 2IW ENGLAND Phone 0293 28844 Telex 87 123 Fax 0293 33453 Products components materials pumps systems BALZERS FL 9496 Balzers LIECHTENSTEIN Tel 075 44111 Telex 889 788 Fax 075 42 762 Products components materials pumps systems VARIAN 12 Hartwell Avenue Lexington MA 02173 USA Phone 617 273 6146 Telex 710 321 0019 Fax 617 273 6150 Products pumps components materials systems technologies technologies technologies 233 PFEIFFERS Arthur Pfeiffer Vacuumtechnik P O Box 1280 D 6334 Assiar GERMANY Phone 06441 802 0 Telex 483 859 Fax 06441 802 202 Products turbomolecular pumps other components member of BALZERS group VAT CH 9469 Haag SWITZERLAND P
69. NT 200 5 200 1 1919 ate KFKI NA 4B 120 13 1o HIGH VOLTAGE LN S 300 2 4x1019 TH ACCEL 150 3 5 3x10l9 us MULTIVOLT NA 150 2 150 1 5 1910 us NA 150 4 150 3 5 gx10 us SAMES J 15 150 1 5 gt 1010 us J 25 150 2 5 2x10 9 us JB 150 2 5 2x10 us TB 300 8 Sli M D 150 1 5 5x1010 us EFREMOV NG 150 I 150 3 19 xis NGP 11 150 2 2x101 NGP 11M 175 5 5x10 i NG 12 1 250 10 10 2 Facility at Chiang Mai University Thailand where the commercially built SAMES J 25 neutron generator has been completed with a post acceleration nanosecond pulsing system and an associated particle target head This effort resulted in a good time of flight spectrometer laboratory based on a modest commercial neutron generator These were originally small compact machines manufactured mainly for activation analysis A comparision of the commercial neutron generators see Tables 9 and 10 can help users to select the appropriate machine for a specific application while knowledge of the technical solutions of these generators may help in the improve ment of their own machines and in repair and troubleshooting Some components can be similar enough to be used in their own systems The main characteristics and 30 Table 9 Comparison of different commercial neutron generators Type No 1 8 9 10 11 12 13 Neutron yield n s 1010 10 lt 10 102 Beam energy keV 150 150 keV F lt 200
70. OURCE EXTRACTION HV POWER SUPPLIES Fig 12 Schematic diagram of sealed tube neutron generator HV CONNECTION VHV CONNECTION RESISTOR TUBE TN26 HOUSING ION SOURCE He ES MEME RAK UW MECHANISM OF INSULATOR OIL TIME COUNTER LV CONNECTION EXPANSION GAUGE AND RESERVOIR Fig 13 Schematic diagram of the SODERN sealed tube neutron generator for bore hole logging 28 protects the ion source power supply This power supply floats at the voltage of the sum of U accelerating voltage and U extraction voltage The U ion source voltage can be regulated by the right hand side variac The D and T mixed ions are extracted by the U extraction voltage into the first acceleration gap and accelerated by the U acceleration voltage within the second gap towards the TiT target The operation of the sealed tube requires a definite gas pressure in the 35 tube The gas pressure is regulated by heating the gas replenisher mainly zircon or titanium The gas pressure is measured mainly by a Penning type vacuum meter Some sealed tube neutron generators utilize feedback control between the neutron monitor and the replenisher to achieve a constant neutron yield Sulphurhexafluoride KAMAN 711 oil several PHILIPS sealed tubes or solid plastic materials SODERN tubes are used for the insulation of the sealed tube generator head 28 The KAMAN 711 sealed tube is placed in a pressure vessel under the insulat ing compressed sulphurh
71. States or of the nominating organizations Throughout the text names of Member States are retained as they were when the text was compiled The use of particular designations of countries or territories does not imply any judgement by the publisher the IAEA as to the legal status of such countries or territories of their authorities and institutions or of the delimitation of their boundaries The mention of names of specific companies or products whether or not indicated as registered does not imply any intention to infringe proprietary rights nor should it be construed as an endorsement or recommendation on the part of the IAEA The authors are responsible for having obtained the necessary permission for the IAEA to reproduce translate or use material from sources already protected by copyrights CONTENTS ONTRODUGCTION sec teria vea eo Ee Uo e Xy ERR EENE E atonal oa Andes QUII add da d QR 9 PRINCIPLES OF OPERATION iesus ntu Ee ER EPARRERAEEXRE RU ru Rr FAV DAL TES 11 DETERMINATION OF THE BEAM ENERGY eee 23 TYPES OF NEUTRON GENERATORS lt j ssveseosnecade seats veo eroe o eX Rd o aod anle dE s 26 4 1 Commerical neutron generators ccci cies es erre ap a Ear R Y Ga o VR stores 28 4 2 Sealed tube neutron generators 1 esee noeh on YEAH xr Vy REP RETE TA 34 4 3 Intense neutron generators eoiooiri i cava oce pU casi PEEL CERO ER Da EON RO RR 38 ION SOURCES OPERATION PRINCIPLES MAINTENANCE AND TROUBLESHOOTING 5
72. The main parts of the valve are made of stain less steel or rustproof material and the valve seat is made of lead In a closed position the valve needle is forced into the preformed conical seat by a spring 67 V Ge NS 2 LOCKING WASHER 3 NEOPRENE O RING 4 VITON O RING HOUSING 2 KNOB 10 BALL BEARING 3 SCALE DRUM 4 SOCKET 5 NEEDLE 6 FILTER 7 NUT 8 SPRING 9 COVER Fig 40 A typical needle valve SCALE TURNSCSE n 100 10 103 102 GAS FLOW mb l s FOR AIR 49 Fig 41 Gas flow rate curves for BALZERS needle valves Sk scale marks of the micrometer like scale drum n number of turns of flow rate adjusting knob 68 which ensures a constant closing pressure Over tightening the valve during clos ing may damage the needle and or the valve seat Moving axially in one direction the needle is coupled to the actuating knob over a ball bearing The fine thread of the knob ensures that the needle is lifted reproducibly from the seat The scale ring and actuating knob are coupled to each other for easy adjustment This allows a simple setting of the opening point at any time and marking a specific gas flow as O point for accurate reproducibility Usually the adjustment of this valve tends from a maximum conductance of ca 130 mbar l s to a minimum of ix10 mbar l s NTP ie the leak rate in closed position The construction of the needle valve is shown in Fig 40
73. Water water inlet Fig 136 Schematic diagram of a water cooled target assembly 184 Various methods to achieve higher source strengths and target lifetimes have been developed 1 A small rotary target is more economical than a stationary one 61 and is ideal for applications with medium ion beams especially to main tain constant high neutron flux over a long period of irradiation The main char acteristics of such targets 105 are maximum beam power 600 W rotating speed 60 rev min active target area 100 cm half life 300 mA h 106 The cost is about 40 of that of the respective number of stationary disc targets The target lifetime can be increased significantly with an off axis deuteron beam by which an annular surface of the target will be used This target beam geometry is sketched in Fig 135 A water cooled target assembly shown in Fig 136 is able to dissipate a few hundred W cm The target assembly is one of the most important parts of the neutron gener ator containing a large amount of tritium and induced radioactivity Therefore the target and the target assembly should be handled with caution 14 5 1 Target replacement Switch on the exhaust of the target room Opening a target assembly involves the exposure of tritium to atmosphere Close the gate valve of the tar get tube if the high vacuum pump s is still working Slowly expose the vacuum part of the target assembly to atmosphere using dry nitrogen If
74. a b in eq 45 the sin can be replaced by TE and so 2R d e 46 175 The single rotating wire scans the beam along the Z axis To get the whole picture of the beam profile a second wire rotating in a perpendicular plane around the Z axis is needed This means that two synchronously rotating wires can scan the beam shape and position 87 These two wires scan the beam current along the Z axis L and along the Y axis I The horizontally I and the vertically I scanned beam currents can be displayed together on the screen of a double trace oscilloscope If the rotating mechanism ensures a 2 2 90 phase difference between the two wires the exact position of the beam can be determined 54 The following oscillographic pictures of I or L can indicate the behaviour the beam I Unstable beam current improper operation of ion source ion optics mw f f J N see beam current III In the case of perpendicular synchronous two wire beam scanning Horizontal scanning Vertical scanning For an absolutely concentric beam in the target tube we have _ _ bz bu yay 47 The well focused beam shows on the oscilloscope a small duty cycle Bre 1 176 Horizontal or vertical scanning It should be noted however that the two wires can be replaced by a single wire with a special curvature 100 for scanning the beam simultaneously in the horizontal and vertical planes 14 2 2 Problems with rotating beam scanner
75. a good vacuum in the acceleration tube in the order of 13 x 10 to 10 Pa Using the electrode shape which shields the insulator ring and bonding fillets from the electron or heavy ion bombardment Constructing the electrode system from aluminium Using the secondary electron suppressor 300V to 400V before the target to protect the residual gas and electrodes from secondary electron bombardment The use of deflecting magnets or electrostatic deflector plates may protect the accelerator tube as well 87 10 5 TROUBLESHOOTING OF ACCELERATION TUBES or In an improperly working acceleration tube the ion beam current will be low diffused The first thing to be done is to check and test the ion source The steps are as follows 1 Check the vacuum 2 3 4 134 Check and test the resistor divider of the acceleration tube As the resis tance between two electrodes is usually in the order of 10 100 MQ the use of a megaohm meter is recommended Check the contacts between the electrodes and the resistor chain A faulty contact between the voltage divider and the electrode may disrupt the ion beam the isolated accelerating electrode will be charged to a positive potential and it will deflect the original ion beam from the original axis of the beam line Test the insulation between the accelerating electrodes with a high voltage insulation tester Testing voltage of several kV is recommended In case of sparks along the s
76. active gases forming stable compounds and a considerable number of bombarding gas molecules will be buried in the cathode Noble gases He Ar Ne are pumped by burial under the layers of titanium on the pump walls and anode while the other gases are buried in the cathode Unfortunately as further sputtering takes place the previously buried molecules can be released giving rise to instability in pumping Various solutions to the pumping of noble gases have been attempted for example the use of differential cathode materials where one is titanium and one is tantalum and the use of slotted cathodes where the bombarding ion arrives at a glancing angle However the most successful has been the triode ion pump configuration Fig 70 in which the whole pump body B is grounded and being at the same potential as the anode cylinder A acts as an auxiliary anode The ions produced as in the diode pump now graze the titanium lattice C giving a high sputtering rate the sput tered titanium forming preferentially on the pump body Energetic neutral particles created by ions glancing off the cathode are bur ied on the surface of the pump body or are reflected and pumped at the anode Any positive ions arriving at the pump body are repelled by its positive potential and do not touch the surface Buried or implanted noble gases covered with fresh 111 Fig 68 Schematic diagram of a single cell sputter ion pump Fig 69 Operation of the di
77. acuum systems containing the above fittings are discussed below Ultra high Medium Low vacuum vacuum vacuum Multi s d dr Rotary plunger H H F Fig 59 The operation pressure range of different vacuum pumps 101 Gas ballast Exhaust Inlet inlet valve Exhaust valve Pump Gas ballast fluid inlet Sliding vanes Rotor Stator Fig 60 The rotary vane vacuum pump 9 2 1 Vacuum system based on a combination of oil diffusion and rotary pumps a The rotary vane pump This is probably the most widely used pump and is well established as a backing pump in many compound systems An eccentrically placed slotted rotor turns in a cylindrical stator Fig 60 driven by a directly coupled electric motor In the slots are two or three sliding vanes which are in continuous contact with the walls of tbe stator Air drawn in is compressed and expelled through a spring loaded exhaust valve The vanes and rotor are sealed by a fluid film and the stator is immersed in the fluid to provide heat transfer to the pump casing Rotary pump fluids are usually selected from high quality mineral oil with low vapour pressure and good lubricating properties Suggested types of oil are Shell Rotary Vacuum Pump Oil Gluvacol R 910 Alcatel 1DO ELF MOVIXA PV100 TURBELF SA 100 ELFLL BARELF F 100 BP CS 100 INLAND 15 INVOIL 2D SHELL VITREA 100 TOTAL CORTIS 100 Two stage rotary pumps are frequently used in which the exhaust
78. ag der Wissenschaften Berlin 1956 in German 70 J Kliman personal communication 71 V M Kellmann SJ Javor Eletronoptics Akad miai Kiad Budapest 1965 in Hungarian 72 M D Gabovich Physics and Technology of Ion Sources Atomizdat Moscow 1974 in Russian 73 M R Shubaly M S de Jong IEEE Trans Nucl Sci NS 30 1983 p 1399 74 J B Hurst M Roche Acta Physica Slovaca 30 1980 p 137 75 A Galejs P H Rose Focusing of Charged Particles Academic Press New York 1967 p 297 76 B Gyarmati E Koltay Nucl Instr Meth 66 1969 p 253 77 G Liebmann Proc Phys Soc London Sect B 62 part 4 1949 p 352 78 J B Hourst et al Nucl Instr Meth 145 1977 p 19 79 E Koltay et al ATOMKI Kozl 22 1980 155 80 J B Hourst M Roche Nucl Instr Meth 92 1971 p 589 229 81 M R Cleland B P Offermann Nucl Instr Meth 145 1977 41 82 J C Davis et al IEEE Trans Nucl Sci NS 26 3 1979 p 3058 83 R Booth et al Nucl Instr Meth 145 1977 p 25 84 K Sumita et al Proc 12th Symp Fusion Technology Vol I Pergamon Press New York 1982 p 675 85 A Kiss E Koltay Gy Szab Nucl Instr Meth 117 1974 p 325 86 A Kiss E Koltay Gy Szab Nucl Instr Meth 212 1983 p 81 87 J B England Techniques in Nuclear Structure Physics MacMillan 1974 88 R G Wilson G R Brewer Ion Beams Wiley Interscience London 1973 89 S Humphries Prin
79. age is suitable The output voltage is usually regulated by a variac or triac on the primary side of the step down transformer 79 The electrolysis should start with low current The voltage and the cur rent of the electrolysis should be increased gradually To prevent the conical taps from falling out the deuterium vessel should not be filled up to the atmo spheric pressure When the electrolysis stops the Vi and v glass taps should be closed and the O ring sealed connection between the electolyzer and the deuterium vessel can be vented into the atmosphere by the valve V3 With two identical deuterium vessels the deuterium supply of the accelerator will be smooth and uninterrupted by the use of the electrolyzer 7 1 THE FLOAT REGULATOR ELECTROLYZER The regulation and stabilization of the deuterium pressure in the ion source need some control circuits related to the different gas valves The combination of an electrolyzer with a mechanical float regulator can serve as a reliable gas To ion source fq fy OLOTE Aa wm Z cd Lora a a YER 7 NERIO e S69 06949000 7 9 FOAL err e ASNASSAASS ASS CCRCICNCICIOY CCCII Y A AV A AF AF AP AV A SSS SF IA Sow wee QZ Fig 47 Construction of the float regulator electrolyzer 53 numbers are explained in the text 80 supply for the ion source The regulating electrolyzer described below has been used satisfactorily for a long t
80. an be calculated by 2X 10 A turns cm f em 38 x Ix N A turns where t is the focal length of the quadrupole lens in the focusing plane N is the number of turns of a single coil and I is the current in amperes The power supply of the quadrupole doublet is based on a simple standard 24 V AC secondary transformer The supply of the quadrupole is asymmetrical using the biased quad rupole principle A diagram of half of this quadrupole is shown in Fig 107 The material of the iron core is manufactured from low carbon content iron C 10 type The pole pieces are plasma welded onto the planed iron sheets and the four iron sheets are screwed together by Allen type bolts The pole pieces were shaped on a lathe and the connecting surfaces were manufactured on a milling machine The wiring diagram of the quadrupole lens pair is shown in Fig 105 Note that the coils of the perpendicular segments are connected in series The coils of 0 8 mm dia double emailed copper wires each coil with 850 turns were wound The surface of each coil is large enough for the dissipation of 10 15 VA The coils are connected to a 12 blade socket fixed to aluminium sheet that fastens the pairs The rectifier circuit serves for the supply of the voltage regulation of the magnet As the dissipation of the coils is low the change of resistance of the coil can be ignored which means that this magnet does not need a constant cur rent
81. and side and decrease at the right hand side While waiting for an easily observable volume of gas consumption say 1 3 ml stop the stop watch If the fall of the silicon oil level took a time At the flow rate through the gas leak the gas consumption of the ion source will be 2 x Oren 20 74 where d is the inner diameter of the U shaped glass tube If the diameter d and the Ah oil level change is measured in cm and the At time in hours the flow rate and the gas consumption of the ion source will be given in ml h NTP This unit is suitable for other purposes as well based on the consumption the normal opera tion time of an ion source neutron generator can be calculated from the D gas container volume The gas flow rate value may also be calculated for other pressures In the case of a needle valve the adjustable flow rate is usually given at lower pres sures e g in mbars This measurement makes it possible to check the characte ristics of leak valves given in manufacturers data sheets and their recalibra tion after repair The U shaped tube filed to half height with silicon oil is also a useful tool for determination of the pumping speed of vacuum pumps Pumping speed mea surements will be found in Section 9 which describes the vacuum systems and vac uum components of neutron generators 75 7 DEUTERIUM ELECTROLYZERS Deuterium gas needed for the operation of neutron generators is commercially available but many
82. are large in comparison with their dimensions and the mutual arrangement of the individual molecules is constantly changing Gas in the Stricter sense is matter in gaseous state which cannot be brought to a liquid or solid state by compression at the prevailing temperature Gas ballast is the inlet of a controlled quantity of a gas usually into the compression chamber of a positive displacement pump so as to prevent condensa tion within the pump Gettering means bonding of gas preferably by chemical reactions Getters getter materials often have large real surfaces Leaks in a vacuum system are holes or voids in the walls or at joints caused by faulty material or machining or wrong handling of the seals Leak rate is the throughput of a gas through a leak It is a function of the type of gas pressure difference and temperature The unit for the leak rate is 1 Pa m s 1W 10 mbar 1 s Maximum tolerable water vapour inlet pressure Pwo is the highest inlet pressure at which a vacuum pump can continuously pump pure water vapour under ambient conditions of 20 C and 1013 mbar The mean free path is the average distance which a molecule travels between two successive collisions with other molecules Outgassing is a spontaneous desorption Partial pressure is the pressure due to a specified gas or vapour component of a gaseous and or vapour mixture Permeation is the passage of gas through a solid barrier or a liquid of finite th
83. ass it is recommended that a copper mesh screen be placed just in front of the quartz As soon as the beam glows on the quartz increase the lens current by the current potentiometers of one of the lenses If the lens current seems to be at optimum a 45 line will appear on the glass target The position of the line focused beam can be changed on the biased quadrupole principle by the symmetry potentiometers These potentiometers control the emitter follower shunt The optimal position of the beam is in the center When the first lens has been tested turn down the current potentiometers and activate the second quadrupole lens A line perpen dicular to the previous one will show the focusing of the second lens in the perpendicular plane When the optimal position for both lenses has been found the focus of both lenses can be activated If the user does not like the two perpendicular 45 plane focusing feature of this quadrupole doublet the doublet should be turned 45 This means that the stand with horizontal table should be replaced by a stand holding the outer magnet planes 45 to the horizontal plane 153 12 3 TROUBLESHOOTING OF A MAGNETIC QUADRUPOLE LENS a The effects of improper operation of a quadrupole lens can be as follows The beam is not focused or is deflected in spite of the activation of the current and symmetry potentiometers This shows the currentless state of the coils Test the interconnection of t
84. becomes coloured especially FREON 113 re place it with a fresh one The whole system is relatively simple The cooling machine can be repaired by a technician from a commercial refrigerator or air conditioner service Trou bleshooting and repair of the two coolant circulating system can be done by the operator of the neutron generator The location of the main components of the cooling unit is shown in Fig 143 The monthly maintenance routine based on the description in the Instruc tion Manual for the A 711 neutron generator KAMAN February 1976 edition is as follows 193 I Ion source cooling loop Check the FREON 113 filter screen Check the flow rate and FREON 113 either monthly if the atmosphere is humid rainy season or at least every three months under normal condition If FREON 113 becomes contaminated or dirty it should be replaced even sooner Don t forget to find the source of dirt and stop it If necessary adjust the FREON 113 pump bypass valve for correct flow rate II Target cooling loop Clean the water filter screen If the water is extremely dirty and rusty check the flow rate and change the water Unless during regular cleaning of the filter the screen shows the water to be dirty and rusty it is not neces sary to drain and refill Check the water pressure If necessary adjust the water pump bypass valve Check the temperature rise of the cooling water during operation of the neutron generator
85. brecen to be published 33 T Sztaricskai Proc XIV th Int Symp Interaction of Fast Neutrons with Nuclei Nov 19 23 1984 Gaussig ZfK 652 INDC GDR 40 G p 10 34 H H Barshall Proc Int Conf Fast Neutron Physics May 26 31 1986 Dubrovnik Yugoslavia p 188 35 J C Crawford W Bauer ANL CTR 75 4 Argonne Nat Lab 1975 p 227 36 H Maekawa JAERI M 83 219 Japan Atomic Energy Res Inst 1983 37 C D Moak H Reese W M Good Nucleonics 9 3 1951 p 18 38 Z Szab Nucl Instr Meth 78 1970 p 199 39 E P sztor Isotopenpraxis 3 1965 p 259 40 K W Ehlers B F Gavin E L Hubbard Nucl Instr Meth 22 1963 p 87 41 J L Nagy Nucl Instr Meth 32 1965 p 229 42 Sun Biehe Wang Fulin Chen Quin Proc XIV th Int Symp Interaction of Fast Neutrons with Nuclei Gaussig 19 23 Nov 1984 ZfK 652 43 D D Armstrong et al Nucl Instr Meth 145 1977 127 44 L V lyi Atom and Ion Sources John Wiley amp Sons London 1977 45 P C Thonemann et al Proc Phys Soc London 61 1948 p 483 46 G Wehner Phys Rev 93 1954 p 653 47 D P Smith Hydrogen in Metal Chicago University Press Chicago 1948 48 W Jost Diffusion in Solids Liquids and Gases Academic Press New York 1960 49 Manual of Needle Valve EVNO1H1 Balzers AG Liechtenstein 50 Instruction Manual of TOSHIBA NT 200 Neutron Generator Tokyo 51 Instruction Manual of KFKI NA 4C Neutron Generator Budapest
86. bunching with chopper and selector plates and X steerers Compression factor gt 10 Pulse width of the neutron pulses 1 ns Repetition rate of the neutron pulses 1 25 10 MHz Target holder 0 3 mm aluminum tube with aluminium backing TiT target Target cooling by compressed air Power consumption of the generator ca 5 kVA Water consumption of the vacuum system ca 5 l min Compressed air consumption ca 5000 1 h 200 102 HF ion Source Focus Focus li Pulsing Unit Acceleration Tube Target Head s iii 1 1 EN Ti Focus Il Power Suppl i Focus 10MHz Power Amplifier 20MHz j Power Amplifier Selector Power Anolifier Phase Phase Synchron Shifter Shifter JL ce Signal Base Oscilldtor f mas Units settted Units placed Units on the Close to the lin the ion Control dask lon optics L Source block ONTEN ndes lon Source 1 HV Control Controt Panel Control Panel Panel Receiver requency Counter i Fig 149 Block diagram of a pre acceleration nanosecond pulsed neutron generator The peculiarity of this generator is that the target is placed at the high voltage As the shadow bar and the neutron detector or gamma detector in time of flight measurements should be placed at a certain distance from the sample and may be placed also at the acceleration high voltage th
87. c Boca Raton Florida 1987 Vol I II 2 S S Nargowalla E P Przybylowicz Activation Analysis with Neutron Generators John Wiley and Sons New York 1973 3 J Csikai Use of Small Neutron Generators in Science and Technology Atomic Energy Review 11 1973 415 4 H Liskien A Paulsen Nuclear Data Tables 1 1973 569 5 J D Seagrave E R Graves SJ Hipwood CJ McDole D dn He and T dn H Neutron Source Handbook LAMS 2162 Los Alamos Scientific Lab NM 1958 6 J Csikai Zs Lantos Cs M Buczk IAEA TECDOC 410 Vienna 1987 p 296 7 V E Lewis K J Zieba Nucl Instr Meth 174 1980 p 141 8 J Csikai in Proc Int Conf Nuclear Data for Science and Technology Antwerp Reidel Dordrecht 1983 p 414 9 M Wagner G Winkler H Vonach Cs M Buczk J Csikai Ann Nucl Energy 16 1989 p 623 10 A Pavlik G Winkler Report INDC AUS 011 LI IAEA Vienna 1986 11 K Kudo T Kinoshito Fusion Energy and Design 10 1989 p 145 12 J Csikai Cs M Buczk R Pepelnik H M Agrawal Ann Nucl Energy 18 1991 p 1 13 Z B dy in Handbook on Nuclear Activation Data Technical Report Series No 273 IAEA Vienna 1987 p 29 14 Cs M Buczk J Csikai T Chimoye B W Jimba Proc Conf Nuclear Data for Science and Technology J lich Springer Verlag Berlin 1992 p 656 15 J Csikai R Raics in Progress Report HUN IAEA Vienna 1991 16 K Kudo N Takeda Matiullah A Fukada Proc Conf Nucl
88. celerator technology for activation analysis prompt radiation analysis irradi ation effects of fast neutrons neutron dosimetry etc Practically no technolog ical development in the field of commercial neutron generators has been reported in recent years and only a few companies are producing these moderate 150 300 kV 1 2 mA machines The existing manufacturers are IRELEC formerly AID and SAMES in France KFKI in Hungary the EFREMOV Institute in Russia and MULTIVOLT in the United Kingdom The sealed tube replacement is still made by KAMAN NUCLEAR in the USA These types of neutron generator are still in operation in many countries As these machines are excellent devices for pure and applied research service and education the IAEA has provided them to the following developing countries Albania Algeria Bangladesh Bolivia Burma Costa Rica Cuba Equador Hungary Indonesia the former Yugoslavia Lebanon Malaysia Mongolia Morocco D P R of Korea Nigeria Pakistan Peru Singapore Sudan Thailand Turkey Zambia Unfortunately some of these generators are now out of order and in addition to repair of the machines it is recommended that they be fitted with some upgrading components For example determination of the nuclear level schemes and neutron cross sections requires microsecond and nanosecond pulsing units These systems have been developed almost entirely in the laboratories where the neutron genera tors were constructed a
89. ciples of Charged Particle Acceleration Wiley Interscience New York 1978 90 T Sztaricskai Regulated analyzing magnet for NGs unpublished 91 L Cs nky personal communication 92 S Williams et al Nucl Instr Meth 185 1971 p 353 93 T Sztaricskai Quadrupole doublet for Thailand unpublished 94 NJ Felici Direct Current 1 1953 p 122 95 E G Komar Fundamentals of Accelerator Technique Atomizdat Moscow 1975 in Russian 96 E Kuffel W S Zaeng High Voltage Engineering Pergamon Press 1972 97 Handbuch der Elektrotechnik Siemens M chen 1971 in German 98 J D Cockcroft E T S Walton Proc Roy Soc Ser A 129 1930 p 477 99 G Pet Izot ptechnika 16 1973 p 561 in Hungarian 100 J Tak cs IEEE Trans Nucl Sci NS 12 3 1965 p 980 101 T Sztaricskai et aL IAEA Int Data Committee INDC GDR 21 6 Spec ZfK 476 1982 p 95 102 D G Shurley et al Nucl Instr Meth 187 1981 p 347 103 T Sztaricskai unpublished 104 C S Zaidins Nucl Instr Meth 120 1974 p 125 105 D D Cossutta Proc Conf Use of Small Accelerators in Technical Research Oak Ridge Tennessee CONF 700322 1970 106 G I Primenko et al Izv Vuzov Physica No 5 1985 p 17 in Russian 107 Radiation Protection Procedures Safety Series No 38 IAEA Vienna 1973 108 R F Boggs Radiological Safety Aspects of the Operation of Neutron Generators Safety Series No 42 IAEA Vienna 1976
90. ctrically by a Teflon ring seal on the vac uum side and a thin Teflon ring on the backing The small amount of scattering material around the target does not disturb the 14 MeV neutron field around the target spot so this arrangement is proposed for accurate neutron data measure ments A ring shaped sample is usually fixed around the target spot for alter ation of the neutron energy The cooling of the target needs compressed air to blow the target through a nozzle air jet The same nozzle is utilized to hold the spring contact of the target current measuring meter This arrangement with the sample holder is shown in Fig 138 1 Samples Sample holder Compressed air Target current Fig 138 Air cooling of an air cooled target holder with ring shape sample holder 14 5 3 Replacement of the target at air cooled target holders Attention As the compressed air is sometimes not dehumidified the air cooling should be checked specially at the beginning because the compressed air can blow water to the target holder During the operation of the neutron gen erators the high voltage power supplies should be interlocked by a flow switch of the target cooling water or compressed air A well collimated ion beam may melt the target backing without cooling releasing the whole tritium content into the vacuum system of the neutron generator The beam heated target can melt the vacu um seals or a well collimated beam can even punch the targe
91. ctrolyZet cce siesey evt yeso xo seu Soo Sl a E aret 80 REMOTE CONTROL OF THE HIGH VOLTAGE TERMINAL 84 8 1 Mechanical control i eoe d roro x rr PUR E V RR E n EE 84 8 2 El ctromechanical control o oce tens tme soe eed vien pit ue e epa Rweb cud des oed 85 8 3 Insulation transformer control ssssssssssssesepoeosrerenerssaereseresrrrerss 86 8 4 Optical insulation control uiii eese rere bo vias Ya ee Sad VEN Y d Ey e va 88 8 5 Computer contiol i uideo EL dre EUER REA SSE ENDE RIEN A EVER ey MEE saw UU 88 VACUUM SYSTEMS OF NEUTRON GENERATORS sees 93 9 1 Important terms and units in vacuum technology sees 93 UIT T PERS vei nas dioses Veveuo ck Pepe tung seeesa cuties et tice tid Nees diner 93 0512 Unts ek n rece suis igs deh ee SceT v es codes P AE Pero btc T tals 99 10 11 12 13 14 9 2 Vacum DUDIDS serit aeee o a E U UE VERA Seta E AA A EO ATI EI ES E ERO I ERE 100 9 2 1 Vacuum system based on a combination of oil diffusion and rotary pumps ssssssererereresesessssesrerreeseere 102 a The rotary vane pump o sepesesekeb redi toig at tea a eco erae rd eed 102 by The diffusion pump eere recto Ex ENEPAKEIXIOS QR had eg 104 c Combination of diffusion and rotary pump 106 9 2 2 Vacuum system based on Ti ion getter pump sese 111 9 2 3 Vacuum system based on turbomolecular pump
92. ctrolysis is changed as well By the cor responding change of the electrolyte level in the float chamber the leakage be tween the rubber seal and the edge of the nozzle is also automatically changed as well as the quantity of q in such a degree that a new equilibrium is achieved ie a state at which the eq 23 is valid When the electric current is switched off the electrolyte level gradual rises up to the level at which the nozzle actually gets completely closed The time behaviour of the equilibrium settling after the change of Q is given by the equation 2 dh dt 4 Q q h z D D 24 where the meaning of D and D is evident from Fig 47 and q h is the function of the escaping deuterium quantity from the float chamber into the ion source during unit time of the electrolyte level h The behaviour of this function is charac teristic for each construction of a regulating electrolyzer since it depends on the geometry and the mechanical properties of the nozzle and the seal as well as on the geometry of the float The experimental function q h for a given geometry and construction of a regulating electrolyzer is shown in Fig 48 The rough surface rubber float shows a faster rising function while the fine surface silicon rubber float gives a re latively easy way to change the gas admittance characteristic of the regulating electrolyzer 53 82 60 50 g cm NTP h 18 22 26 30 34 38 40 himm Fig 48 The ion source
93. ctrostatic deflection so the Wien filter forms a straight line ion selector When s dn only a single ion of specific charge e m may pass through the exit slit of the filter 88 The value of v is constant and depends on U 5 voltage m 2eU B E The Am m mass resolution of the Wien filter as for other magnetic ana lyzers strongly depends on the width of the entrance and exit slits of the vacuum chamber and the length of the filter Magnetic poles side view Fig 91 Schematic representation of magnetic deflection 136 eu S Magnetic pole piece Electric Va deflector Separated ions ie heavy ions Desired Entering Ms ions dg Fn Fe d 44 43 ions E 2eUocc Yg m 0 lons with V gt W d Fg Fe ie light ions Fig 92 Schematic diagram of a Wien filter The capability of a Wien filter is determined by D the dispersion between the selected mass M and M AM The dispersion D of the filter is determined by the expression lq a E AM DRM 36 a where ly is the distance of the exit slit from the filter a is the length of the filter magnetic poles and static deflectors and M is the mass number A sche matic diagram of a Wien filter is shown in Fig 92 11 2 TROUBLESHOOTING OF ELECTROSTATIC DEFLECTORS In the case of improper operation of a static deflector pulser the problems can be of mechanical or electrical origin The mechanical problems are mainly in the va
94. cuum chamber of the deflector so it is necessary to open the vacuum system 137 and inspect the deflector plates A broken contact or deflector plate holder can be found easily and the electric contacts of the deflection voltage should be tested During conductivity measurements the occasional resistive conductivity between the deflector plate to the vacuum vessel should also be tested Sometimes the isolators can be covered by conducting evaporated metal or carbon layers sO a proper resistance test should be carried out at high voltage of the same or der as the value of the original deflection voltage Every breakdown between the deflector plate and the ground means a reflection HV pulse along the high voltage cables connecting the HV supply to the deflector chamber The cable end should be tested carefullp because an unterminated cable end could carry a doubly high voltage due to the reflection from the shorted cab le end This means the cable short circuits are usually at the HV power supply end connector of the high voltage cables Test careful the HV vacuum feed through for conductivity and HV insulation 11 3 ANALYZING MAGNETS OF NEUTRON GENERATORS The analyzing magnets of neutron generators are as follows jon species selectors for the D D3 and D3 components of the accelerated beam selectors of charged particles from neutral particles e g oil vapour oxy gen and nitrogen molecules from the residual vacuum As the ma
95. d ion beam Low acceleration voltage 120 kV Mixed ion beam Relatively low yield High D consumption 33 technical solutions are given in Table 9 where the numbers correspond to the following manufacturers and types SAMES D J T JB TB Type No 1 2 3 4 5 TMC A 111 Type No 6 KAMAN A 1254 A 711 Type No 7 8 TOSHIBA NT 200 Type No 9 KFKI NA 4 Type No 10 MULTIVOLT NA 150 02 and NA 150 04 Type No 11 12 EFREMOV NG 150 I Type No 13 4 2 SEALED TUBE NEUTRON GENERATORS The sealed tube neutron generators or neutron tubes are 14 MeV neutron sour ces used in in situ geological measurements bore hole logging in hospitals and in chemical biological and industrial laboratories Most neutron tubes have a Penning ion source a one or two gap acceleration section a tritium target and deuterium tritium gas mixture filling system The pressure of the gas is con trolled by a built in ion getter pump and or gas storage replenisher The tritium displaced from the target during the operation is absorbed by the gas occlusion elements and is accelerated later back into the target resulting in an extended target life 26 The operation principle of a typical sealed tube generator is shown in Fig 12 The first sealed tube neutron generator was developed by PHILIPS 27 and its originally low neutron yield could increase up to over 10 n s The advantage of the neutron tubes is their small size which makes them suitabl
96. d radiation originating from the deuteron beam bombarded target sur face is taken out by means of a mirror and a sapphire window to the outside of the vacuum beam duct The thermal image is transformed by the analyzer into video signals and they are transmitted to the digital processor of the analyzer in the control room after the necessary data processing in the commercial model The thermal image of the object the target surface is displayed on a colour monitor screen in a 189 190 OFHC COPPER DISC O RING SEALS TiT COATING ANNULAR _RADIATOR SHAPED COOLING SYSTEM 9 4 PT E m PLACE FOR SAMPLES Fig 140 The schematics of the MULTIVOLT rotating target with water cooling and magnetic fluid seal feedthrough Cogwheel Bh hl hha N G Beam line Wobbling target tube Lith dh b ithstdheshede sg Ball bearing EP TIN Cogged ring Fig 141 Operation principle of a wobbling target holder 16 colour temperature scale The use of this equipment at the higher kW power beams is important to increase the target life and decrease the tritium pollution round the neutron generator and in the vacuum exhaust Similar equipment is used in Dresden 110 14 5 4 Rotating and wobbling target holders The thermal load of the tritium targets can be decreased by rotating or wob bling the targets around the beam The targets rotate at a speed of several hun dred revolutions per minute The vacuu
97. d they are suitable for general applications down to a pressure of 107 mbar They will not withstand repeated exposure to atmosphere in a hot state because the exposure will produce carbonaceous compounds with high vapour pressures which decrease the performance of the pump Their deposits on the inner surface of the accelerator system will produce conducting layers 105 Silicon oils These oils are exceptionally stable compounds at high temperatures and they will provide an ultimate pressure between 10 to 10 mbar Their deposits on the electrodes of an accelerator will produce an insulating layer These fluids are poor lubricants Pentaphenylaether These fluids have exceptionally low vapour pressure and are thermally very stable They will not backstream in a properly designed pump and baffle and they are chemically stable Their break up on the surfaces of the electrodes of an accelerator will produce a conducting layer They are good lubricants but they are expensive Vacuum chamber Ionization gauge Roughing valve High vacuum isolation 6L valve Pirani auge Cold Backing nh sd Te Fore line Diffusion Sepa eet a trap pump chamber Vent Rotary pump Fig 64 Typical diffusion pump vacuum system c Combination of diffusion and rotary pump A typical system consisting of an oil diffusion pump and a rotary pump can be seen in Fig 64 As mentioned earlier selection of the appropriate
98. determination of the mean neutron energy at 14 1 MeV is about 20 keV This method is also applied to determine the mean neutron energy for an extended sample In this case the Zr and Nb foils are placed back to back in different positions inside the sample The calculated energy angle functions E for D D reaction are shown in Fig 5 for thin and thick targets at Ey 200 keV The measurements of the mean energy and its spread require appropriate en ergy and fluence monitors The angular yield of the D D neutrons can be measured either by the U n f using a depleted U layer in a fission chamber or by the 5In nv P In reaction The Um Ey curve is relatively well known 13 and its change between 2 and 3 MeV is within 2 0 see Fig 6 For In n n P In reaction the high cross section value in this energy range is advantageous however this inelastic process is sensitive for the scattered neutrons Measurements carried out in Debrecen J lich Dhaka and Khartoum have indi rated that the discrepancies in the o E D data between 2 and 3 MeV originate mainly from the presence of room scattered neutrons and self target build up in beam apertures The contamination of the neutron spectrum by the scattered neu trons can be decreased if a scattering free arrangement is used The contribution of the scattered neutrons to the activity can be checked 14 via the 1 r rela tion between the apparent activity and the flux values EMISSION AN
99. diffusion pump is deter mined first of all by the gas consumption of the ion source On the other hand for the selection of the rotary pump to be fitted to the diffusion pump the pumping speed of the diffusion pump as well as the maximum permissible pressure at the exhaust have to be taken into account 106 For example in the case of a diffusion pump having 1000 l s pumping speed if a pressure as low as 10 mbar is assumed for the pumped volume in fact it should be higher than that then a 3 6 m h pumping speed is necessary at the exhaust side with a 101 mbar pressure Such a pumping speed can be achieved even by the simplest rotary pumps with a pumping speed of 6 8 m h in practice see the pumping speed diagrams Cooled vapour traps The so called foreline traps liquid nitrogen traps between the rotary and diffusion pump protect the diffusion pump from the oil vapour of the rotary pump This must be impeded in order to prevent mixing the rotary pump oil with the much finer diffusion pump oil This is necessary be cause such mixing would spoil the diffusion oil the molecular size being much larger for diffusion than for rotary pump oils A vapour trap has another very important role it prevents the vapours es pecially steam general present in vacuum systems from getting into the rota ry pump A rotary pump cannot completely remove such water or even steam Prac tical experience shows that the use of a trap improves
100. dry nitrogen is not available use the vent valve of the target tube The following description of target exchange is based on the hypothetical target holder shown in Fig 136 Close the water inlet of the target cooling Be sure that the water has left the target cooling cup blow out the water with compressed air or a blower Dis connect the target current meter cable Opening the target cup should be done with the same tools used only for target assembling These tools should be stored separately in a vented glove box they are expected to be polluted with tritium During the dismantling of the target assembly rubber gloves should be worn Put polyethylene foil on the floor under the target assembly Two polyethylene bags should be kept in the vicinity of the target assembly when the vacuum sys tem has been opened the target assembly should be put into the first plastic bag to avoid further tritium pollution The second plastic bag should be pulled onto the open end of the target beam line tube A tritium gas monitor should be ready to monitor the tritium contamination during the assembling of the target holder 185 Hold the tritium target with tweezers or with medical rubber glove covered fingers The removal of the old target and its replacement with a new one should be done by a person wearing gloves and mouth mask When replacing the old target with a new one make sure that the gray side the titanium layer is on the side of the vac
101. e benefit of this system is the easy pulse control of the extracted and pre accelerated deuteron beam The use of the deuteron beam pulse pick up signal needs optical insulation The tar get current is similarly converted into frequency and optically connected to the integrator placed at the ground potential As the beam chopper the beam buncher and the selector electrodes are at the ground potential the ion source is floating at the voltage of the first focus pre accelerating power supply This focus lens is an immersion type cylindrical lens focusing the extracted and pre accelerated deuteron beam through the main vacuum manifold into the entrance of the second Einzel focus lens The Focus II unit consists of three diaphragms with a short focus This lens focuses the deu teron beam into the twin gap buncher The chopper steerer deflector plates two pairs in front of the buncher chops the steady deuteron beam with a frequency of 10 MHz The position of the beam can be controlled by the UD and UD steering voltages The second deflector plate pair deflects and chops the pre accelerated deuteron beam whose energy is determined by the sum of the extraction voltage and the voltage of the first focus electrode As the chopping frequency is 10 MHz the buncher electrode works at a 20 MHz frequency The phase control be tween the 10 MHz and 20 MHz signals is made in the pulse control unit As the en ergy of the pre accelerated deuteron beam is re
102. e control of the SAMES J 25 54 The only advantage of insulating transformers is the easy variac control at the ground potential 86 L8 10n 1220n 100k S8yu0ZSNVUL O Opto OP AMP LM 324 receiver HV TERMINAL Fig 52 Optoisolated triac control of mains transformers to the power control at HV terminal The disadvantage of this type of remote control in addition to the high probability of electrical breakdown along the surface of the transformers is that repair of the insulated secondary coils is impossible in practice 8 4 OPTICAL INSULATION CONTROL The control of HV terminal equipment by optical fiber insulation can be ANALOG or DIGITAL Analog control is mostly used for neutron generators How ever some sophisticated neutron generators RTNS II OCTAVIAN etc use micro computer control Analog control of equipment at the acceleration high voltage HV terminal requires power supplies and other units where the regulating input needs DC volt ages This input is the usual remote input of the medium frequency power sup plies and other electronically controlled devices In a simple power supply using one mains transformer the variac type input voltage regulation can be changed to a DC voltage input controlled triac control circuit This circuit is the usual power electronic circuit the required DC input control voltage can be produced after the optical insulation by simple circuits A typical triac cont
103. e for geological bore hole logging or isocentric cancer therapy in hospitals The dis advantages are their limited lifetime and the relatively high neutron production cost Typical applications of the neutron tubes with built in high voltage power supplies and neutron detectors are as follows Bore hole logging deposit evalu ation uranium exploration and processing mineral exploration coal exploration and processing oil well logging gas well logging under sea exploration and such utilization where the small diameter and the remote detection are impor tant For cancer therapy the isocentric irradiation is most important the neu tron source and the neutron collimator are rotated around the malignant tumour in the same way as in linear accelerators or at radioactive sources The deuteron ions bombarding the tritium target are produced in a Penn ing ion source The ion source is powered by a 0 10 kV HV power supply consist ing of an insulating high voltage step up transformer and single wave rectifier The discharge current of the ion source can be observed by an Ic ion source amp meter Owing to the negative resistance of gas discharges a damping resistor 34 PENNING VACUUM GUAGE REPLENISHER TiT TARGET PENNING ION SOURCE DAMPING RESISTORS ION SOURCE CURRENT AERE UP aes em Li STEP DOWN METER STEP UP TRANSF CIRCUIT NANSEOERERS PRESSURE CONTROL FEED BACK REGULATEO VARIA MAINS ION S
104. e is controlled by a switching Darlington power transistor The on off ratio of the transistor is controlled through a LED phototransistor optical link by an astable multivibrator at the ground potential The optical connection can be a simple 3 mm diameter Perspex rod or plastic insulating fi ber cable which endures the high voltage differences between the electrolyzer on the HV terminal and the LED driving pulse generator at the ground potential of the control desk The effective heating current of the Pd leak is controlled by the duty cycle of the astable multivibrator pulse generator The heating current of the Pd leak is displayed on the electrolyzer by a simple linear 5 LED bar in dicator Similarly the D level status of the hysteresis switching relay is shown by a green and a red LED The electrolyzer Pd leak systems are very economical solutions for the deu terium supply of a neutron generator The deuterium supply of commercial neutron generators can be solved also by separate heavy water electrolyzers at the site of the neutron generator laborato ries An electrolyzer for filling the ion source deuterium vessels can be con structed on the basis of Fig 46 This electrolyzer can produce 100 500 ml h NTP deuterium gas with a simple sometimes not water cooled electrode stucture and can serve as the deuterium supplier for the neutron generator and other utiliz ers The whole glass system of the electrolyzzer can be manufactured
105. e neu tron generators that can meet the criteria of the high neutron yield and relati ly long life time The MARCONI has a mixed D T beam containing a refillable sealed tube with typical high frequency ion source and a homogeneous field acce leration tube The target material is Er to achieve high thermal stability The manufacturer can renew the system and this is advantageous for situations where regular change of used sealed tubes is required For cancer therapy the sealed tube in the collimator is changed regularly every week The KARIN tube is manufactured by HAEFELY in Basel This generator has an annular shaped Philips ion gauge PIG ion source at the ground potential while the cylindrical or conical target is on the high voltage potential and in the 36 Type Name Manufacturer A 3045 6 A 3041 4 A 3043 18601 18604 TN 26 KARIN TN 46 center of pneumatic rabbit system sample transfer ALPHA PARTICLE vacuun PINCH OFF ALPHA SCINTILLATOR iy IALO P SLE EL Ta aii ZZ NEUTRON PHOTO MULTIPLIER TUBE D T ION BEAM ACCELERATING ELECTRODE TUBE ENVELOPE HIGH VOLTAGE INSULATOR lah 4 Yj HIGH VOLTAGE ELECTRODES FOCUS ION ELECTRODE SOURCE Fig 14 A sealed tube neutron generator combined with APM head for time correlated measurements KAMAN KAMAN KAMAN PHILIPS PHILIPS SODERN MARCONI HAEFELY SODERN the annular Table 11 Compa
106. e pumping ability The value of the necessary forevacuum at the exhaust passage of a diffusion pump is at least 101 10 mbar However the pumping speeds of the rotary pumps in this pressure range are usually quite low see later It is usual to insert a so called booster pump in between the two pumps if the gas intake is high 104 In older vacuum systems an overheated diffusion pump only at the pressures 1 10 mbar was used as a booster In this way the exhaust pressure of the 10 booster increased into the mbar region where the rotary pumps also have high enough pumping speeds In contemporary diffusion pumps the booster stages are originally built in the housing stator Fig 63 shows that the outstreaming of the oil vapour from the pipe directed towards the exhaust passage operates as an oil va pour ejector pump Table 18 Technical data for diffusion pump fluids 64 Mineral oils Silicon oils Pentaphenylaether BALZERS 61 71 DC704 AN175 SANTOVAC 5 Theoretical vapour pressure mbar 2107 2x109 zx108 4x19 10 paot Viscosity mm s 171 410 39 175 1000 at 25 C Chemical resistance good good better better best Thermal resistance good good better better very good Pressure range 107 5x10 9 102 107 193 1097 105 108 103 108 Price low low medium medium high Suggested types of diffusion pump fluids are as follows Table 18 Mineral oils These oils are manufactured by molecular distillation of crude oil an
107. e the length of the metal probe alumini um sonde and the length h of the quartz sleeve over the metal probe see Fig 24 An extraction arrangement of a quasi Pierce geometry is shown in Fig 26 with electrodes of approximately ideal shape 50 A po27755 2722 7 LAKA 777 EIAS S UG LL 7 Magnet BaFel Coating fin Tin Extraction voltage 3L T Rubber NS Sse B Steel tak i S E NNS SSS RTI Lat SiSs ZA JAN ene Lille LA IA isulafer als EN Extracting electrode Al Fig 27 Capacitively coupled HF ion source with quasi Pierce extraction The quasi Pierce type extraction systems consisting of two conical elec trodes are used in duoplasmatrons Penning PIG and duopigatron type ion sources with oscillating electrons The quasi Pierce type extraction system is used with HF ion sources as well especially in those neutron generators where the HV terminal and the accelerator are placed in a tank under pressure of SF or oil Fig 27 shows such an HF ion source with high 5 10 mA extracted ion current In an arrangement shown in Fig 19 by applying a frequency of 45 MHz at 100 W output power and U_ 6 kV an ion current of 5 mA with a gas consup tion of 15 cm h has been achieved 31 5 3 MAINTENANCE OF GAS DISCHARGE PYREX BOTTLE After several hundred hours of operation a thin metallic layer is deposited on the inner wall of the discharge bottle which can
108. e the nominal value The voltage drop across the secondary coil and thermomechanical leak contacts There should be a small 1 10 mV voltage drop along a good contact If the contacts are defective clean them and test again If there is a grayish pink discharge in the RF ion source check the vacuum and deuterium connections If these connections are in good condition there may be some cracks along the body of the leak 6 2 3 Needle valves Needle valves are used for controlled admittance of fine gas flows into the ion sources They can be operated either manually or electromechanically Re liable and popular needle valves for ion sources are the BALZERS EVN 010 H1 and EDWARDS FCV 10K types The variable leak needle valve of VARIAN includes a mov able piston with an optically flat sapphire which forms a variable seal complete ly free from friction seizing and shear All these valves can be used for very fine gas streams for adjustment of the gas pressure in the ion sources The fine adjustment of the stream is performed by adjustment of the position of the needle or the sapphire piston The movement of the needle is controlled through a threaded shaft or a threaded shaft and lever mechanism having a mechanical advan tage of up to 10 000 to 1 49 For a BALZERS EVN 010 H1 valve an exchangeable easy to clean dust filter is fitted into the lateral small flange port by which the valve can be connected to the gas bottle of the ion source
109. ear Data for Science and Technology J lich Springer Verlag Berlin 1992 p 533 17 N Jarmie R E Brown Nucl Instr Meth D10 11 1985 p 405 18 New Uses for Low Energy Accelerators National Academy of Sciences Washington D C 1968 19 T Freye A Lorenz Wirzba B Cleff H P Trautvetter C Rolfs Z Physik A281 1977 p 211 20 J Szab P Raics in Proc XIX th Int Symp on Nuclear Physics Zf K 733 Dresden 1989 p 266 21 G M Griffiths E A Larson L P Robertson Can J Phys A40 1962 p 402 22 C Rolfs S Rodney S Durrance H Winkler Nucl Phys A240 1975 p 221 227 23 X ray and Gamma Ray Standards for Detector Calibration IAEA TECDOC 619 Vienna 1991 24 Z E Switkowski P D Parker Nucl Instr Meth 13 1975 p 263 25 D F Hebbard J L Voglh Nucl Phys 21 1960 p 652 26 F M Penning J H A Moubis Physica 4 1937 p 1190 27 C W Elenga O Reifenschweiler Proc Symp Pulsed Neutron Research Karlsruhe May 10 14 1965 Vol 2 IAEA Vienna 1965 p 609 28 C M Gordon C W Peters Nucl Geophys 2 1988 p 123 29 SODERN Data Sheets on Sealed Tube Neutron Generators France 1990 30 C M Gordon C W Peters T K Olson Conf Accuracy in Trace Analysis Gaithersburg MD USA 28 Sept 1 Oct 1988 31 T Sztaricskai ATOMKI K zl 22 1980 p 47 in Hungarian 32 T Sztaricskai Neutron Generators Proc IAEA Advisory Group Meeting on Small Accelerators 1 5 June 1992 De
110. ecoil effects the energy of detected gammas E differs from E The relation between E 3 and E is 2 1 2 E E MeV E E UB ___ B 14 cos6 9 053678 keV 7 5 9 1 Bcos 2M c M u where f v c and is the angle between the directions of M and the y ray By measuring E at 90 the energy of protons can be determined from eqs 6 and 7 Equation 7 shows that at 90 only the recoil effect must be taken into account The calibration of the Ge detector for high energy gammas is a difficult task If the measurements are based on the D p y He reaction the 6 12917 MeV gamma line with its single and double escape peaks at 5 61817 and 5 10717 MeV respectively can be used for calibration Such a gamma line is emitted by N produced in the O n p reaction The Ga isotope is also a good energy standard in the 833 6 4807 0 keV range High energy gamma ray standards for detector calibration are summarized in Ref 23 Procedure of the energy calibration by non resonant reactions has been described in detail in Refs 1 19 20 22 24 Typical yield curves for the py reactions on thick B 1 and thin C 25 targets are shown in Fig 9 There are a number of resonance reactions recommended for energy calibra tion However below 200 keV proton energy only a single calibration point the B p y C resonance at E 163 1 0 2 keV is available Precise energy calibration of low voltage neutron generator
111. ed by experiment using a point source in a scattering free arrangement 1 and the Zr Nb Zr Au Zr Ta activity ratio method 7 8 The E a functions at E d 50 150 and 300 keV for a thick tritium target are shown in Fig 2 Table 1 Recommended parameter values for calculation of thin target angular distributions of D T source yields in laboratory system equation 3 normalized to 90 a 90 a E E 20 1 0 0220 0 00025 4 2942 30 1 0 0227 0 0093 19 6126 40 1 0 0310 0 0007 52 8382 50 1 0 0344 0 0010 105 4180 60 1 0 0518 0 0035 173 2630 70 1 0 0407 0 0011 249 3768 100 1 0 0482 0 0011 393 3834 150 1 0 0599 0 0009 316 3704 200 1 0 0678 0 0005 198 4180 250 1 0 0685 0 0104 132 8133 300 1 0 0818 0 0005 95 0878 350 1 0 0904 0 0028 71 3864 400 1 0 1003 0 0008 63 4112 450 1 0 1140 0 0101 53 0290 500 1 0 1273 0 0187 45 5970 Data obtained from the zr Np above activity ratio 6 produced in n2n reactions prove the possible use of the analytical expressions The cal culated energy spread E D 6 of neutrons 1 2 FWHM as a function of emission angle for a thick TiT target is also shown in Fig 2 at Eq 150 keV Typical shapes of the distributions 9 calculated by the Monte Carlo simulation code PROFIL 10 are shown in Fig 3 The neutron energy spreads refer to the following circumstances Eq 190 10 keV point like beam spot Dt analyzed beam 11 5 cm sample target distance and 8 mm x 16 mm sample dimension Recently K
112. ed functions for an oil diffusion pump are shown with different cooled traps For neutron generators it is advisable to carry out the pumping speed measurements of the vacuum system according to the arrangement shown in Fig 80 The most important task is first of all to check whether the required vacuum can be guaranteed by the available pumping system along the whole length of the accelerator tube having dimensions negligible compared to the mean free path with the deuterium consumption necessary for normal operation of the accelerator For pumping speed measurement it is advisable to connect the U tube to the gas inlet of the ion source via a needle valve and to let an amount of deuterium gas flow into the system which corresponds to the actual gas consumption If for economic reasons D cannot be used hydrogen gas may serve the same purpose like at the installation of a new or repaired neutron generator Most commercial neutron generators with a deuteron beam of 1 2 mA have a deuterium gas consumption equivalent to 4 5 cm h at NTP Therefore the pumping speed measurement should be carried out at this gas inlet with very great preci sion The pumping speed of the pumping systems can change for the following rea SODS Decreased heating power of the oil diffusion pumps This can be an electrical failure but the problem may arise even if the water cooling loop is very close to the heater of the pump or if the oil level in the pump is
113. ed in any vacuum system Valve seat Evacuate the closed needle valve to a low pressure and close the gas inlet flange with a blanking plate Open the valve slowly about five turns to evacuate also the valve interior Close the valve and wait until the pressure stabilizes Remove the blanking plate abruptly and observe the meter If there is a leak at the valve seat there wil be a pressure rise in the vacuum chamber which will be indicated by the meter Valve interior The valve interior is sealed by a K shaped ring the K ring Again the gas inlet flange must be closed by a blanking plate and the valve opened If the valve interior is leak tight the same pressure must be reached irrespective of whether the valve is open or closed As mentioned above a leaking valve interior only means a certain loss of the gas admitted Compared with the results obtainable with sensitive leak detectors this testing method yields a relatively low detection limit For qualitative testing this is normally sufficient because leaks which would be detrimental to the vac uum system can be detected easily by a vacuum gauge Leaking valve seats can only be repaired by the manufacturers Leaks in the valve interior are mainly caused by contaminated or damaged seals 72 g Testing leaks with halogen or helium detector Valve seat Connect the closed valve to the leak detector and admit test gas into the gas inlet port via a fine dust filter Valve interi
114. egulations During the operation of neutron generators a certain amount of radioactive waste will be accumulated used targets components sorption and getter pumps tritium contaminated oils etc and disposal must be in accordance with the regulations 20 3 HIGH VOLTAGE HAZARD The high voltage power supply housing should always be connected to a good ground A neutron generator uses 150 200 kV therefore care should be taken when working in its vicinity without protective covers or devices Discharge all capacitors of the HV units before attempting power supply maintenance or re pair The use of an isolated handle discharge rod is recommended for such pur 219 poses Ground the HV terminal with the same rod if the generator doesn t operate with HV The 1 20 kV power supplies of ion sorces and extraction and focus system and ion getter pump are lethal voltages necessitating extreme caution when working on the HV terminal or ion pump supplies 20 4 IMPLOSION HAZARD The neutron generator is basically a vacuum vessel and presents the same im plosion hazard to accidental breakage as does a TV picture tube 20 5 PRESSURE HAZARD Some neutron generators have about 2 bars SF isolation gas on the high voltage and in the HV power supplies The pressure must be reduced to atmospheric before opening the pressure dome Removing the dome without reducing the pressure could cause a serious accident Similarly the pressure used in pneumatic syste
115. em will decrease with time after switching on the system as shown on the cur ves in Fig 75 If at all the connections of the system the gaskets are well fit ted to result in hermetic sealing and the inflow is very small the pressure will decrease from the initial Po value as described by curve 1 The pressure decrease continues until the ultimate pressure whose value depends on the pump ing speed and the hermeticity of the vacuum system is reached The time neces sary to attain the ultimate pressure is determined by the pumping speed of the pump t Fig 75 Pressure change in vacuum chamber vs pumping time Curve 2 shows the case when the usual ultimate pressure can be reached la ter after switch on of the gas ballast of the rotary pump This phenomenon indi cates the presence of different vapours Curve 3 describes a situation when the ultimate pressure cant be reached even after switching on the gas ballasting valve This indicates a higher than usual intake from the outer atmosphere In this case the pumping speed of the pump is not high enough to decrease further the pressure in the vessel to be evacuated The dynamic equilibrium will be reached when the gas outflow pro duced by the pump is the same as the leak current that is the intake from the outside 118 In case 3 it is not advisable to leave the vacuum system in operation A leak test should be carried out search for the defective gaskets and change
116. en indicate some malfunction in the cir cuit Do not use higher rating fuses than the original value this could cause some problems and as the condensers in the HV power supplies usually store high energy the use of a higher rating fuse can even cause a fire Recommended tests are described below a Testing the main components Testing a high voltage power supply should start from the power mains side Check the primary side of the HV transformer The test of the HV trans former starts with measurements on the unloaded transformer Remove the load from the secondary side of the transformer Test the primary current versus primary voltage In the meantime the secondary voltage should be measured by an AC HV meter The primary current should be in the interval indicated by the parameters and the ratio of the secondary to primary coil voltage should be almost con stant If there is some short circuit between the windings primary or second ary the temperature of the transformer will rise If the HV transformer is found to be operating normally test the rectifiers and the buffer condensers b Testing HV rectifiers The data sheet usually contains all of the parameters which should be tested and measured to find the trouble with the rectifier The HV rectifiers are sili con diodes and usually in series connected controlled avalanche diodes A recti fier diode with one pn junction may withstand a 1000 1500 V reverse avalanche breakdown v
117. er LLLLLLLI AEE T IERI LIII ILI OAR LEE TT Pe IN TTT es Scere SS Pepe Peer GEECEDDE a hn EE ESEIOERELS Hite acto Beige Wm eco 1 V ERERSEEE FEhEEREP BREELSE ERI Vlas The current stabilizer series circuit is shown in Fig 95 It is based on the commercial voltage regulator circuit LM 317 The LM 317 load current is shun ted by the usual PNP transistor equivalent complementary Darlington circuit The magnet current can be regulated by the helical potentiometer connected parallel to the 1 Q reference resistor at the output of the LM 317 This resistor should be a high power resistor with low thermal coefficient The resistor itself is as sembled into a cylindrical hole of the radiator of the regulating LM 317 and 2N3055 transistor A second 2N3055 transistor with 36 V Zener diode in the basis circuit protects the current regulating circuit against transients The voltage drop on the current regulator can be read by a voltmeter connected parallel to the current controlling unit The optimal voltage drop on the current regulator is between 10 and 30 V An advanced voltage regulator circuit the LM338K may replace the LM317 and 2N3055 based circuit in the above described magnet current regulator circuit As the absolute maximum rating of the voltage difference between the input and the output is the same the overvoltage protection ZF36 and 2N3055 should not be changed The operator alarm circ
118. etector to measure the energy of the direct capture gammas emitted in the PC py N non resonant reaction the absolute proton beam energy between 150 and 350 keV could be determined with a precision of about 0 4 keV 19 The abso lute energy calibration is possible even below 100 keV by using the D p He nonresonant reaction 20 This process was observed 21 as low as E 25 keV The Q values of the D py He C py N and O py F are 5 4936 1 9435 and 0 60035 MeV respectively The nonresonant direct capture method has been developed 22 in an experimental arrangement as shown in Fig 8 The y ray yield curves are measured at target location 1 with a NalI Tl detector At the second location the y ray transitions can be measured with a high resolution Ge detector at 0 and 90 degrees According to the kinematics of an X p y Y reaction the energy of a gamma photon emitted by a nucleus decaying at rest is M E Q E 6 o P M my 23 TARGET LOCATION 1 TARGET LOCATION 2 90 po f FON 1 t x x Y BEAM P a z 72 cm Ge Li td ZA DETECTOR 1L p VIEWER SHROUD 8x8cm Nal TU DETECTOR Fig 8 Experimental setup for measuring proton beam energy via the y ray detection emitted in non resonant p y reactions 1 0 0 5 RELATIVE GAMMA INTENSITY J fAo3keV RES 459keV RES 0 0 01 02 03 04 05 06 07 Ep MeV Fig 9 Yield curves of p y reactions on Ig and c 24 Because of the Doppler and r
119. exafluoride gas The ion source is cooled by electrically insulating liquid FREON 113 while the target is cooled by circulated water out side the pressure tank The arrangement makes it possible for the small so called accelerator head which has only two pairs of coolant pipes and a couple of cables to be moved easily and adapt to the circumstances of the investigations This sealed tube neutron generator delivers over 1010 n s during its guaranteed life time of 200 hours The PHILIPS sealed tube type 18601 has a builtin Dushman type ionization vacuum gauge a pressure regulating replenisher and a high voltage damping resis tor in its 737 mm long 70 mm diameter usually oil filled metal tube container This typical bore hole logging neutron tube has a pulsing capability between 5 and 1000 us A schematic diagram of a recent solid insulation tube packed SODERN neutron generator is shown in Fig 13 28 Therapeutic use of neutron generators assumes a minimum 14 MeV neutron yield of gt 10 n s The relatively small size of the neutron tubes is an advantage in fast neutron therapy they can be easily installed in the neutron collimators of the isocentric treatment geometry The problems related to tritium handling in pumped neutron generators do not exist in the case of the sealed tube so sealed tube intensive neutron generators are ideal for fast neutron cancer therapy The MARCONI ELLIOTT 30 and the HAEFELY the KARIN or KORONA are sealed tub
120. f the duct section and the trans mission probability of the molecules In this flow range it is independent of the pressure Degassing is a desorption which is accelerated by physical processes Desorption is the liberation of gases absorbed by a sorbent material The liberation can be spontaneous or can be accelerated by physical processes Gas diffusion is the movement of a gas in another medium due to its concen tration gradient The medium may be gaseous liquid or solid Flow Viscous flow is the passage of a gas through a duct under conditions such that the mean free path is very small in comparison with the smallest internal dimension of a cross section of the duct The flow is therefore dependent on the viscosity of the gas and may be laminar or turbulent In the case of viscous flow the resistance is a function of the pressure 93 Turbulent flow eddy flow is a viscous flow with mixing motion above a critical Reynolds number For circular cylindrical pipes Re 2300 Laminar flow parallel flow is a viscous flow without mixing motion at small Reynolds numbers Molecular flow is the passage of a gas through a duct under conditions such that the mean free path is very large in comparison with the largest internal di mensions of a cross section of the duct In the case of molecular flow the re sistance is independent of the pressure Gas is matter in a state of aggregation in which the mean distances between the molecules
121. ff generators 13 1 ELECTROSTATIC FELICI HIGH VOLTAGE GENERATOR The Felici generator 94 replaces the belt of the Van de Graaff generators by a rotating insulating cylinder which can sustain a perfectly stable movement against the rubber belt which tends to vibrate even at high speeds The princi ple of the Felici generator is shown in Fig 109 The main components of the Fe lici generator are The rotor which is a tube like cylinder made of insulating material The ro tor is driven by an electric motor and charges are deposited on the surface of the rotor The rotating cylinder is the only moving part of this electrostatic generator 155 Fig 109 Diagram cross section of the Felici generator Discharging ionizer xeu EO Metallic inductor P a 2h Stator 7 Glass cylinder Insulating cylindrical roto Charging ionizer 1 HV Output 200kV 49 0 4 So ef Compressed hydrogen Voltage regulator _ i Exciter 30kV Fig 110 Principle diagram of a regulated two pole Felici HV generator 156 The ionizer electrodes which are very thin metallic needles blades placed in close proximity to the rotating cylinder The charging needles spray the electric charges by corona discharge onto the surface of the rotor while the discharging electrodes needles collect the charges by drawing them off the surface of the rotor The segments or inductors which induce a strong elect
122. fferent gases For example for electrons of 100 eV energy in H gas x 500 cm n 20 in He gas x 800 cm n 32 and in N gas x 140 cm n 6 In the case of the Penning source the extracted beam contains only about 40 to 60 of atomic ions as a maximum In spite of the low atomic ion fraction PIG ion sources are often applied in neutron generators even for deuteron energies lower than 200 keV because of their simple construction cooling power supply system inlet gas flow requirement and long operating lifetime The U is usual ly about 5 10 kV the I is in the range of mA The current from the ion source the density of the plasma can be regulated by changing the anode voltage The Penning ion sources are usually equipped with permanent magnets so they are ideal ion sources for sealed tube neutron generators The extraction of the ions from the discharge in the Penning ion sources is usually made by a diaphragm type extraction system The self maintaining discharge is influenced by the cathode material through the secondary electron emission induced by positive ions striking the surface Al Mg and Be cathodes coated with oxides require low ignition voltage U 300 to 400 V The oxide layer can be regenerated by operating the ion source for 10 to 30 min with oxygen gas The lifetime of oxide cathodes substantially increases if 2 to 10 oxygen gas is admitted to the ionized gas Low ignition voltage 400 to 800 V is requi
123. fier circuit will produce a V o output voltage so the n stage of rectifiers will produce nV o Output voltage The advantages of the circuit are the loadabili ty and that the highest stack and its secondary coil should be insulated only from the transformer core and primary coil over the nV output voltage 1 Further stages up ton i Stage 2 Stage 1 Uin Fig 119 HV cascade circuit with cascaded transformers 165 b Allibone voltage multiplier The Allibone voltage multiplier is based on higher voltage transformers Each stage comprises one HV transformer which feeds two half wave rectifiers As the storage capacitors of these half wave rectifiers are series connected the HV secondary coil T cannot be grounded This means the main insulation between the primary and the secondary coils of Ti has to be insulated for a DC voltage of Vina the peak value of the secondary coil of T The same is necessary for T but here the HV secondary coil is at a potential of SY aye Increasing the number of stages will increase the insulation problem related to the secondary to pri mary insulation as in the case of the insulated core transformer The Allibone HV multiplier circuit is shown in Fig 118 c DC cascade with cascaded transformers The increasingly better insulation between the transformer core and the sec ondary coils in the ICT HV generators can be moderated by the cascaded transform er method In this circuit every trans
124. fore starting up the turbomolecular pump it is important to check the operation of water cooling In most vacuum systems water cooling starts automatically and the operation of the turbomolecular pump is in terlocked by a pressure switch on the drain leg of the water cooling pipes If the water cooling is not automatically switched on by the fore vacuum pump wait until the pressure in the vacuum system has reached the forevacuum level The interlock circuit of the forevacuum controller will inhibit the manual switch on of the turbomolecular pump c The use of an isolation valve quarter swing or butterfly valve is advisable between the inlet of the turbomolecular pump and the vacuum manifold of the neutron generator This butterfly valve is common in most neutron generators The operation of the isolation valves is manual electromagnetic or pneumatic For smooth operation of the neutron genera tor the vacuum in the manifold and in the acceleration tube beam line etc should be kept but the housing of the turbomolecular pump has to be vented to atmosphere to avoid oil infiltration from bearing lubricat ing oils The use of dry nitrogen is advisable d After switch off the rotor of the turbomolecular pump remains whirling for a period of 10 15 minutes During this time it is advisable to leave the rotary pump running and only at the end of this period to vent the pump housing to atmosphere Experience has shown that uninterrupted opera
125. foreline trap has to be filled by liquid ni trogen b Open the backing valve 2 c Open the isolation valve 1 d Switch on the heating of the diffusion pump when the correct forevacuum value in the whole system is reached e After another period of 10 15 minutes has elapsed this depends on the heating up time of the diffusion pump the cold trap on the top of the diffusion pump has to be filled up with liquid nitrogen B Switching off the vacuum system a Close the isolation valve 1 b Empty the cold trap by warming or compressed air During this opera tion the greater part of the adsorbed gases and vapours wil be re leased and the diffusion pump that is still working will transport them to the foreline trap within about 10 20 minutes depending on the size of the traps c Switch off the heater of the diffusion pump let it cool down below operation temperature d Close the backing valve 2 e Switch off the rotary pump now the valves 3 and 1 will automatically be turned off and on respectively This vacuum system as can be seen in Fig 67 involves the possibility of the tritium target exchange and the exchange of the old ion source components using a second rotary pump without switching off the diffusion pump In such a case the required part of the vacuum system after closing the isolation valve 1 can be exposed to the atmosphere After the necessary changes have been made the uppe
126. former per stage consists of a low volt age primary a high voltage secondary and a low voltage tertiary coil The third low voltage coil excites the primary coil of the next stage The operation of the circuit may be understood from the circuit diagram shown in Fig 119 The advantage of this circuit is that each stage is identical there are no higher insulation problems between the primary and secondary coils than V Al though there are limitations on the number of stages as the lower transformers have to supply the energy for the upper ones this circuit excited with mains frequency provides an economical DC power supply with moderate ripple factors and high output power capability d The Deltatron HV generator A sophisticated cascade transformer system is the Deltatron HV DC power supply These generators might be limited in power output up to about 1 MV and some mA The very small ripple factor the high stability the fast regulation and the small stored energies are essential capabilities of this circuit The circuit shown in Fig 120 consists of a series connection of transformers which do not have an iron core Connected to every stage is the usual Cockcroft Walton cascade circuit which has only a small input voltage some kV but produces output voltages of some 10 kV per stage The storage columns of these cascades are then connected in direct series providing the high DC output voltage for the whole cascade HV generator unit T
127. gas consumption q vs height of electrolyte for the rough surface 1 and fine surface 2 silicon rubber seal on the top of the float Fig 48 shows a relatively easy way to change the gas generation It can be done by adjustment of the electrolyte level When a regulated current source is used for the supply of the electrolyzer the gas admission into the ion source will be stable The advantages of this float regulating electrolyzer are It enables the gas supply of the ion sources to be stabilized by the regula tion of the current in the electrolyzer The insulated distance control of the current is relatively simple If the electrode current is switched off the ion source will be closed 83 8 REMOTE CONTROL OF THE HIGH VOLTAGE TERMINAL The ion source the ion source power supplies beam handling facilities etc placed on the HV terminal of a neutron generator require normal control during operation Regulation of the ion source and the related devices needs a control through the acceleration high voltage The terminal control of a neutron generator can be carried out by mechanical control using insulators manual drive by Perspex rod or nylon fiber electromechanical control motor gear driven insulating rod or nylon fiber insulation transformer control of the power supplies on the HV terminal optical insulation in the control line of the HV terminal units computer microprocessor control of the terminal uni
128. ge multiplier system of the Dynamitron is usually placed in the SF pressure vessel of the accelerator and the stages sometimes power the homogeneous field acceleration tube directly A Dynamitron type high voltage pow er supply utilized in a neutron generator is in operation for 14 MeV neutron therapy at the Eppendorf Hospital in Hamburg A comparison of the main parameters for the single wave full wave Cockcroft Walton cascades and the Dynamitron is shown in Table 19 99 TABLE 19 Main parameters of Cockcroft Walton cascades and Dynamitron Circuit Single wave Full wave Dynamitron NU ve U 2NU U 2mU U T 4C JC unloaded I N 1 M IN l5 Ripple U U U p S 2fC efC Io 3 Io 3 Io 4C Voltage drop OU ic 2m 3 N 3 U IC N 6 N 3 D SRL o9 1 2 C5 Usual Vout 0 1 1 MV 0 1 2 5 MV 0 7 7 MV Liaz 1A 500 mA 50 100 mA 13 3 TROUBLESHOOTING OF HIGH VOLTAGE POWER SUPPLIES The malfunction of a high voltage power supply is usually caused by improper operation of the components e g transformer rectifier s buffer condensers filter resistive or inductive regulating system electronics contacts or in sulations 168 In troubleshooting of a neutron generator with HF ion source an analog without active semiconductors multimeter is recommended The output power of an HF oscillator is a few 100 W which can easily kill an LSI based digital or FET transistor amplifier based electronic multimeter Similarly
129. generator 107 The shielding material for a laboratory often costs as much as or more than the actual neutron generator and care is needed in designing a building to house such equipment The maximum recommended weekly dose to personnel for all types Table 20 Maximum permissible neutron fluxes and fluences to personnel EME CON MN VN CECI HEN MEN E n MeV Flux n cm s Fluence n cm Thermal neutrons 268 38 8 x 108 0 1 MeV neutrons 32 4 8 x 10 0 2 MeV neutrons 8 1 16 x 10 10 30 MeV neutrons 4 6 0 x 10 215 of radiation is 4 x 10 v 40h 40 mrem 40 h or 10 x 10 Sv h 1 0 mrem h Particle fluxes n cm s equivalent to 1 0 x 10 Sv h and fluences n cm delivering 4 x 10 Sv are given in Table 20 The maximum recommended dose for the public is 0 5 x 10 Sv h that is about 0 2 n cm s Neutron generators can produce 3 MeV and 14 MeV neutrons in D D and D T reactions respectively The yield of D D reaction is lower than that of D T by a factor of 100 and the energy of neutrons is also lower for D D Therefore shielding should be constructed for a radiation hazard of 14 MeV neu trons The flux of neutrons is given by the following expression jlE ge 48 4znR where S is the source intensity e g 2 5 x 10 n s R is the distance between the source and the given point where the radiation exposure should be determined x is the thickness of the shielding and 2 is the removal cross section Table 21 Removal
130. gh voltage test equipments insulating transformers mains frequency HV power supplies sealed tube neutron generators 238 TECHNICAL UNIVERSITY BUDAPEST Institute for Automation H 1117 Budapest HUNGARY Tel 36 1 166 4527 Fax 36 1 166 6808 Telex 225 931 Products high voltage and high current stabilized power supplies for accelerators and X ray equipment Contact person Dr I Ipsits PULSE ELECTRONIC ENGINEERING 3 15 Tatekawa 4 chome Sum ida ku Tokyo 130 JAPAN Tel 03 633 6101 Fax 03 634 0636 Products high voltage DC powers supplies stabilized constant current power supplies special HV equipment INSTITUTE OF EXPERIMENTAL PHYSICS KOSSUTH UNIVERSITY H 4001 Debrecen Bem ter 18 A HUNGARY Tel 36 52 415 222 Fax 36 52 315 087 Telex 72 200 univk h Products accessories components for neutron generators and related equipment target holders quadrupole lenses associated particle target heads wobbling target holders ion source components neutron monitors and sample holders pneumatic sample transfer systems for neutron generators design and manufactur ing services for neutron generator laboratories Contact person Head of the Institute ALFAX Malmo Lundanvagen 143 212 24 SWEDEN Tel 040 189 000 Telex 32504 Products heavy water compressed deuterium gas in gas cylinder INSTITUTE OF NUCLEAR RESEARCH Prospekt Nauki 119 252650 Kiev 28 UKRAINE Products tritium and deuter
131. gnetic field selects the ion species by e m even a slight deflec tion larger than the diameter of the beam on the original place may select the charged and uncharged components of the beam The selection of the charged and neutral beam components increases the target lifetime protecting the target surface from oil vapour contamination in the vacuum system The analyzing magnets are made of soft iron The main problem in the constr uction of an analyzing magnet is procurement of the proper soft iron Iron sheets with carbon content less than 0 06 are suitable material for deflection analyz ing magnets 89 The power consumption of the coils is usually in the range of a couple of hundred watts so correct cooling of the coils is recommended As the metal wires of the magnet have positive temperature coefficient regulation of the magnet current is advisable Two magnet constructions are shown in Fig 93 and in Fig 97 The first is easy to construct having two half disc shaped pole pieces This solution allows a relatively high deflection angle with the given coil data up to 60 70 The pole pieces have a shape that is easy to manufac 138 Fig 93 Schemat Fig 94 Power s m S E oe s of a 60 deflecting magnet sizes in mm upply of the deflection magnet shown in Fig 93 1N 4001 Helipot 300k Fig 95 Current regulator circuit of a magnet power supply ture and they can be machined on a
132. h 2385 and 732th foils long counters and liquid scintillators are used as prompt monitors for re cording the source strength as a function of time The spectra of background neutrons are determined either by the activation threshold foils method or by prompt spectrometry based for example on an NE 213 liquid scintillator The ideal 14 MeV neutron field of the neutron generators is contaminated in practice by lower energy groups to some extent The most important sources of these non 14 MeV neutrons are 1 Elastic and inelastic scattering of the original 14 MeV neutrons in the target target assembly sample holders bulk samples APM head and electronics air construction materials of the target room 2 The D D neutrons from the drive in target resulting in a 0 1 1 of the D T yield depending on the condition of the tritium target 3 D D neutrons from the beam apertures ie beam accelerating and transport systems diaphragms etc 4 Contribution of the D3 and Dj ion induced reactions in the case of nonana lyzed beam In order to decrease the influence of these parasitic neutrons the target should be placed in the center of the target room at equal distances from the walls floor and ceiling Thin wall air cooled target holders 1 are ideal to get low contributions of the background neutrons The thick water cooled targets in a heavy set target holder and the mixed beam neutron generators ie sealed tubes may produce a contamina
133. h a yield higher than 10 7n s were developed for fusion related applications neutron cross section measurements production of long lived isotopes investigations on radiation effects cancer therapy and elemental analysis of small samples The original DC intense neutron generators were equipped later with pulsing systems to measure the secondary and leakage neutron spectra Such experiments require about 100 times higher neutron yields than that of today s intense neutron generators to achieve the required accuracy of data for fusion reactor design These programs became more important after the first successful experiment with DT fusion in the Joint European Torus in Abingdon UK 1991 The design of a fusion reactor needs accurate data for tritium breeding nuclear heating bulk shielding secondary reactions and gas production The accuracy of the energy and angular distributions of secondary neutrons DDX measurements for blanket and other structural materials should also be increased 31 33 34 For calculations of the induced activity more accurate activation cross sections are needed not only for the materials of the major components but also for their impurities Precise activation data are required mainly for the dosim etry reactions and for unfolding the neutron field in different parts of the fu sion reactor During the early intense neutron generator period second half of the seven ties when several such machines had been co
134. has promoted the develop ment of a control system for processes related to accelerators An up to date 88 ION SOURCE H V AREA MEASURING CONTROL AND Fig 53 Block diagram of the ion source control by CAMAC at the neutron generator using intelligent CAMAC crate controller 577 graphic compute terminar for communication electronics for matching and transfer NEUTRON GENERATOR Fig 54 Hardware system block diagram of a neutron generator control 89 10k Fig 55 Optical transmitter of analog signals from control console to HV terminal computer control system for such purposes represents an on line closed loop sys tem consisting of one central computer and additional microcomputers for special tasks Such a system reduces operating expenses and is necessary for testing and operation of the automation of the accelerators The microprocessor system for neutron generators requires solution of problems related to the operation and construction of a cascade generator The open loop systems can be utilized at the manual control of the generator while the closed loop systems can handle the entire control of the neutron generator operation The modules of the accelerator neutron generator control are almost stan dard CAMAC or more recently PC cards A block diagram of a neutron generator with duoplasmatron ion source control is shown in Fig 53 57 In this neutron generator the ion source is floati
135. he magnet and the power supply the output voltage of the power supplies the wall outlet of the mains the continuity of the coils The repair of the faulty component depends on the fault found b c d e 154 The doublet lens does not focus in one plane Check the corresponding cir cuits The lens does not focus in both planes and it works only horizontally or ver tically Activation of both quadrupole lenses results in a beam which cannot be focused by the two lenses together This can cause a faulty rectifier Test the buffer condensers of the rectifier the rectifier diodes the voltage regulator IC Bad focusing conditions can also be caused by poor vacuum conditions Test and improve the vacuum in the system Strange magnetic fields may cause the focusing properties of a quadrupole lens to deteriorate If the quadrupole lens pair is close to the beam analyzing magnet the deflection magnet may magnetize the quadrupole lens This effect can be observed from the strange focusing behaviour of the quad rupole lens while the power supplies and coils work normally In this case a magnetic shield may help to improve the focusing properties 13 HIGH VOLTAGE POWER SUPPLIES High voltages are extensively used in physics accelerators electron micro scopy etc for electromechanical X ray equipment and industrial applications precipitation and filtering of exhaust and in communication radio
136. hone 085 70161 Telex 855 162 Fax 085 74 830 Products Vacuum valves gate valves ALCATEL 4 rue Perkier F 92120 Montrose FRANCE Tel 1657 1100 Telex 270 431 Products components materials systems pumps technologies HUNTINGTON LABORATORIES 1040 L Avenida Mountain View CA 94043 California USA Tel 415 964 3323 Fax 415 964 6153 Product vacuum components MAC VACUUM 23842 Cabot Boulevard Hayward CA 94545 1651 California USA Phone 415 887 6100 Telex 910 383 2023 Fax 415 887 0626 Products vacuum components materials systems SHADIER SCIENTIFIC 2976 Arf Avenue P O Box 57287 Hayward CA 94545 California USA Tel 415 783 0552 Fax 415 783 7245 Products vacuum materials components systems 234 MEGAVOLT Cornhill minster Somerset TA19 OAH ENGLAND Tel 44 460 57 458 Fax 44 460 54 972 Products services accelerator tubes ion sources beam handling devices target assemblies repair maintenance second hand accelerator components TECHNABEXPORT Starimonetni pereulok 26 1091800 Moscow RUSSIA Tel 2392 885 Fax 70 952 302 638 Telex 41328 TSE SU Products neutron generators tritium and deuterium targets Contact person Mr Borodulin and Mr Basov Mr Grigorjev targets ATOMKI Bem ter 18 C H 4001 Debrecen P O Box 51 HUNGARY Tel 36 52 317 266 Telex 72 210 Telefax 36 52 316 181 Products Diffusion pumps vacuum me
137. i rre citps Vra eH IRE e kPa SEP RE SEE QUID a o K tenisin FE MER 43 5 1 High frequency ion SOUTCES iet eese n otv i o UR en REPRE ERE E ORENSE ERAN E ER NES 43 5 2 Extraction of ions from ion sources sssseesseeee ene 48 5 3 Maintenance of gas discharge pyrex bottle sse 51 5 4 Hiph frequency Oscillators i uecsesehdeerevex v e Ie eess Poe er uid ise auxi Econ o en 52 5 4 1 Troubleshooting of high frequency oscillators sesse 55 5 5 Penning ion SOUTCES ooierbuss exe xso a cR En ve Reo Sup a aS XVn ek Y awit org aire ED 58 5 5 1 Troubleshooting of Penning ion sources sseseeen 61 s DEUTERIUM LEAKS eite eroe oto e ERR SR os eH NUEVAS erg 63 6 1 The palladium leak rp io ERE QU rh ae ERE EN eese e anri 63 6 2 The thermomechanical leak and the needle valve ueeesess 65 6 2 1 The thermomechanical leak valve cccce cess ese neta eee eeneeeenes 65 6 2 2 Maintenance and troubleshooting of thermomechanical leaks 66 6 2 3 Needle Valves tai9 cacensuriceeseasregante eX Y PIA FR Nd Aqu DU COE cU M eet s 67 6 2 4 Maintenance of needle valves 11 eese err br rra hh hn 70 6 3 Calibration of leak valves Gas consumption measurements of ion sources 73 6 3 L Measurement scere oe ee xo cepas v E cen ena VRs EA d AERE TRAN LETS 74 DEUTERIUM ELECTROLYZERS eelbzioisashkeeeo ano e m odexo gore Ra BEAT e IR RR 76 7 1 The float fepgulator ele
138. iam Application 107 n s kV mA mm RTNS I 6 400 22 6 irradiation TOF RTNS II 30 380 130 10 irradiation LANCELOT 6 160 200 50 irradiation OCTAVIAN 4 300 35 30 irradiation TOF LOTUS 5 250 500 50 cm irradiation Chalk River 4 300 25 10 irradiation Lewis NASA gt 1 300 30 54 therapy Sandia 4000 200 40A 200 cm irradiation 6x6 10 250 250 1800 radiobiology NRPB gt 1 600 10 6 irradiation DYNAGEN 3 500 12 20 therapy FNS 5 400 20 15 irradiation TOF AWRE 2 5 300 12 10 Bratislava gt 1 300 10 10 irradiation TOF Kiev gt 1 250 15 10 irradiation INTTF gt 10 180 200 16 target devel Dresden gt 1 300 20 15 irradiation Debrecen gt 1 200 20 10 irradiation Wisconsin gt 1 gas therapy Lanzhou lt 1 300 5 10 irradiation Note the high current 40 A for the Sandia generator stable titrides than TiT has many advantages The target cooling is a fundamental problem for intense neutron generators and therefore the use of a technically perfect gas target assembly is recommended to increase the yield of present day neutron generators At an equilibrium pressure of 100 Pa the corresponding tem perature for the TiH ZrH ScH ErH and YH systems are 390 590 700 790 and 860 Celsius respectively The behaviour of the TiH ScH and ErH systems is shown in Fig 16 35 and a survey of these generators is given in Table 12 Most of these neutron generators have solid rotating targets and minimal beam line components to achieve the h
139. ical leak valve The operation of the thermomechanical leak is based on the differential thermal expansion coefficients of different materials The leak itself consists of a metal ball held by a ceramic rod against a precision seat with an orifice The orifice and the ball form the valve seal The outer cylinder the container wall around the ball and the ceramic rod are directly heated electrically The expansion of the cylinder is greater than that of the ceramic rod so the seal between the ball and its seat will be gradually opened The leak rate of the thermomechanical leak valve depends on the temperature of the outer wall of the leak ie on the electric current used for heating the wall In the case of a heating current switch off or power failure the ball will be reseated to the seat and the valve closes In the case of an electric power cut off this valve will protect the neutron generator against exposure to atmo 65 Gas outlet ion source metal ball ceramic rod Gas inlet Direct heated metal cylinder Bottle Good electrical contact 0 1 V AC Fig 38 Schematic diagram of a thermomechanical leak valve z Latm ml hour 10 20 30 mV AC Fig 39 The leak rate vs heating voltage of a thermomechanical leak spheric or D pressure The operational principle of a thermomechanical leak is shown in Fig 38 The thermomechanical leak is a precise component and therefore the concur rent exposure to cor
140. ickness Permeation involves diffusion and surface phenomena The pressure of a gas on a boundary surface is the normal component of the force exerted by the gas on an area of a real surface divided by that area The legal pressure units are Pascal as the SI unit abbreviation Pa and bar as a special unit designation for 10 Pa 1Pa 1Nm 1 bar 1000 mbar 10 N m 10 Pa The unit commonly used in vacuum technology is the millibar 94 Quantity of gas pv value is the product of the pressure and volume of a specified quantity of gas at the prevailing temperature If the pv value is to be used as a measure for the quantity of substance or gas this must be an ideal gas whose temperature must be specified The resistance is the reciprocal of the conductance The Reynolds number is the nondimensional quantity Re Pv 25 where p Density of fluid v Average flow velocity 1 Characteristic length e g pipe diameter n Dynamic viscosity Re lt 2300 Laminar flow Re gt 4000 Turbulent flow The saturation vapour pressure is the pressure exerted by a vapour which is in thermodynamic equilibrium with one of its condensed phases at the prevailing temperature Sorption is the taking up of gas sorbate by a solid or a liquid sorbent Sorbents are also called sorption agents The standard reference condition is the condition of a solid liquid or gaseous substance determined by the standard temperature and standard pres
141. ighest possible neutron output The mixed DT beams were used to increase the target life at LANCELOT and LOTUS A gas 40 It 80 BEAM LINE e WORKSHOP cH SHIELD PLUG o TARGET ROOM I j TARGET dns p pA 8EAM PROFILE MONITOR aoe e VACUUM PUMP ao ea i QUADRUPOLE LENS 9 9 i e J A GAS ANALYZER ar GATE VALVE Q q o f P E g FARADAY CUP 0 o rc 6 EXPERIMENTAL PORT 1 n BUNCHER LJ Y DEFLECTION MAGNET MOTOR ALTERNATOR X DEFLECTOR V x wl ACCELERATION Tai Sy TUBE i C Es In 2 ig F C ACCELERATOR ROOM Fig 17 Top view of the FNS facility at JAERI Tokai mura Japan target is used only at Wisconsin University The target problems the tritium handling and the shielding require special buildings in the case of intense neu tron generators RTNS II OCTAVIAN and FNS were constructed in their own building together with the supporting electronic and mechanical workshops The relative amount of associated equipment inside and outside the generator hall can be estimated on the basis of the top view of the FNS facility in Tokai mura see Fig 17 36 42 5 0 ION SOURCES OPERATION PRINCIPLES MAINTENANCE AND TROUBLESHOOTING 5 1 HIGH FREQUENCY ION SOURCES Low voltage D T generators employ three types of ion source radiofrequency 37 39 Penning also called PIG Philips Ion Gauge 26 27 40 41 and duopla smatron
142. ime at Bratislava 53 The construction of the regulating electrolyzer is shown in Fig 47 The main component is a float 4 filling almost the whole volume of the cylindrical cham ber 1 The electrolyzer is closed from the upper side of the flange 2 in the center where there is a nozzle 3 The float has small points 5 round its cir cumference that prevent the float jacket from directly touching the inner surface of the chamber and at the same time allows the float to move freely along the axis of the chamber In the center of the upper lid of the float there is an elastic rubber seal 6 On the inner and outer walls of the lower edge of the float are the platinum electrodes 7 of the electrolyzer The chamber and the float are placed in a glass vessel 8 partly filled with the electrolyte D4O with the addition of KOD The material of the float the nozzle and the float chamber is Perspex Plexiglas In order to understand the operation of the regulating electrolyzer let us assume that the whole free space of the float chamber is filled with the electro lyte so the float is pushed upwards til it closes the inlet of the nozzle by its elastic seal If DC voltage of appropriate polarity is applied to the elec trodes the electric current starts to flow between them and deuterium gas pro duction starts at the inner negative electrode The deuterium gas gradually fills the space between the float and the inner surface of the float chamber
143. imental Physics Kossuth University Debrecen Hungary Yogyakarta Nuclear Research Centre Indonesian Atomic Energy Commission Yogyakarta Indonesia International Atomic Energy Agency Vienna Austria Institute of Nuclear Physics Pyongyang Democratic People s Republic of Korea Atomic Energy Establishment Dacca Bangladesh Institute of Experimental Physics Kossuth University Debrecen Hungary Nuclear Engineering Department National Polytechnic Institute of Mexico Mexico City Mexico Institute of Experimental Physics Kossuth University Debrecen Hungary Institute of Experimental Physics Kossuth University Debrecen Hungary International Atomic Energy Agency Vienna Austria 247
144. intenance troubleshooting and repair on the HV terminal must be done only after first grounding the HV terminal If the terminal is not grounded the 500 1000 V anode voltage can be lethal The neon lamp and incandes cent lamp may only be used with a sufficient long isolator Perspex rod When the indicators do not show HF oscillation the HF oscillator should be checked Test the following components and parts 1 All of the electromechanical connections of the oscillator 2 At power on check Filament voltage and current Anode voltage and current Suppressor voltage and current if it exists 3 At power off check Conductivity between the contacts coils and along the condensers Resistance of the resistors 4 The operation of vacuum valve s of the oscillator 5 The operation of the anode power supply For capacitively coupled ion sources of some 100 MHz the twin tetrode QQE 06 40 is used Because the two anodes are connected by a thick rectangular sil ver plated copper bar which can span the two anode pins the glass metal solder ing of the anode pin sometimes cracks leading to the exposure of the vacuum valve to air If this happens a larger white area can be observed on the glass wall of the tube which is caused by the chemical reaction between the getter ma terial and the air In push pull HF oscillators used usually in inductively coupled ion sources the changing of the two tubes should be carried out simul
145. into the vacuum vessel at that area of the wall where a leak exists Therefore the Pirani gauge will show a higher pressure due to the higher heat conductivity of the hydrogen indicating the position of the sus pected leak Ionization vacuum gauges may also be used to search for a leak if vapour of some organic liquids e g ether is used as a test gas In this case of course a vacuum value has to be attained which is at least within the range of applicability valid for the given gauge this value is between 10 and 108 mbar depending on the type of gauge Both methods described above are suitable for a quick search for rather coarse leak sites ie intense air intakes this quick and rough method is the most expedient to detect what has happened to a neutron generator during dis mantling and reassembly Vacuum chamber Rubber ring Needle valve Vent v ab S FREON Penning T isoLATION Fo GAS gauge VALVE Pirani gauge Fig 81 Vacuum test stand with halogen leak detector 124 When an ion getter pump is used in the vacuum system this pump can also be used for leak detection This is because the pumping speed of an ion getter pump is different for different gases for example it is three or four times smaller for argon than for oxygen Therefore if argon is used in the test gas jet the ammeter of the pump power supply will show a higher power consumption Leaks causing smaller inflow
146. ion fragments spectrum has changed check the over pressure in the detector box The bubbling gas flow indicator indicates the prop er pressure in the chamber If there are no bubbles check the built in manome ter of the gas cylinder If the cylinder is not empty the manometer shows over pressure Close the polyethylene tube by a clip between the fission chamber and the regulator and check the leak in the fission chamber or between the chamber and the bubble vessel 214 20 SAFETY HAZARDS RELATED TO NEUTRON GENERATORS The main sources of hazards related to the operation maintenance trouble shooting and repair of neutron generators are as follows Neutron and X ray radiation Open radioactive source tritium Residual radiation of the neutron irradiated construction materials High voltage power supplies for ion source extraction focus and acceleration Mains voltages Pneumatic pressure vessels and tubes Vacuum vessels and chambers Poisonous gas SEQ pressure Flammable deuterium gas D5 Flammable insulating transformer oil 20 1 Radiation hazard Maintenance and repair of neutron generators must be performed either by personnel trained by the manufacturers or other by experts in high vacuum and high voltage techniques and in handling tritium In designing the shielding consideration should be given to the fact that radiation levels from neutrons as high as 2 Sv h 200 rem h are common at 1 m from a neutron
147. iron The neutron induced radioactivity of structural materials can become a sig nificant problem In the air and the cooling water the 14N n 2n N and 160 n a 3N reactions will take place Some expected exposure rates at 10 cm following 1h of operation at 25 x 10H Backstreaming electrons can produce bremsstrahlung at the high voltage ter minal with a dose rate of about 600 to 800 mR h for a nonanalyzed beam The dose rate of bremsstrahlung can be decreased significantly if an electron suppressor n s produced by the target material are summarized in Table 22 is applied close to the target During the operation of D T neutron sources contamination or radiological hazard can occur in the environment caused by tritium outgassed from the target A potential tritium hazard exists also when the system is opened to the at mosphere for any reason ie repair maintenance target replacement deuterium leak replacement HF ion source balloon replacement breakage pump exhausting into the generator hall etc It has been proven that the tritium in gas and T5O vapour forms takes part in chemical reactions in the same manner as the hydrogen gas and H O vapour The half life of tritium is relatively long it is about 12 y The specific activity for T4O is 9 99 x 10 Bq g Tritium is uniformly distributed in the body within 90 min The biological half life of tritium is short about 12 days The gaseous T H O vapour atmospheric exchange rate
148. isadvantage and does not change the fundamental efficiency since two HV windings of the transformer are 160 Vy D h t transformer Vyn Fig 114 Full wave single phase rectifier now available With reference to the frequency f during one cycle now each of the diodes D The ripple factor is therefore halved Thus single phase full wave circuits can only be used for HV applications if the HV coil of the transformer can be earthed at the midpoint and if the DC output is single ended grounded and D is conducting for one half cycle with a time delay of T 2 13 2 2 Cascade generators The first voltage multiplying circuit was published by Greinacher 97 in 1920 and was improved in 1932 by Cockcroft and Walton in the first accelerator Such cascade circuits are known as Cockcroft Walton 98 generators An n stage cascade circuit of Cockcroft Walton type is shown in Fig 115 together with the main working parameters From Fig 116 it can be seen that the potential at all nodes 1 2 n are oscillating due to the voltage oscillation of V t the potential at the nodes 1 2 n remain constant with reference to the ground potential the voltages across all capacitors are of DC type the magnitude of which is 2V max amp CTOSS each capacitor stage except the capacitor Cy which is stressed with V max only every rectifier D D D D Da D B is stressed with ZN ax Or twice AC peak voltage and the HV o
149. ith a direction indicated in the figure is also present the electrons will move along expanding helical trajectories The maximum radius of the electron trajectory To max at given geometrical arrangement depends on the magnitudes of E and B as well as on the direction of the electron velocity to the magnetic field With a sufficiently high B value the r e max lt R requirement can be assured and thus the electrons from K will continue to proceed towards the K electrode The negative potential of the Ki electrode prevents the electrons which lost a part of their energy in Fig 34 Operating principle of the Penning type ion source 58 elastic and inelastic collisions from reaching the surface of Kj and they will return towards the K electrode This process is repeated at the K and K elec trodes and the electrons create an intense gas plasma by further collisions If the energy of electrons decreases below a critical value they can strike the an ode The number of oscillations n of the primary electrons can be calculated from the x mean freee path covered by the electrons in the oscillation 44 n X 15 2d where d is the distance between the cathode surface and the anode ring The value of depends on the pressure p and the probabilities of the in elastic w and elastic Oe collisions i 1 16 The number of the collisions will decrease with increasing pressure in the ion source The mean free path x can vary for di
150. itium target The tritium is radioactive and decays to He by beta emission During one year about 6 of the tritium will be replaced by He causing a background in the associated alpha particle detector from the re action 3He d p He The energy of the alpha particles from the 3H d n He and He d p He reactions are almost the same and can only be separated by a detector of high resolution Fortunately the reaction on He has a resonance at 440 keV while the reaction on tritium is at 110 keV On the other hand the He particles can escape from the target depending on the temperature At about 400 keV inci dent deuteron energy this is not rare at neutron generators the cross sections of the two processes become equal which can cause large errors in the determina tion of the neutron yield if this effect is neglected 120 For a 400 kV neutron generator using a one year old tritium target an ex cess count of about 6 will come from the helium reaction As the energy of the 206 Scintillator Aluminium sheet foil Ti T target Deuteron beam Fig 153 Associated alpha particle target head using thin plastic scintillation detector COUNTS CHANNEL o o 300 200 100 0 120 130 140 150 160 170 CHANNEL Fig 154 Associated alpha particle spectrum showing effect of the 3He d p He and H d p H reactions 207 incident deuteron energy is lowered the effect is reduced but even at energies of 200 keV in a fairly new target this
151. its original German name 69 A typical single gap diaphragm lens acceleration tube is shown in Fig 85 This acceleration tube has an extraction gap focusing gap due to the focusing behaviour of the first diaphragm and an acceleration gap The size of the electrodes for 150 kV acceleration voltage is indicated in the figure 70 An immersion lens is an electrostatic lens which has equipotential re gions on both sides of the lens It can be constructed of diaphragms or cylin ders Fig 86 shows an immersion lens consisting of two equal diameter cylinders The immersion lens depending on the polarity of the voltage drop of the lens gap can be analogous with convex and concave or concave and convex lenses 71 KAA LS SIS XA 6 SRRI IERIE RRES lon source base Acceleration HV Vacuum manifold d beam direction 100mm ORASINI XP 9101019101919 95 X oo A Fig 85 Immersion lens equivalent single gap acceleration tube for 150 kV neutron generator 128 Fig 86 Electrostatic immersion lens consisting of two cylinders with its optical analog 10 2 UNIPOTENTIAL OR EINZEL LENS The Einzel lens is an electrostatic lens in which the energy of the beam does not change because of the symmetrical potential of the electrodes The uni potential lens focuses the ion electron beams with positive or negative Uc Po tentials of the middle electrode Fig 87 represents the optical analog convex c
152. ium targets Contact person Mr Kolomentzev 239 RADIOISOTOPE CENTRE POLAND Foreign trade office POLATOM Ottwock Swierk 05 400 POLAND Tel 4822 798 435 Telex 812202 Fax 4822 797 381 Products deuterium and tritium targets deuterium gas CHINA INSTITUTE OF ATOMIC ENERGY P O Box 275 Beijng 102413 PEOPLES REPUBLIC OF CHINA Tel Beijing 86 1 935 7312 Telex 222 373 IAE CN Fax 86 1 935 7003 Products radiochemicals labelled compounds targets for neutron generators Contact person Mr Yu shan Wang neutron generator targets WALLIS HIVOLT Dominion Way Worthing Sussex BN14 8NW ENGLAND Tel 44 903 211 241 Fax 44 903 208 017 Telex 877 112 Products medium frequency high voltage power supplies up to 200 kV 500 2000W MARCONI AVIONICS Neutron Division Elstree Way Borehamwood Hertfordshire WD6 1RX ENGLAND Tel 44 1 953 2030 Telex 22 777 Product sealed tube neutron generators 240 z x e mj 8 PU Po im v lec Check for power lines f ANNEX B TROUBLESHOOTING FLOW CHART FOR NEUTRON GENERATORS WITH RF JON SOURCE START YES YES YES YES YES Are all power Pressure 1s below Ion source works supphes OK 10 mbar jnormally Is the FINISH YES YES Is any target Neutron lcoop SYSTEM current shown production f on the meter Extraction and Acceleration focus voltages voltage over 100 kV
153. k properly the synchronization of the y and z scanning wires will be unstable The proper operation of the contact can be checked by the os cilloscope observing the signals of the beam scanner while the synchronous motors are switched on and off several times The phase position of both the Z and Y signals should be in contact The oscilloscope should not indicate any DC level in the case of a stopped beam scanner 14 3 WIRE ELECTRODE MATRIX BEAM SCANNERS The development of microelectronics and the spread of personal computers have made beam scanning easy even at the lower end of accelerators The typical multiwire sensors of a beam monitor used earlier in the expensive machines consist of a vertical and horizontal grid of thin wires These wires collect the ion beam and the current is converted into voltages These voltages are multi plexed into an analog to digital converter The multiplexing and the processing of the measured current data are carried out by computer 101 In this Manual only a general description of such systems will be given The wire chamber of a beam scanning electrode consists of two normal glass laminated epoxy printed circuit boards which hold the vertical and the horizontal wires The wires are made mainly of tungsten electroplated with gold The usual diameter of the gold plated tungsten wires which can be soldered easily is about 20 wm Depending on the number of the input of the analog multiplexer the wires c
154. kind of error could result in an uncer tainty of 0 5 Fig 154 shows an associated particle spectrum around the alpha peak for an old tritium target at 400 keV bombarding deuteron energy Peaks 1 and 2 corre spond to the alpha particles from the 3H and the He reactions Peak 3 is the re sult of the proton buildup from the H d pj H reaction The energy difference of the two alpha peaks is about 350 keV but the energy resolution of the detector does not allow a better separation 120 18 1 SELF TARGET FORMATION BY DEUTERON DRIVE IN The associated particle target head with semiconductor detector is a good tool for studying the buildup of the deuterons in the target and the beam aperture materials The presence of the self target will contaminate the 14 MeV neutron spectrum through the D D reaction A sketch of an APM target head made for the study of self target formation and with plastic scintillators time of flight measurements is shown in Fig 155 Based on an APM head the neutron yield and contamination of the D T neu tron spectrum can be determined by observation of the charged particle spectrum The charged particle spectrum measured by a surface barrier semiconductor detec tor can give information on the condition of the tritium target and the back ground neutrons A typical APM spectrum is shown in Fig 156 which shows the Detector Fig 155 Associated particle target head used for self target formation and fast neutron
155. laboratories produce their own D gas to supply their accel erators The deuterium consumption of a RF or Penning ion source is below 10 ml h NTP while a duoplasmatron needs about 20 100 ml h These gas consumptions can be satisfied by a simple laboratory electrolyzer The Toshiba and the KFKI neutron generators even have their own electrolyzers with palladium leak on the HV terminal to supply the HF ion source The Toshiba neutron generator uses a heavy water electrolyzer of a few ml heavy water with a built in palladium leak The construction of this Pd leak with heavy water electrolyzer is shown in Fig 44 50 The production rate of the electrolyzer can be adjusted by setting the con ductivity the pH value of the heavy water electrolyte The solution is usually alkaline by putting a couple of KOD or KOH tablets into the heavy water If the electrolyzer produces more deuterium gas than the consumption of the ion source the water level will sink in the central glass tube and the cathode contact will be cut off This electrolyzer works normally if the conductivity of the electrolyte is set up properly i e in the 02 Q1 em range A part of the tip of the cath ode electrode is immersed in the electrolyte which can sometimes cause a KOH de posit formation on the cathode surface This insulating layer can prevent the D 2 to leak Anode 02 Mercury cut Electrolyzer Fig 44 Heavy water electrolyzer of the TOSHIBA NT 200 neutro
156. lathe The manufacture of the rectangular components does not even require a milling machine The coil is about 5000 8000 windings of 1 6 mm dia wire 90 This coil requires a 150 180 V DC power supply with an output current of 3 5 A The power supply can be made from a variac Tr in Fig 94 controlled isola tion transformer Tr rating 1 kVA The rectifiers are air cooled mounted on a suitable radiator The wire of the filter coils is made of 2 0 mm dia wire The condensers are parallel connected and inexpensive similar to those used in TV power supplies An additional current regulator can be attached to the system for example instead of the F3 fuse This current regulator may be constructed on the basis of power transistors cooled effectively by a fan A principle diagram of the power supply is shown in Fig 94 The power consumption of the relatively large coil is about 500 600 W in the case of separation of a 200 keV ion beam at a deflection angle of 60 Water cooling of the coil does not compensate for the growth in the resistance of the coil due to warming up A simple current regulator can be attached to the origi nal full wave rectifier power supply corresponding to the marked places in Fig 94 140 Iti RED I uu T 33Q 2200pF 33k 100k CH 162 GREEN IN 4151 Fig 96 Operator alarm circuit for the magnet current regulator shown in Fig 95 e C O f T DONN a LL QiilliLP c o pT et
157. latively low the diaphragm be tween the chopper and the buncher electrode does not need any extra water cool ing This diaphragm is made of stainless steel and its chopper side is covered by a titanium sheet Similarly a titanium sheet covers the selector side of the post selector diaphragm The pulses of the bunched 20 MHz repetition rate ion beam can be selected at 1 25 10 MHz As this selection is controlled by normal frequency dividers and phase shifting units the additional use of a pseudo random pulse selecting unit is easy 116 The geometry of the ion optical system of this nanosecond pulsed accelerator is shown in Fig 150 The first focus lens was necessary to get pre accelerated deuteron beam and to decrease the ion beam scattering of the residual vacuum al ong the 80 cm long path in the main vacuum manifold The Einzel lens as it does not change the energy of the focused beam easily focuses the pre accelerated deuteron beam into the chopper buncher selector acceleratortube ion optical line 202 Pulsing unit immersion lens Einzel lens Chopper Buncher Selector Acc tube 25cm 90 cm TiT Target Pick u d Suppr xU Fig 150 Geometry of the pre acceleration bunched nanosecond neutron generator 0c BEAM LINE s BUNCHER nd 30n 2 SLIT NEN ELECTRIC FIELD 500 ns TIME DEUTERON INTENSITY TIME TIME TIME Pd DEUTERON INTENSITY TIME Fig
158. layer loaded with tritium or deuterium up to this atomic ra tio corresponds to 0 23 cm cm or 0 6 Ci cm for tritium only For a thick tar get 1 2 cm cm gas is absorbed into Ti or Zr The range of D ions in a TiTi 7 target is between 0 12 and 1 05 mg cm for incident deuteron energies from 25 to 400 keV respectively 1 Therefore a TiT target of 5 10 mg cm is about 10 times thicker than the range of the deuteron ions of a few 100 keV For the cal culation of the range a simple approximation is given in Ref 104 Cu backing Thermal ae target tube insulator Stainiess steel e d Thin tube Fig 134 Heavy ice deuterium target for neutron generators 183 Heavy water D40 in frozen form may be used as a target material The prin ciple of heavy ice target formation in the vacuum system of a neutron generator is shown in Fig 134 The target holder is made of a thermally well conducting backing directly cooled by liquid nitrogen Control of the amount of heavy water vapour will balance the amount of the evaporated heavy ice Target disc Fig 135 The off axis target arrangement to utilize the whole surface of TiT targets Water outlet Insulating Target ring cup ZA Sta f Insulated water tight electrical contact for target current S N DTIIDITILLDLDALIIPYTIXLIS measurement ALLG ald ait 7 NNN teenie Rubber O rings Cooling
159. lead to a reduction in the atomic ion ratio of 40 to 50 Metals are good catalysts for the recombination of atomic ions this should be considered in construction of ion sources The recombination coefficient on a relative scale for most metals is about unity while for Pyrex 2 x 10 quartz 7 x 104 aluminium 0 3 The surface recombi nation of the ions at the exit canal is decreased by applying a quartz sleeve 51 that acts as a virtual anode and focuses the positive ions into the channel in the presence of extracting voltage During the operation aluminium sputtered from the extraction channel is deposited onto the inner surface of the quartz sleeve causing an increase in the fraction of molecular ion component to well over 40 96 This means that after a long term operation 50 to 100 h the glass balloon and the quartz shield surrounding the canal must be cleaned or changed For cleaning a solution consisting of 80 HF 40 and 20 HNO 100 is recommended The layer deposited from the inner surface will disappear within 10 15 minutes if the discharge balloon is rotated around its axis horizontally in the presence of a few mm thick layer of the solvent After cleaning the bal loon should be washed carefully with distilled water The same procedure is re quired to clean the quartz sleeve The metal extractor tips soldered into the ion source balloons should not be etched with the HF HNO solution The brownish deposit carb
160. lowing The colour of the discharge of the HF ion source if the discharge is weak and is slightly pink the gas supply is not adequate e g the electrolyzer does not work properly the filling valve is leaking and the D gas is ex hausted or the heating power is not enough contact failure The palladium tube leaks this can be observed if the discharge in the ion source bottle cannot be extinguished by switching off the filament A hole in the thin wall palladium tube can be detected by usual vacuum leak testing methods and the repair requires soldering with silver at the workshop of a precision goldsmith if there is no replacement for the defective Pd 6 2 THE THERMOMECHANICAL LEAK AND THE NEEDLE VALVE These two mechanical leaks have two major problems they can fail to close properly or they cannot be opened Their repair requires a well equipped preci sion mechanical workshop and an experienced tradesman The electrical trouble shooting of a thermomechanical leak is similar to that of a palladium leak The high flow rate in the case of a closed valve can be caused by stain or dirt in the canal of the leak The dismantling cleaning and assembly should be carried out with precautions Anybody who is not familiar with the structure of these leaks must not open them for cleaning or repair The best solution for the re pair of these elements is to send them back to the manufacturer for repair and readjustment 6 2 1 The thermomechan
161. m feedthrough of these rotating targets al lows a maximum 1000 rpm The commercially available rotating and water cooled target holders are manufactured by MULTIVOLT Crawley United Kingdom The schematics of a MULTIVOLT rotating target are shown in Fig 140 The wob bling target holders do not require vacuum feedthrough Below 200 W target load the wobbling system is recommended for use at the whole target surface and in or der to increase the target lifetime As the target moves around a circle the beam will utilize an annular surface of the tritium target The speed of the ro tation is a few revolutions per minute so the mechanical and vacuum problems of the target rotation can be solved by simple bellows or other elastic tube 105 A typical wobbling target is presented in Fig 141 The circular rotation of the target is carried out by a cam ring This cam is fixed onto a cogged ring driven by a cogwheel The cogged ring is rotated around a ball bearing on the standing part of that ball bearing The target tube is forced by springs onto the cam The connection of the wobbling target tube to the beam line tube is made of stainless steel bellows This bellows allows a slight circular movement caused by the rotating cam The excentricity of the target rotation wobbling can be ad justed by the position of the wobbling target tube by holding the flange of the target holder In this way the total surface of the tritium targets can be uti lized
162. m the control desk is utilized at the Toshiba KFKI and MULTIVOLT neutron generators 85 TERMINAL POTENTIAL 150 kV Pd LEAK MAGNET g POWER disces 5 SUPPLY FOCUS txTR prs POWER POWER SUPPLY SUPPLY VOLTAGE DIVIDER Te INSULATING PT FT oF P O m TO THE VARIACS ON THE CONTROL DESK Fig 51 Insulating transformer control of HV terminal SAMES J 15 and J 25 neutron generators 8 3 INSULATION TRANSFORMER CONTROL This type of ion source control is a solution for the control of a Penning ion source or an extraction voltage For RF or duoplasmatron ion sources the in sulation transformer control is less reliable due to the high number of insula tion transformers connected to the HV terminal The KAMAN TMC A 111 A 1254 and A 711 and the SAMES type D neutron generators utilize this method for insulation remote control of the equipment at the HV terminal In the case of a Penning ion source this is a simple method while the RF type requires a number of separate insulating transformers as many as five or six as at the SAMES J 15 and J 25 neutron generators The probability of sparking along the surface or between the primary and secondary sides in a single transformer is much lower than in the five or six insulating transformers The utilization of a single high power in sulating transformer is typical for neutron generators and small accelerators Figure 51 shows a block diagram of the insulation transformer remot
163. mping speed of a 2000 1 s diffusion pump 195 REMARK As the evaporation of the water from the soil is utilized for chill ing the coolant circulated in the closed circuit the area of the buried pipes should be sprayed with water about one hour before the operation of the cooling system so as to start with water chilled in the soil This system has some extra taps in case of emergency As the neutron genera tor needs only electric energy the cutoff of the electric power and the utiliza tion of a diffusion pump require some precautions The normal flow of the water is detected in the return pipe to the water tank by a flow actuated switch as shown in Fig 145 Water return Microswitch Closed circuit water tank or sink Fig 145 Water flow detection switch for water cooling systems The water from the return pipe of the cooling water loops flows through a hollow cylinder The weight and pressure of the flowing water pushes down the cylinder and through it the contact of a microswitch The threshold of the switching depends on the water flow rate of the coolant and it can be adjusted by the balancing weight of the lever The tank of the cooling water should be pro tected against dirt with a filter mesh In case of emergency cutoff of the electric power when the diffusion pump needs further water cooling the water can flow through the circulation pump ow ing to the height difference between the tank and the diffusion pump In this
164. ms should be reduced to atmospheric pressure before opening the system The SF tanks should be opened in well ventilated rooms Handle with care the hermetic units of the SAMES HV Felici generators 20 6 FIRE HAZARD The deuterium gas and some transformer oils are flammable Avoid electric sparks and avoid open fire while opening D gas vessels and oil transforms The organic solvents acetone benzene alcohol n hexane etc used in cleaning the neutron generator components are flammable use them with caution In the case of an accidental fire in the neutron generator hall use carbon dioxide fire extin guishers Water foam or powder extinguishers can cause fatal damage in the neu tron generators and the related instrumentation 220 21 CONSTRUCTION OF A NEUTRON GENERATOR LABORATORY 21 CONSTRUCTION DETAILS Many factors must be taken into account in establishing a neutron generator laboratory the most important topics are listed based on tbe proposal for the local staff in a developing country below 1 2 3 4 5 6 7 8 The thickness of the biological shielding can be calculated In general around the source at a distance of at least 2 m an additional 1 5 m wall of poured concrete p 23 g cm is needed The inner surface of the neutron generator room NGR must be covered with washable oil or plastic paint A thin plastic foil is advised for covering surface constructed of concrete blocks
165. n generator 76 220V IN 4101 V Pd Filament TIP 122 D 0 ELECTROLYZER 5V AND Pd LEAK INFRA LIGHT TRANSMITTER INFRA RED LED Fig 45 Optical fiber isolated Pd leak control and electrolyzer for the deuterium supply of ion sources on the HV terminal flow of the electric current from the electrolyte especially if the neutron gen erator has been out of action for a long time This disadvantage of the simple electrolyzers was eliminated by KFKI 51 in their U shaped electrolyzer Pd leak system by using two cathodes The upper cathode electrode No 2 in Fig 45 switches on a relay which remains switched on through the lower cathode of the electrolyzer electrode No 3 in Fig 45 until the electrolyte level sinks under the level of the tip of the lower cathode As 77 the electrolyte level rises again up to the upper cathode due to the gas con sumption of the ion source the relay switches on and the electrolyte level sinks again due to the operating condition Using this hysteresis principle of the KFKI electrolyzer a heavy water electrolyzer Pd leak system with optical fi ber connection for the regulation of the deuterium flow into the ion sources has been constructed This system can be used for the gas supply of the HF Penning and duoplasmatron ion sources The circuit diagram of the system is also shown in Fig 45 52 The electrolyzer works on the hysteresis principle described above The heater of the Pd tub
166. n oscilloscope This test can only be carried out in simple variac controlled mains powered transformer HV power supplies The shape of the rectified voltage and the reverse conducting not only due to the charge storage of the pn junctions can be observed on the screen of the oscilloscope As this method requires some experience in electric and elec tronic measurements it is recommended only for personnel well trained and experi enced in electronics d Testing the buffer condensers The capacitance of the condensers should be in the 30 and 20 range of the nominal one As the low capacitance of a buffer condenser may cause a higher ripple in the output voltage the correct value of the buffer condenser is important In high voltage condensers the stored energy is usually high so a breakdown inside the condenser may cause blowing and evaporation of the outer contact of the condenser Therefore the capacitance test is an important duty The stored energy in the condenser can also be tested by a discharging resistor the resistor should be fixed onto an insulating rod and the two arms of the re sistor should touch the two terminals of the high voltage condenser If there are healthy sparks the condenser can store higher energies This discharging method always gives an indication on the condition of the condenser especially when the person troubleshooting is experienced in discharging condensers in nor mally operating power supplies
167. n the system must not be changed during operation for example at a neutron yield of 19019 n s in the case of a commercial neutron generator with a beam cur rent of 1 2 mA the D gas intake is 4 5 cm h 100 c Economic considerations The greatest possible simplicity in handling the pumps and the entire vacuum system start stop The lowest possible probability of failures the pumps may break down espe ally during long irradiations The least possible reconstruction requirements during maintenance to repair any parts of the system might be difficult owing to tritium contamination The best possible accommodation for the existing laboratory conditions vari ous supplies eg for electric power water and liquid nitrogen replacement possibilities of failed parts of instruments workshop background for maintenance and renewal etc Fig 59 shows the best known pump types made by well known manufacturers Taking into account both the pumping rates and cost during operation the manufacturers of neutron generators use in practice one of the following pump P 109 mbar ultimate pressure necessary for a combinations to obtain the 10 neutron generator oil diffusion pump rotary pump turbomolecular pump rotary pump Ti ion getter pumps rotary pump The working principles of these pumps vacuum gauges and other vacuum tech nical equipment as well as the most important details of the operation of v
168. nd are not available commercially Therefore strong co Operation is required between the developing and advanced laboratories for up grading the commercial neutron generators with pulsing units and other compo nents As a number of manufacturers have closed down in the last decade some second hand neutron generator companies POTENTIAL in the USA or MULTIVOLT in the UK have started to buy used machines and restore them to their original conditions POTENTIAL specializes in neutron generators manufactured by TEXAS NUCLEAR 29 while MULTIVOLT deals with those made by SAMES The lack of spare parts and components causes many problems for the users especially in the industrially less developed countries in the field of maintenance and repair Dealing with second hand neutron generators is particularly important for the developing countries Table 8 lists the most popular commercial neutron generators Improve ment of the original characteristics of a commercial neutron generator ie ana lyzing magnet quadrupole lenses pulsing systems associated target assemblies etc needs local or international co operation with experienced laboratories An excellent example of such international co operation is the Fast Neutron Research 29 Table 8 Commercial pumped neutron generators Manufacturer Type U I Yield Pulsing kV mA n s TMC Ami 180 1 5 1019 us TEXAS NUCL 280 7 gt 10 i KAMAN A 1254 190 22 gt 10 nsus TOSHIBA
169. nd high value resistors can be tested by using a nuclear detector high voltage power supply A component can only be tested under working conditions the total test of a component can be carried out in its own circuit The housing of the high voltage power supplies should always be opened in a switched off and output grounded condition Even a switched off and output shortened power supply may contain charged high voltage capacitors precautions should be taken in dismantling the covers of the HV power supplies the body of the power supply should be grounded and the repairing personnel should wear shoes with insulated toes After removal of the covers try to discharge every high voltage condenser The troubleshooting of a high voltage power supply usually starts with visu al or small inspections If there is a lack of output voltage the output wiring and or connectors should be inspected The measurement of the continuity between the output of the power supply and the load should be done without output volt age and the unit should be grounded if it floats at the terminal voltage of the neutron generator The conductivity can be measured with the usual multimeter 169 digital or analog Discharging of the output buffer condenser is advisable If there is a lack of output voltage and the built in HV voltmeter does not indi cate any output voltage discharge shorten the output and inspect the fuse or fuses in the circuit Blown fuses oft
170. nded between 2 1 and 2 9 MeV range for the determination of the D D neutron fluence For D D reaction the neutron energy at O 100 is almost constant in the 50 lt Eq lt 500 keV range A value of E 6100 2 414 0 010 MeV can be accept ed for normalization of the energy scale with D D neutrons using 100 200 keV in cident deuteron energies Considering the possible sources of the energy spread the FWHM value must be in the 100 and 25 keV range between 0 and 100 degrees respectively From the pulse height spectra measured at different emission angles by a He proportional counter the mean energy and its spread can be derived 16 21 The absolute source strength of the H d n He and H d n He reactions can be measured accurately lt 0 5 by means of the associated particle method APM ie by the registration of the He and 3He particles from the D T and D D reac tions respectively by an alpha detector in a given solid angle The differential cross sections of the H d n He and H d p H reactions are the same at 90 up to a few 100 keV deuteron energies 17 Therefore the source strength can be determined by observing the recoil tritons or protons with a silicon surface barrier detector In Table 7 energies of the residual particles produced in the D T and D D reactions are summarized In addition to the 93Nb n 2n the 27 a n a activation detector is also a good fluence monitor for D T neutrons Fission chambers wit
171. nding on the dimensions and distance of the transfer system tubes Working pressure of the network must not be less than 6 bar A buffering air tank of about 1001 should be placed close to the pneumatic transfer system 221 9 10 11 12 13 14 15 16 17 18 19 222 A pipeline for cooling water in the NGR with a flow rate of about 100 l h is needed If there is no outlet to a canalized water network forced water circulation is necessary through a cooling system Another pipeline at nor mal pressure should be introduced into the measuring center The same pipe line can be used for the whole neutron generator laboratory NGL Neutron generators in general are operated and controlled by a central Generator Control Unit control desk which can be located at a distance of 10 15 m far from the machine The power requirement of the whole labotory is not more than 10 kVA including the operation of a compressor which needs about 4 kVA 220 380 V 3 phase For the generator 3 phase 5 kVA 220 380V 50 Hz are required Considering the further development of a final NGL as well as the energy consumption of the measuring and operating equipment it is advisable to design the supporting cables and transformers for about 25 kVA Every room in the NGL should be supplied with a one and three phase network system The cables from the control room to the NGR can be placed either along the wall or through channels
172. ng MEASUREMENT Cooling 2 START STOP START STOP I lt T A le 6 Tes TM TOC 1 I t I I lt t gt l sample transfer sample transfer a Fig 147 Timing diagram of a pneumatic sample transfer system for periodic irradiation and measurements Fig 148 Twin rectangular tube pneumatic transfer system for activation analysis by neutron generator 198 The time programmer should ensure the change of the irradiation cooling and measuring times in a wide range A typical cyclic timing diagram of a pneumatic sample transfer system is shown in Fig 147 The cycle starts with the activation time TA followed by the first cooling TC measuring TM and the second cooling TC times 113 The schematics of a pneumatic transfer system with twin rectangular tubes developed for the special requirements of activation analysis with neutron gene rators are shown in Fig 148 The two separate rectangular tubes transporting the sample and the standard sample are controlled by two electropneumatic valves 1 The slowing down or breaking of the samples at the irradiation or detector position is carried out by the valves 2 and 3 These valves ensure the soft arrival of the samples in the irradiation or measurement positions The microswitches 4 detect the ar rival of the samples in the correct positions The compressed air supplied by the compressor 5 is buffered by the buffer tank 6 The whole sys
173. ng ion source is shown Fig 36 Since the ion source requires a U 10 kV potential this high voltage is usual ly produced by a voltage doubling circuit 5 5 1 Troubleshooting of Penning ion sources The problems causing improper operation of this ion source can be divided into three groups magnetic mechanical electric The magnetic problems are caused by the decreased field strength of the per manent magnet so that the radius of the electron trajectory increases resulting in a lower discharge current I than during normal operation at the same Up volt age Similarly every shunting of the magnetic circuit will have similar ef fects Penning Primary ion source from variac Fig 36 A voltage doubling HV power supply of a Penning ion source 61 The mechanical problems may be caused by contaminated cathode surface due to the oil vapour in the vacuum system alteration of the original anode ring cathode geometry a change in the geometry or surface properties of the extraction slit The electrical problems can be caused by the HV power supply transformer rectifiers condensers limiting resistor Ri HV connectors on the HV supply or on the Penning ion source the HV feedthrough HV cable between the power supply and the ion source Testing the magnetic circuit needs some magnetic field measuring instrument but the field strength in the ion source must not be tested directly Some iron objects bolts screws
174. ng on the HV terminal at the potential of the acceleration voltage extraction voltage The optical fiber cables allow excel lent insulation between the HV terminal and the subterminal of the ion source The sensors and controllers are at different potentials in the HV area and the ion source area so they are connected by different fiber cables The intelligent CAMAC controller at the ground potential controls the ion source power supplies and the gas supply The measured parameters e g arc current arc voltage pressure in the ion source are transmitted back to the ground potential The software of the system has been designed to meet requirements like starting the generator operation controlling the operational parameters of the generator as well as operator aided control and safety control in breakdown situations 90 12V Fig 56 Optical receiver of analog signals at HV terminal with a switch on option 12V LM 331 39k 1N4148 Fig 57 Optical transmission circuit of signals from HV terminal to ground HV terminal to control desk A similar system using CAMAC data way is shown in Fig 54 58 The lower part of this diagram shows the microcomputer configuration Z 80 based The mea sured ion source parameters and the output control values are using the CAMAC data way in digital form The analog values measured at the HV potential are con nected to a multiplexer between the ADC to obtain more input The analog output of
175. ng surfaces of a rotor an impulse in a required flow direction The surface of the rotor usually disc shaped forms with the stationary surface of a stator intervening spaces in which the gas is transported to the backing port In the original Gaede molecular pump and its modifications the intervening spaces were very narrow which led to construction difficulties Fig 71 Schematic diagram of the turbomolecular pump 113 Using a turbine form of blading of the rotor the so called turbomolecular pump was developed as a technically viable pump A typical double flow pump can be seen in Fig 71 The rotor revolving at a very high speed carries the air by molecular impacts towards the backing pressure channel In the case of lower vacuum needs eg for neutron genera tors mainly rotary pumps are utilized as backing pumps The turbomolecular pumps have the advantage over oil diffusion as well as ion getter pumps in many respects they can be put into operation quickly in only a few minutes the vacuum system is much less polluted by oil vapours an unexpected exposure of the vacuum to the atmosphere does not damage the pump Important instructions on the use of turbomolecular pumps are as follows a It is worth while to insert a cooled trap between the turbomolecular pump and the rotary pump this improves the ultimate vacuum and protects the turbomolecular pump and the recipient from the oil vapours of the rotary pump b Be
176. nical staff leave the laboratories and when the new operators have less experience in accelerator technology This manual is intended to assist operators in troubleshooting and upgrading of neutron generators It is directed particularly to operators and technicians in less experienced laboratories and therefore the descriptions of the principles and techniques of these machines are operator oriented In addition to a discussion of the main characteristics of neutron generators detailed information is given on the function of particular commercial units on common problems related to specific components of accelerators and on methods of troubleshooting and repair Detailed schematic and circuit diagrams are provided to help operators in the development and improvement of the generators The problems treated in the Manual have been collected during several IAEA missions in developing countries The IAEA is grateful to 1 Sztaricskai who performed the major part of the drafting of the manuscript and also to J Csikai and S Szegedi for their contribution to the drafting The IAEA officer responsible for this publication was R L Walsh Physics Section Division of Physical and Chemical Sciences EDITORIAL NOTE In preparing this publication for press staff of the IAEA have made up the pages from the original manuscripts as submitted by the authors The views expressed do not necessarily reflect those of the governments of the nominating Member
177. nly for activation analysis neutron therapy or materials research may have a bulky water cooled target holder espe cially for a few mA target current The intense neutron generators having sever al kW target loads can manage this load only with water cooled rotating tar gets The thin wall low mass target holders with air jet cooling 1 are recom mended for clean neutron work around 2 and 14 MeV The vacuum system of a neutron generator consists of prepumps mechanical cryogenic and high vacuum diffusion ion getter turbomolecular pumps The ad vantages of the diffusion pumps are as follows low cost high pumping speed simple maintenance long lifetime no mobile components Their disadvantages are oil vapour contamination of the accelerator components and the target which can be decreased by using of FLOMBIN or SANTOVAC oils and liquid nitrogen traps see Fig 11 not only at the inlet of the diffusion pump but also along the beam lines The titanium getter pumps can assure clean vacuum and high pumping speed for hydrogen deuterium gas however the high absorption rate for tritium released from the target makes it difficult to handle the used pump elements because of their high activity even in the case of moderate commercial neutron generators The cleanest vacuum can be achieved by using turbomolecular pumps Their disad vantages are the relatively high cost and the possibility of mechanical damage In general Pirani and the
178. nstructed for fusion oriented re search and neutron therapy it became clear that these originally DC generators would be more suitable for fusion studies if a pulsing system helped the DDX and neutron transport measurements The Osaka OCTAVIAN generator and the JAERI FNS were equipped with nanosecond pulsing units as the first intense neutron genera tor the RTNS I at LLL There are some limitations of the recent generators in determination of the data required for fusion reactors low yield small irradiated volume large gra dient of the neutron fluence in the sample Therefore new machines are under 38 YIELD 10 n s o 30 0 7 TARGET LOAD kW Fig 15 Specific neutron yield vs specific target load for solid tritium targets 35 EQUILIBRIUM PRESSURE mbar 100 300 SOO 700 TEMPERATURE C Fig 16 The equilibrium pressure for some metal hydride systems vs temperature construction or improvement e g the conceptionaly new Osaka generator and the Los Alamos supersonic gas target system A 200 kV acceleration voltage and 50 mA current produce a load of 10 kW on the target which gives about 10 5 n s yield if the beam is selected for pt see Fig 15 34 Since the thermal stability of the tritium targets depends on the equilib rium pressure of the metal titride system at a given temperature the use of more 39 Table 12 Some characteristics of intense neutron generators 32 Name Yield Ae et Irarget Beam d
179. nt of non 14 MeV neutrons of about 10 22 3 DETERMINATION OF THE BEAM ENERGY According to eqs 3 and 4 the angular yield and energy of neutrons depend on the incident deuteron energy The absolute energy of an ion beam can be measured by using an electrostat ic a 180 magnetic or a velocity analyzer In addition to the absolute methods the energy calibration of particle accelerators is usually performed by a 90 magnetic analyzer which must be calibrated with nuclear reactions having accu rately known resonance and threshold energies A magnetic analyzer containing entrance and exit slits is transparent only for ions with charge Q and energy E E Kf Q m 5 where K is the analyzing magnet calibration factor f is the NMR frequency if the strength of the magnetic field is measured by a nuclear magnetic resonance probe usually a proton probe and E is the nonrelativistic kinetic energy 18 i e the analyzer is a tool for Q m separation and for measuring the energy E In addition to the absolute methods measurements of current through a bank of resistors is widely used because of its simplicity below a few hundred kV terminal voltage However at high voltage discharges on the surface of the resistors and their unstable values can cause an uncertainty of about 5 kV During the last decades new methods based on nuclear reactions have been developed for precise beam energy measurement For instance by using a Ge Li d
180. ode type sputter ion pump Titanium atom O Gas molecule 9 Positive ion Electron Neutral particle Fig 70 The triode type ion getter pump 112 titanium are left generally undisturbed leading to a higher net pumping speed for these gases Before the titanium ion getter pumps start it is necessary to reach the fore vacuum in the vacuum system of the accelerating tube by a rotary pump A cooled trap between the rotary pump and the sputter ion pump adsorbs the vapours and the humidity of the air As soon as the rotary pump reaches the ultimate vacuum the ion getter pump should be switched on for a short time The starting current of the Ti getter pump at the forevacuum pressure is usually high which leads easily to over heating of the ion pumps The switch on and off procedure should be repeated sev eral times as the loading current of the getter pump reaches the normal work ing current range During this time the isolation valve between the rotary pump and the getter pump should be closed and the rotary pump should be switched off exposing to the atmosphere its inlet through an air admission valve The normal work of the ion getter pump jn a leak free vacuum system will be indicat ed by a decreasing getter pump current 9 2 3 Vacuum system based on turbomolecular pump The principle of the molecular pump is based on the fact that the gas par ticles to be pumped receive through the impact with the rapidly movi
181. oltage while the controlled avalanche rectifier stacks withstand vol tages in the range of 100 150 kV Depending on the speed of the diodes they can operate at up to 20 30 kHz frequencies if the charge storage in the pn junction is low enough Testing these rectifiers diodes in the forward direction is sim ple the reverse voltage bias requires a reverse bias voltage equal to the opera tional voltage of the HV power supply itself or higher The forward characteris tics of these diodes can be measured by a slightly higher voltage 300 500 V power supply and the reverse current of the rectifiers can only be checked by a nuclear detector power supply c Testing forward characteristics The forward characteristics and the reverse bias current of the HV rectifiers can be tested by a circuit shown in Fig 122 170 5x1MO 2W DIODE STACK DETECTOR BIAS P S 7 kV METER 0 SkV FORWARD REVERSE n mA METER BIAS Fig 122 Circuit for HV rectifier testing For forward characteristics of a 100 150 kV crest working reverse voltage rectifier stack the forward voltage drop of the rectifier stack at the nominal say 50 mA forward current is about 100 180 V indicating an equal voltage drop at all of the rectifier tablets The rectifier stacks can be tested in their original circuit in such a way that the rectifier diode should be loaded only resistively and the shape of the rectified current should be observed by a
182. oltage and other power supplies neu tron and tritium monitors and shielding arrangements The operation maintenance and troubleshooting of a neutron generator require well trained technicians who can successfully undertake not only preventive main tenance of the machine but also its upgrading Troubleshooting and the locating of faults in components can be just as difficult as their prevention Thoughtless component exchange in an accelerator may cause additional problems the correct choice of component for a given func tion in the machine requires complete understanding of the operation of the gen erator as well as the role of the component in the machine In troubleshooting for neutron generators as with other sophisticated equipment it may be that the cause of malfunction stems from a single fault and that an investigation of the whole system is both time consuming and unnecessary However an electrical failure in the high voltage power supply or a discharge in the accelerating tube could be caused by trouble in the high vacuum system An experienced troubleshooter can avoid unnecessary investigation of a number of subunits This Manual has been prepared not only for operators but also for those who are dealing experimentally with the upgrading of neutron generator laboratories Principles of operation and output characteristics of neutron generators can be found in Refs 1 3 This Manual is restricted to practical information required b
183. omic to molecular ions depends on the operating time of the ion source the pressure in the discharge volume and the purity of the gas en tering the plasma A carefully designed pressure regulator must be applied to guarantee the stable flow rate and the purity of the gas during long operation time The three most commonly used deuterium leaks in neutron generators are pal ladium leak thermomechanical leak and the needle valve The palladium leak can be utilized for the regulation of hydrogen gas iso topes flow which discriminates against other gases The thermomechanical leak and the needle valve are suitable for the regulation of all types of gases As the neutron generator uses deuterium for feeding the ion sources all of these leaks may be found in the vicinity of the ion sources on the high voltage termi nal 6 1 THE PALLADIUM LEAK One of the best gas regulators is the palladium leak constructed of pal ladium silver alloy 2 Palladium has temperature dependent permeability to the passage of hydrogen isotopes and discriminates against contaminant gases between 300 and 400 C Characteristics of the H Pd system make it possible to supply the low intensity RF and PIG ion sources with the necessary amount of hydrogen or deuterium gases at a consumption of 10 to 20 cnr h NTP Permeation of hydrogen through a palladium tube of 2 mm diameter 100 mm length and 0 1 mm wall thick ness is about 5 cm h at 400 C which value is enhanced
184. on layer from the oil vapour is a very adhe sive layer on the surfaces of the glass and it can be removed only by mechanical procedure The aluminium extractor tip canal should be changed in every case The mechanical cleaning of its surface is simpler but the diameter of the hole must remain Cleaning the dirty glass parts of the HF ion source usually results in a shorter lifetime of the component Use the cleaned balloons and quartz sleeves only in case of emergency because after cleaning they will always give a lower beam current Before starting a long irradiation 20 hours always change the extractor tip quartz sleeve and Pyrex bottle 5 4 HIGH FREQUENCY OSCILLATORS The high frequency oscillators in the 15 100 MHz range are coupled to the discharge balloon inductively while at frequencies gt 100 MHz they are coupled capacitively The high frequency oscillators are fed by a ca 500 1500 V 0 5 A anode power supply The active component of the oscillator is an HF triode tet rode or pentode type transmitter electron valve The circuit diagram of a simple but reliable oscillator of about 100 W output power is shown in Fig 28 The cir cuit is a three point type oscillator and it oscillates at about 27 MHz frequen cy The coil L is the inductive coupling coil of the oscillator surrounding the Pyrex discharge balloon of the ion source The 5 nF capacitor and the coil L protect the anode power supply against the high frequency
185. on of an electrically suppressed beam current collector Faraday cup is shown in Fig 130 The grid placed in front of the beam collector cup is usually also replaced by diaphragms The external cylindrical mesh screens secondary electrons emitted from the the outer side of the Faraday cup due to X ray production by the ion beam 87 In a neutron generator the Faraday cup is the target holder 180 RELAY REFERENCE CAPACITOR PRECISION VOLTAGE SCALER SOURCE TRIGGER CIRCUIT AN FLIP D FLOP INPUT CAPACITOR Fig 131 The principle of the target current integrator 12V 2219 10 Hz 10kH2 Fig 132 Circuit diagram of a V f converter based current integrator RELAY DRIVER 181 14 4 1 Beam current integration In all of the experiments carried out with charged particle beams the mea surement of the total beam charge or the number of incident particles that reached the target during a given time is required This requires an integration of the beam current detected in the given time interval For the integration of the beam current the Faraday cup target is fed to a large high quality capaci tor The voltage across the capacitor increases with the collected charge and reaches some predetermined level At this voltage a fast acting flip flop will be triggered This trigger operates an electromechanical relay for counting the charge or in a more advanced form it is fed an accurately known quanti
186. on the surface of the insulator of the focus electrode from hydrocarbon oil contamination in the system may produce a bypass current and a change in the focus voltage As the focus power supply is usually protected against breakdowns due to discharges between the focus electrodes the voltage test of the focus voltage may give an indication on the surface contami nation in the focus electrode region If malfunction of the focus power supply is detected troubleshooting and repair should be done on the basis of the instruction manual in which the neces sary procedures will be described in detail in the section on high voltage power supplies 10 4 THE ACCELERATION TUBE The acceleration tube of neutron generators consists of annular electrodes sep arated by a hollow cylindrical insulator of glass or ceramic The insulators and the metal electrodes are bonded together either with epoxy resin or with polyvi nylalcohol PVA glue to form vacuum tight joints There are various simple de signs for accelerating tubes with homogeneous or inhomogeneous field of low volt age accelerators 74 77 The transport of the beam over long distances is sim ple if the currents do not exceed a few milliamperes The homogeneous field ac celeration tube ie the multigap tube consists of more acceleration gaps using diaphragms cylinders hollow cones etc as electrodes which are fed by an equiresistance voltage divider resistor chain The inhomogeneous field or
187. oncave convex lens system of the Einzel lens for a deuteron beam The Einzel lens is symmetrical the focal length on the object and the image side have the same f value The d focusing power and the f focal length for a unipotential lens is shown in Fig 88 Fig 87 The unipotential Einzel lens with its optical analog 129 Fig 88 The focusing power and focal length of the unipotential lens The focal lenses of the low energy accelerators neutron generators are usually immersion or unipotential lenses In a unipotential lens the diaphragm or cylinder close to the ion source serves as an extracting electrode The exit electrode of the unipotential lens is connected to the potential of the first electrode of the multielectrode or homogeneous field accelerator tube The fo cus voltage source is on the acceleration high voltage terminal see Fig 11 As the focus electrodes and the accelerator tube electrodes should be aligned the beam position at the exit of the acceleration tube is almost off axis The steer ing and positioning of the beam after acceleration is usually carried out by bi ased quadrupole lenses For RF PIG and DP ion sources the current can be sub stantially increased in a wide range by increasing the extracting voltage 72 ie the current is proportional to the energy of extracted ions For typical ex traction geometries the maximum voltage V is given by the relation 73 Ve5x 104a 2 where d is the ga
188. ong counter sizes in mm 211 LONG COUNTER Fig 160 Electronics for a neutron long counter counter detects the neutrons by the lbn Li reaction after thermalization the pulses from the gamma rays associated with the neutron field can be well separated The long counter is a direction sensitive detector its construction is shown in Fig 159 122 The setup of a long counters electronics is shown in Fig 160 As can be seen in Fig 160 setting the optimal discrimination level can be helped by a multichannel analyzer For monitoring and recording the neutron flux a multi channel analyzer may be used in the multiscaler mode 123 The mechanical construction of a long counter depends on the neutron modera tor material In the case of polyethylene or similar good machineable materials containing high concentrations of hydrogen the internal cylinder and the exter nal shield can be machined by lathe 19 2 FISSION CHAMBER MONITORING The fission chamber is a cheap and reliable piece of equipment for recording fast neutrons through the detection of fission fragments from the fission of 232Th or 238 Thorium dioxide or uranium tertafluoride layers 0 2 mg cm thick and 15 20 mm dia are used The chamber shown in Fig 161 was constructed from a thin wall aluminium cosmetic cream box The pressure of the counting gas methane or argon is just slightly over the atmospheric pressure 124 The pressure in the fission chamber can be regulated
189. or Cover the gas inlet port with a blanking plate and slowly open the valve about five turns This also evacuates the valve interior Now blow the outside of the valve with test gas If the valve is in good condition the leak tightness values must reach the technical data Similar halogen or helium leak testing procedures can be used in thermomechanical valves 6 3 CALIBRATION OF LEAK VALVES GAS CONSUMPTION MEASUREMENTS OF ION SOURCES In the case of abnormal operation of leak valves their transmission should be tested The gas consumption of the ion sources is usually measured in a practical unit of ml h NTP Normal Temperature and Pressure i e 20 C and 1 bar the gas consumption unit is ml bar hour which can be converted into the usual flow rate pumping speed unit of mbar s The gas consumption of the HF or Penning ion sources at the neutron generators is in the range of 1 to 10 ml h NTP while the duoplasmatrons and other high current ion sources consume 10 to 100 ml h NTP gas This means the leaks should assure the gas flow into the ion sources in the range of 1 to 100 ml hour NTP gas The U shaped manometer which is the usual equipment for determination of the pumping speed of the vacuum pumps can be used for the measurement of the gas consumption and the character istics of the gas adjusting valves For the determination of the gas consumption Vae um Rubber gas balloon system 77 H9 05 Fig 42 Setup for
190. or low pressure atmosphere Compactness small room for Safe operation in humidty TYPE SAMES D Simple construction Simple Electrostatic HV unit SAMES J Easy maintenance safety system SAMES T safety system Electrostatic HV unit SAMES JB Compactness Simple installation and position change SAMES TB installation Simple position change TMC A 111 Compactness easy achievement of horizontal or vertical beam Simple closed circuit cooling KAMAN A 1254 Compactness small room for TOSHIBA NT 200 KFKI NA 4 MULTIVOLT NA 150 installation Simple position change SF insulation FREON 113 cooling Long life D supply Compactness 200 kV acceleration voltage Compactness Long life D supply Small room for installation Compactness Availability of spare parts and components DISADVANTAGES Mixed beam Not easy maintenance and repair of HV terminal Horizontal use only Mixed beam Unreliable and too numerous insulating transformers Electrostatic HV unit Mixed ion beam The maximum HV can be achieved only in an environment that is not humid AC power at the HV terminal Mixed ion beam Not easy maintenance and repair Special tools Mixed ion beam Not easy dismantling repair and maintenance Special tools Mixed ion beam Not easy dismantling repair and maintenance Unreliable needle valve Mixed ion beam Unreliable needle valve Mixed ion beam Unavailable spare parts Mixe
191. oth cases indirect and di rect is resistive the heating can be increased by changing the heating voltage or by reduction of the resistance of the heater In the case of an indirectly heated Pd tube the heater spiral should be shortened cutting off 10 20 of length In direct heating the palladium tube itself is the load of the low voltage power supply transformer the Pd tube re sistance can be decreased by sliding the isolated F contact towards the lid of the D tank Shortening the original tube length by 20 50 is a successful solu tion for improvement of the maximum D transparency of the Pd leak At a given potential difference the energy is dissipated in the resistor at a rate DR o and so if R R 2 then I 2I ie the energy is doubled The glass ware palladium leaks may break If they do lower pressure vacu um or ion source side should be tested by usual vacuum testing if there are improper gray coloured gas discharges in the HF ion sources The vacuum testing of directly heated palladium leaks is carried out in a similar way In the case of an overpressure greater than 1 bar D in the tank the sealing of the tank should be adequate All of the seals at the connection of the manometer the tank closing valve and the heater electrode should be checked regularly 64 In the case of defective operation of the palladium leak if the operation of the ion source shows some problems related to the gas supply test the fol
192. over an active area of about 50 mm x 50 mm square The vertical and hori zontal wires are mounted through the square using conventional soft soldering technique directly onto the printed circuit board The wire holding the printed circuit board will be connected to the screening diaphragms and vacuum system flange holders by epoxy resin Without epoxy resin bonding the printed circuit boards can be vacuum sealed by Teflon PTFE gaskets The printed circuits are 178 Al Endcap Survey Markers Endcap Fig 128 Assembly drawing of a multiwire beam scanner 102 conventionally etched and connected by the usual ribbon cables to the individual amplifiers and to the multiplexer of the data processing unit The assembly draw ing of a multiwire beam scanner is shown in Fig 128 102 14 4 THE FARADAY CUP Most experiments with neutron generators and charged particle accelerators require knowledge of the beam intensity and the position at the target The sim plest method for detection of a charged particle beam is to intercept the beam with an insulated metal plate of sufficient thickness The ion current striking this plate is measured by an analog meter if the mean current is greater than 109 A which is the case for neutron generators Complications arise because the ion beam to be detected is accompanied by a diffuse secondary electron beam The utilization of beam line components like deflection magnets and quadrupoles will remove the elec
193. overload of the insulating rod fiber or variac and needle valve should be avoided This is why position limiting switches are used for the electromechani cally driven variacs potentiometers needle valves etc These switches protect the system against mechanical overdrive and in the case of the lower or upper mechanical limit allows the opposite direction drive only The limit switches are usually microswitches with NO Normal Open and NC Normal Closed contacts The principle of such a mechanical overload protection is shown in Fig 50 LIMIT SWITCHES INSULATING ROD VARIAC m Bs zi ETC o NO NO D j j oaa UT HV i TERMINAL NO DOWN MOTOR zn o NO Fig 50 Principle of mechanical protection of the electromechanical drives This mechanical protection utilizes a twin DC power supply for the DC motor gear driving insulating rods or fibers The motor can be switched to the for ward or backward direction by the UP DOWN switch This switch is a key with nor mal open position For forward motion the motor will be powered by positive voltage and for backward direction it wil be connected to the negative voltage The two limit switches break the positive or negative supply when the position of the rod fiber reaches the upper or lower limit These switches ensure the re turn of the gear in the opposite direction after the positive or negative supply has been switched off This type of electromechanical remote control fro
194. owards the anode and they ionize air molecules along their way Positive ions produced in this process are collected by the grid having negative potential and therefore the grid current intensity in the circuit will be proportional to the gas pressure in the gauge 116 The normal operation range is in the 10 108 mbar range in general At higher pressures the incandescent filament may burn out on the other hand the extension of measuring ranges towards smaller pressures would be limited by sec ondary electrons released from the collector by X rays generated by the electron bombardment of the anode The electronics connected to the gauge usually has builtin filament protection which interlocks the switch on of the gauge when ever the pressure is higher than 10 mbar c EMI 2kV Fig 74 Operation principle of cold cathode ionization vacuum gauge b Cold cathode or Penning ionization gauge The construction of this gauge can be seen in Fig 74 The operation princi ple is very similar to the ion sources with the same name Electrons emitted by cold emission from the two flat cathodes as a result of having some kilovolts on the ring anode ionize the gas molecules in the gauge head The positive ions pro duced in this way run up towards the negative electrode and therefore they change proportionally to the gas pressure the electric current in the anode or cath ode circuit The magnetic field B generated by a permanent magne
195. p IRK exp CBNM eval Pavlik 1989 vt 907 n 2n zr A DA 8 mb mb For 7 For 52er 2 LI SEn Z aE SIE ag aE ay 13264 5 dy 2210 0 a 1668 16 aq 176 32 a 48 543 5 52er n 2n dcr p l 13 5 14 14 5 15 Ent MeV Fig 4 Parameters of energy standard reactions 12 3 0 Eg 7 200 keV ui eo SE keV w e 2o ett Pitt ee ee NEUTRON ENERGY MeV N n RETES 2 0 10 o 30 60 9 120 150 180 EMISSION ANGLE 0 Fig 5 Neutron energy vs emission angle for D D reaction at E riz 200 keV 3 the energy spread of ions 4 the slowing down and scattering of projectiles in the target Therefore a value of 14 00 0 07 MeV at 96 98 is recommended as a reference for absolute normalization of the energy scale with D T neutrons Shapes and parameters of the o E curves for Zr n2n Zr and Cr n2ny Cr energy standards given in Ref 12 are shown in Fig 4 A value of 460 S mb is recommended for the cross section of the Nb n2n Nb fluence monitor around 14 MeV The error of this method is determined by the shapes of the Zr n 2n or the Cr n 2n cross section curves and the source sample geometry but it does not exceed 50 keV while the sensitivity between 13 and 15 MeV is on average 50 MeV and 64 MeV for Zr and Cr respectively The activity ratio is measured with an accuracy of better than 1 and so the error in the
196. p spacing in centimeters 10 3 TROUBLESHOOTING OF ELECTROSTATIC FOCUS LENSES If the properties of the electrostatic focus lens are unsatisfactory the following should be tested Vacuum in the system Focus voltage on the focus electrode The correct operation of the ion source and the extracting system 130 Bad vacuum oil vapour high pressure in the focusing space can easily lead to surface contamination of the focus lens electrodes In the case of high pressure a glow discharge can be built up between the focus electrodes and can contaminate the surface of the electrodes In the case of oil vapour the surface layer may insulate the electrode silicon oil and the distribution of the po tential between the lens electrodes will be changed This can lead easily to the deterioration of the focusing properties of the lens Observation of the focus electrodes after a long term operation of the vacuum system of the accelerator is important In the case of blackish brown electrodes clean them with organic solvents and polish their surfaces After dismantling the focus lens the pol ished surfaces should be washed if possible in organic solvent and an ultrasonic bath and carefully reassembled in the vacuum system Test the high vacuum feed through of the focus electrode If the surface is contaminated clean the feed through as well Test the conductivity and the isolation of the assembled system A thick conducting carbon layer
197. parator will start the second red LED flashing and the continuous alarm signal starts to be pulsed This indicates that the absolute maximum permissible voltage has been reached 143 As the magnet coil warms up its resistance increases normally the current regulator circuit should be set below the upper voltage drop limit 27 V The voltage drop along the current regulator circuit is displayed by the 100 A ana log meter as shown in Fig 95 A deflection angle as small as 30 is usually enough to separate the ions and the uncharged components of the beam A simple 30 analyzing magnet is shown in Fig 97 The deflection coil is divided into two parts to allow a more effec tive cooling of the coil The same solution applies in the case of the previous deflection magnet however it is more difficult to wind a semicircular coil than a rectangular coil The coil is about 3000 windings if the power supply of Fig 94 is utilized Both magnets may use other coils with thicker wires and lower resistance but they suffer from the disadvantage that they would need larger buffer condenser banks and higher electronic current regulation The power supplies of the magnets if they are regulated can be calibrat ed for beam energy utilizing the monoatomic ions of the neutron generator tar get beam line In a properly regulated magnet power supply the change in beam energy due to the change of accelerating voltage or extraction voltage may be detec
198. pose this relay stops or inhibits switching on the heating of the diffusion pump if the gas intake exceeds the upper limit set in the controller If a neutron generator with its vacuum system is operated for a long time it is advisable to control continuously the temperature of the diffusion pump by a thermocouple or thermal switch Pirani lonization vent 4 gauge gauge vent GUL valve EX Pirani valve EX 3 gauge Isolation valve 2 Target assembly d A a Acceleration Isolation wen Cold trap 0 Buffer Fore line Diffusion vessel trap Pirani pump es HST i Backing poni Boos X valve 2 valve 1 HT Vent X Z Loan A valve 2 Backing FP O valve 3 O valve 1 Rotary Rotary pump pump Fig 67 General vacuum system for a neutron generator 109 During operation of the vacuum system of a neutron generator consisting of a rotary as well as diffusion pumps see Fig 67 the following important instruc tions should be followed A Switching on the vacuum system a Switch on first the rotary pump backing valve 3 is opened and the vent valve 1 is closed automatically Let the rotary pump run for 5 10 min utes with gas ballast valve open During this time the pump house warms up to operation temperature the vapours do not condense later and in addition it will be possible to release the gases and condensed vapours from the foreline trap After this cleaning the
199. power supply while the biasing of the upper or lower pole pairs is done by the symmetry potentiometer controlling the bypass of the upper or lower pairs of coils The strong focusing property of quadrupole lenses makes them particularly suitable for use along the beam line in neutron generators The commercial neu tron generators which are mainly manufactured for fast neutron irradiation and activation analysis are not usually equipped with this strong focusing compo nent As the magnetic quadrupoles are simple they are therefore recommended in addition to the beam deflecting or analyzing system for upgrading commercial neu tron generators 12 2 THE BIASED MAGNETIC QUADRUPOLE DOUBLET The quadrupole lens described above has been designed and constructed for focusing the D beam of 150 keV 200 keV energy used in small neutron genera tors The pole face aperture of 82 mm diameter fits the beam tubes of almost all commercial neutron generators The properties of this lens allow the D beams to be focused down to 40 50 cm for 150 keV beam energy The 850 winding of the coils were designed for a 50 cm focal length Changing the standard coils of the quad rupole lens pair will ensure a different focal length Increasing the current 150 220 mm 177 tron generators Fig 107 Mechanical diagram of a quadrupole magnet for neu flow through the coil will shorten the focal length The E focal length of this quadrupole lens c
200. pq ee po 50 where Z P is calculated by the data in Table 21 of the i th compound cm g and p is the effective density of the i th element g cm Neutrons scattered by air can contribute significantly to the total dose If a barrier shield were used air scattering above the source skyshine would contribute more to the dose than to the transmitted radiation Thus there is no point in attempting to shield the direct radiation unless skyshine is also re duced by the provision of a shielding roof The presence of ducts voids pas sageways and safety doors all require careful consideration in the design of the final facility 1 Table 22 Expected levels of induced radioactivity at 10 cm after 1 h of operation with a neutron generator yield of 2 5 x 10 n s Exposure rate at 10 cm Reaction mR AC h kg h Half life 27 Aln p Mg 200 52 9 5 min 7 Ana Na 30 73 14 9 h 63 Cu n 2n f Cu 60 15 9 8 min 65Cu n 2n ffCu 60 15 12 8 h 217 Gamma radiation produced by the scattered and captured neutrons in the shield can contribute significantly to the radiation hazard Calculation and mea surements show that for thick 2 1 cm concrete shields the gamma dose rate is about half that due to epicadmium neutrons In water the gamma dose exceeds the fast neutron dose if the thickness is greater than 75 cm The prompt gamma emis sion of 4 to 7 6 MeV from the 5S6 Fe ny Fe reaction is a problem if the shield ing contains
201. pushing the electrolyte out With a gradual decrease of the electrolyte level in the float chamber the force P by which the float presses the seal to the nozzle will also decrease The force P can be described by P 4p sh G d p py 21 where D diameter of the float s specific density of the electrolyte h height of the electrolyte level measured from the bottom of the float G weight of the float d diameter of the inlet of the nozzle pj deuterium gas pressure above the electrolyte p deuterium pressure at the inlet of the nozzle 81 The size and mass of the float as well as the diameter of the nozzle inlet should be chosen so that the following relations for each term of eq 21 are fulfilled 2 7 D H gt G and G gt gt d pypy 22 where H is the height of the float If the construction of the regulating electrolyzer obeys these conditions the force P at a certain electrolyte level h h a decreases so much that a leak age appears between the rubber seal and the nozzle edge and deuterium flows from the float chamber into the vacuum system of the ion source The leakage grows with further decrease of the electrolyte level until the gas quantity q that flows through the leakage into the ion source equals the quantity Q originated by the electrolysis i e until q Q 23 becomes valid If the electric current in the electrolyzer circuit is changed the quantity of the gas achieved by ele
202. r before starting any leak detection it is worth checking the whole system again Most problems are caused by rubber gaskets it may happen that the gaskets are not fitted properly to the metal surfaces in the joints or the gas kets might be missing by mistake It is advisable to check the valves and the air admittance valves A particularly careful test should be carried out on any part of the neutron generator where some alteration has been made recently e g target exchange exchange of glass balloon in the ion source placing or replac ing a new vacuum gauge etc If the check seems to be unsuccessful some method of leak detection should be carried out Some methods which can be easily accomplished in a neutron generator labo ratory are described below 123 The simplest solution is the use of a vacuum gauge which is a normal acces sory in any vacuum system The use of Pirani gauges for leak detection is based on differences between thermal conductivities of different gases e g the con ductivity of hydrogen is much higher than that of air The Pirani gauge uses the thermal conductivity in the pressure measurement of gas therefore the leak de tection should be carried out as follows Using a cylinder of hydrogen with the corresponding pressure regulator a narrow jet of H should be blown onto the wall of the vacuum vessel where the leaks are being looked for Test carefully the gaskets and the joints The hydrogen creeps
203. r from the left side of the U shaped glass tube will flow into the vessel under evacuation therefore the liquid level will rise on this side Using a stopwatch the time interval can be measured during which the liquid lev el rises from the lower to the upper mark in the U shaped glass tube The volume of air v having the external atmospheric pressure between the marks can also be determined From the expression p ext 7 PV it follows that the volume of air that flowed into the vessel p v V a 30 120 and therefore the pumping speed Pext V Typical pumping speed versus pressure functions are shown in Fig 77 and Fig 78 for a rotary and an oil diffusion pump respectively It is characteristic for both cases that the real pumping speed becomes zero at the ultimate pressure ie when the pumping causes a dynamic equilibrium then the pump removes just as much air from the system as the air intake due to the ineffective sealing The pumping speed is given for both types of pump at the constant section of the pumping speed function in l s for a diffusion pump and in m h for a rotary pump in general These pumping speed values given by the manufacturers are measured at the inlet of the pumps but the realistic pumping speed far from the inlet is below this catalog value because the resistance of different elements of the vacuum system decreases the pumping speed This can be seen in Fig 79 where changes in the pumping spe
204. r grid to the collec tor of the BU 208A transistor and ground the collector of the transistor The op tical coupling between the ground and the HV terminal may be light cable or screened Perspex rod The circuit diagram of the oscillator and the pulsing unit s receiver is shown in Fig 30 The transmitter of the light pulser may be a commercial pulse generator fed infrared LED 5 4 1 Troubleshooting of high frequency oscillators The discharge tube ion source balloon is made of glass so the colour of the HF discharge gives information on the vacuum condition of the system the condition of the gas supply the power of the HF oscillator The HF oscillator should be tested if the discharge in the ion source bottle is poor or totally absent but the vacuum system and the gas supply operate cor rectly The operation of the oscillator can be detected by a neon lamp or a tube light If a small neon lamp fixed on a 20 30 cm long Perspex or any other iso lator rod is placed near the oscillator close to the anode or to the coil the lamp should light when the HF oscillator operates see Fig 31 A tube light 18 40 W of power already indicates the output power of the oscillator placed in the vicinity of the HF coil Fig 32 The tube light will light up if the oscillator works The intensity of the tube light depends on the output power of the oscil lator Neon lamp Perspex rod 20 25cm Fig 31 The neon lamp indicator of HF
205. r part of the vacuum manifold can be evacuated by the second 110 rotary pump in order to reach the required forevacuum value and finally it can be connected by opening the isolation valve 1 or the isolation valve 2 to the high vacuum manifold 9 2 2 Vacuum system based on Ti ion getter pump The sputter ion pump is a getter ion pump in which ionized gas is accelerat ed towards a getter surface continuously renewed by cathodic sputtering The basic sputter ion pump consists of two flat titanium cathodes a cylindrical an ode and an axial magnetic field as shown in Fig 68 A typical Ti getter pump consists of two flat rectangular titanium cathodes with a stainless steel anode between them consisting of a large number of open ended boxes see Fig 69 The pump mounted inside a narrow stainless steel box and attached to the vacuum system is surrounded by permanent magnets The anode is operated at a po tential of some kV whereas the cathodes are at ground potential The cold dis charge is initiated by stray electrons produced by cold field emission These el ectrons execute a helical motion around the magnetic field lines and they oscil late to and fro in the axial direction between the cathodes as in the case of PIG ion sources or Penning vacuum gauges The positive gas ions formed by ioniza tion bombard the titanium cathode sputtering titanium to form getter films on the anode and the opposite cathode The titanium reacts with all
206. ratio of atomic to 96 98 emission angles The distribu molecular ions Table 4 Values of parameters in equation 4 for the calculation of thin target neutron energy vs emission angle in laboratory system Ea D T D D keV E Ei E E E E E4 50 14 04814 0 47679 0 00834 2 46073 0 24848 0 01282 0 00031 100 14 06732 0 67488 0 01719 2 47303 0 35237 0 02524 0 00062 200 14 10711 0 95596 0 03320 2 49771 0 50072 0 05044 0 00242 300 14 14704 1 17282 0 04923 2 52289 0 61581 0 07530 0 00589 400 14 18670 1 35640 0 06527 2 54798 0 71456 0 10013 0 00757 500 14 22569 1 51899 0 08249 2 57246 0 80285 0 12592 0 01024 Table 5 Values of parameters in equation 4 for the calculation of thick target neutron energy vs emission angle in laboratory system Ea D T D D keV E Ei E E E E E4 50 14 06520 0 42329 0 00682 100 14 07883 0 57613 0 01222 2 46674 0 30083 0 01368 150 14 08942 0 66776 0 01600 200 14 09680 0 72427 0 01908 2 47685 0 39111 0 04098 0 02749 250 14 10286 0 76661 0 02167 300 14 10803 0 80001 0 02374 2 49712 0 47697 0 05124 0 02957 325 14 10723 0 79477 0 02347 400 2 50981 0 55825 0 07125 0 02474 500 2 52140 0 62147 0 09816 0 03307 Ey 190210 keV TiT target RELATIVE YIELD OF NEUTRONS 13 2 13 4 13 6 13 8 14 0 14 2 15 4 14 6 14 8 15 0 15 2 NEUTRON ENERGY MeV Fig 3 Calculated neutron energy distribution profiles for D T reaction 9 ex
207. rator or their equivalents should be procured The usual stock of active and passive components are recommended to complete the HV and HF components and insulating materials such as epoxy resin As laboratories especially in the developing world do not usually have staff skilled in vacuum technology and since vacuum materials and components are difficult to obtain the person in charge of the neutron generator should be careful to maintain the supply of frequently needed components and materials Spare oil for the diffusion and mechanical pumps silicon high vacuum grease sol vents the most important O rings rubber sheets spare Pirani and other vacuum gauges should be kept in store The store room attached to the mechanical and electronic workshop should be clean and equipped with shelves and cabinets As it is usuall locked the store room is the best place to store radioactive materials targets and radioac tive litter in locked bins and cabinets The operator of the neutron generator 223 usually a technician is a skilled worker so he may use both the workshop and the store The keys of the radioactive material storage cabinets and venti lated glove box in this store room must not be available to the general personnel in the workshop but the neutron generator operator should be able to get into the workshops at any time especially during the long irradiations at night to perform maintenance and repairs A glove box for t
208. rcuitry is it OK Check tube to power supply connections Check for replen isher and vacuum Check related circuitry Check for power hnes gauge circuit Are interlocks cooling system OK Check the elec Check the elec Check loads High load and tnc components tric components and related low voltage circuitry i discharge in the tube Tube overloads Check the interlocks and cooling circuits i 3 P gt 99 zm e DEAD SEALED TUBE CHANGE FOR A NEW ONE rc ANNEX D TROUBLESBOOTING FLOW CHART FOR NEUTRON GENERATOR VACUUM SYSTEM START Is the pump switch in ON position YES Check the forepump and related system OK YES Check the high vacuum pump and related system YES Check the vacuum meters Are they OK YES Check the parts of the system arethey OK YES Are the seals and the OK joints YES FINISH GOOD SYSTEM NO Switch it on NO _ ss i Troubleshooting and repair NO Troubleshooting exchange or repair NO Troubleshooting and exchange NO _ Leak testing and repair NO Leak testing and exchange 245 CONTRIBUTORS TO DRAFTING AND REVIEW Csikai J Darsono Dolnicar J Li Gwang Nyong Molla N I Raics P P Sanchez A A Szegedi S Sztaricskai T Walsh R L Scientific Secretary Institute of Exper
209. red also in the case of Fe U and Ti cathodes Metals suitable 59 Ferrous cathode Annutar o permanent Nonmagnetic magnet stainless steel ASS QAAAASSS Extracting electrode Vacuum gauge Gas intet i i Cr on me m Magnets Magnetic circuit Pyrex Cathodes tube it Anode Exit canal y NA NA pi A ae WE RY to Yj oe a EN Rubber seal Base CUOCO xtraction Fig 35 Typical Penning ion source schematic left and actual scheme right for use as cathode materials with high Ui values are as follows 44 Ni Zn AI Cu C W Mo and Ta for which the ignition voltage is between 3600 and 1700 V In most cases Ta cathodes are applied Ui 1700 V however Mg Al and alu minium alloys have also been used as cold cathode material because of their high sputtering thresholds 46 A cold cathode PIG ion source with axial extraction is shown in Fig 35 A magnetic field of about 0 03 to 0 7 W m supplied either by a permanent magnet or by a solenoid is applied between the electrodes The PIG sources are operat ing with gas pressures in the discharge volume of 0 1 to 2 5 Pa at gas consump tion of 20 to 500 cm h NTP A typical PIG source can operate continuously over 200 h at about 2 mA beam current without the need for replacing the cathode or anode A typical HV power supply feeding a Penni
210. ric field on the sharp edge of the ionizers The inductor electrodes are placed behind a slightly conductive ca 1012 2 cm special glass cylinder The excitation inductors lay the electric charges onto the surface of the rotor whilst the extracting ting inductor withdraws them The charge collecting ionizer electrode and the inductor pair on the opposite side are called a pole of the machine The diagram cross section in Fig 109 shows a single pole machine and Fig 110 shows a two pole machine The maximum number of poles is usually 8 The inductor and the conductive glass cylinder are together called the stator The stator the rotor and the ionizer electrodes are closed hermeticallly in a tank under pressure of compressed hydrogen The U aig excitation voltage in Fig 110 produces a potential difference be tween the excitation inductors and the charging electrodes inductors sufficient to get an electric field on the edge of the charging ionizer high enough to cre ate the local ionization on the surface of the rotor The insulating rotor trans fers the electric charge towards the collecting electrodes which collect the charges The C capacitance between the discharging ionizer and the inductor is several times less than the capacitance between the charging ionizer electrode and its inductor so the U out Output voltage will be several times higher than the Usxc charging potential The Usut potential of the collecting ionizer at any
211. rison of sealed tube neutron generators Life time Acc voltage Beam current Yield kV mA n s 200 h 200 kV 5 mA 3x1010 2x10 pulses 5x10 100 h 5x107 1000 h 125 kV 0 1 mA gt 108 400 h 200 kV 8 mA gt 10 2500 h 125 kV 0 1 mA 2x108 200 h 200 kV 20 mA gt 10 400 h 200 kV 500 mA 5x10 gt 1000 h n lon source 225 kV 2 mA gt 10 and spherical collimator A cylindrical can be introduced into the center of the target along its axis by an insulating polyethylene tube to achieve a homoge neous sample irradiation especially if the sample is rotated during irradiation in the hollow target The conical targets are used almost only for neutron thera py The target is produced from scandium titride The tube can give gt 10 n s yield during its guaranteed life time of 400 hours 37 The long life time criterion of the sealed tube neutron generators seems to be fullfilled by the new generation of SODERN France neutron tubes Some para meters of these neutron generators are summarized in Table 11 The combination of the sealed tube accelerator with the associated particle method APM 28 Fig 14 for field analysis of bulk mineral samples verifica tion of chemical and nuclear weapons 29 30 and three dimensional elemental analysis in solids 28 has great importance A schematic diagram of such a com bined generator is shown in Fig 14 4 3 INTENSE NEUTRON GENERATORS Recently 14 MeV neutron generators wit
212. ritium target storage may be constructed by experienced local personnel 21 3 LABORATORY LOG BOOK The operation of neutron generators like all complicated equipment requires regular maintenance duties and regular exchange of the short lived components The operator of the neutron generator should record the working parameters in a log book This log book should show the time of every operation and record the exchange of short lived components e g target ion source balloon quartz sleeve The neutron generator log book is a good basis for planning maintenance and repair as well as operation 224 A typical laboratory log sheet Date Supported by Project Project leader Operator Beam branch Direct Deflected Target None Deuterium Tritium Others Dose during the operation Vacuum Gauge No 1 Gauge No 2 Gauge No3 10 mbar 10 mbar Starting time hours minutes Ion source gas High frequency Magnet current Extraction Focus Accelerating high voltage Deflecting magnet Quadrupole lenses Target current Target and target holder activation relative unit V mA relative unit or A V mA relative unit V mA relative unit kV mA A relative unit A A A A or relative units mA direct mA deflected relative unit Beam current on isolated slit mA Monitor counter Remarks counts 225 REFERENCES 1 J Csikai Handbook of Fast Neutron Generators CRC Press In
213. rmocouple vacuum gauges are used for measuring the forevacuum The high vacuum is measured mostly with Dushman type or Penning type gauges The use of the vacuum controllers makes possible the automation of the neutron generator control The rubber or Viton O rings are changed these days for metal gaskets the contemporary neutron generators use bakeable vacuum compo nents as well With neutron generators used for research purposes control is mostly manual but the principle of minimal interlock and other control inputs is also followed with commercial generators The multipurpose and intense neutron gener ators use microcomputer and computer control systems The choice of neutron gen erator depends on its construction and purpose This Manual outlines the commercial medium intense high current pulsed and sealed tube neutron generators dealing with their operation technical solu tions maintenance and repair as well as with updating with a view to extending their utilization in science and technology 4 4 COMMERCIAL NEUTRON GENERATORS The commercial neutron generators are modest machines with production yields of about 10 n s and 10 n s for D T and D D reactions respectively They are 28 utilized in basic nuclear research education and technology for measurement of nuclear data and in laboratory exercises to study the different interactions of neutrons detection of charged particles and neutrons They are also used in ac
214. rol circuit with optical input is shown in Fig 52 based on the AID SAMES T type neutron generator 55 This circuit has a pulse gener ator unit at the ground potential which drives the infrared light emitting diode This light source transmits the light pulses to the optical receiver photo tran sistor by an electrically insulating optical fiber cable The optoreceiver is on the HV terminal The output pulses of the optoreceiver are integrated and the integrator output drives the DC input of the triac or thyristor controller chip The pulse generator works at constant repetition rate constant pulse fre quency The duty cycle of the pulses will be transformed into voltage at the integrator Figure 52 shows this universal triac controller circuit utilized by SAMES The transmitter can be a TLA94 pulse width modulation circuit or a NES55 timer circuit in astable multivibrator mode 56 From a practical point of view it is very important to use transient sup pressors at the small transformer giving a phase control signal and parallel to the triac The operational amplifiers may be any of the usual types The thyris tor controller circuit may be one of the commercial type IC proposed by the semi conductor manufacturers The optical fiber can be substituted by Perspex rods the optotransmitter and receiver are recommended to be a matched pair like op tical gate 8 5 COMPUTER CONTROL Increasing application of microcomputer techniques
215. rosive gases and atmospheric moisture may damage the valve A typical leak rate torr l s or gas consumption bar ml h is shown in Fig 39 as a function of the heating voltage of the outer wall of a thermomechanical valve 6 2 2 Maintenance and troubleshooting of thermomechanical leaks The thermomechanical leak valves should be connected to the vacuum system ion source in a correct way relative to the direction of the gas flow The di 66 rection of the gas flow is usually indicated on the valve Flush the thermome chanical leak valve with dry nitrogen or other dry gas in the case of uncertain operation Check the electric contacts of the leak valve As the heating current of the cylinder is usually higher than 10 A the electrical connections should be checked and cleaned regularly As accurate AC current measurement in the range of several tens of amperes is not simple it can be done by a digital clampmeter for AC heating current calibrate the heating voltage along the cylinder while it is working properly The voltmeter contacts should be separately connected to the cylinder in case of a bad contact between the filament transformer and the leak body this voltmeter will indicate the condition of the contacts Opening the welded body may kill the whole leak valve In case of lack of heating test the following The primary side of the stepdown transformer voltage and current The secondary voltage across the leak valve it should b
216. rtain branches are recommended see Table 15B For convenience a conversion table is given in Table 16 9 2 VACUUM PUMPS When selecting the most appropriate vacuum system for a neutron generator the following important factors should to be taken into account a The value of the ultimate pressure to be attained for normal operation The aim of all vacuum systems for a neutron generator as for a charged par ticle accelerator is that the accelerated deuteron ions should reach the tritium target without collision To do this it is necessary to keep the pressure in the accelerating tube so low that the mean free path of air molecules should exceed the length of the accelerating tube Data at different pressures as can be seen in Table 17 show that the above requirements for the mean free path values for neutron generators at pres sures of 105 106 mbar are in the range of some meters These ultimate pressure values in Table 17 have to be taken into account at least when selecting the most appropriate type of vacuum pump 63 Table 17 Mean free path vs pressure Pressure mbar Mean free path cm atmospheric 6x19 1 5x103 10 5x100 106 5x102 10 5x10 b Gas intake due to lakage the ion source etc to be accounted for during the run of the accelerator The pumping speed is an important consideration Its required value should be determined first by the regularly occurring gas intakes Note that the pressure i
217. s 1 Vacuum problems As the rotating beam scanners require rotating feedthroughs the usual seal testing should be carried out Similarly if the stator and the rotor of a synchronous motor are in the atmosphere or in vacuum respectively the problems observed are sometimes at the electric feedthrough of the scanning wires 2 Mechanical problems As this scanner requires a smoothly rotating wire every mechanical problem stain dirt in the bearings may make the operation of the scanner unnstable These mechanical instabilities can be observed on the screen of the oscilloscope VI 177 The length of the pulse and the period of time will appear jerky on the screen If there is no pulse the wire may have stopped If the wire intercepts the beam some DC current can be observed by the oscilloscope Dismantling and cleaning the mechanical part of the scanners should be done carefully 3 Electrical problems The isolation of the wire output and the conductivity between the scanning wire and the off vacuum connector should be tested periodi cally The ceramic insulation of the wires is usually covered by a sputtered met al layer they should be cleaned both mechanically and chemically The usual synchronous motors rotating the wire scanners should stopped off beam to avoid sputtering due to the beam This is assured by an electric contact fitted to the axis of the synchronous motor end position switch If this con tact does not wor
218. s which cannot be detected by common vacuum gauges will occur due to mechanical imperfections e g imperfect welding This could occur e g when a new target holder or a new cold trap is assembled onto the vacuum system It is advisable to test them with the help of a sepa rate simple vacuum system before installation Such a vacuum system equipped with a halogen leak detector is shown in Fig 81 The measuring gauge of the halogen leak detector contains an indirectly heat ed platinum probe like a cathode emitting alkali ions and their current is measured by an ammeter or is indicated by audio signal The ion emission in creases considerably whenever halogen gas gets into the measuring gauge This phenomenon can be used for leak detection in any vacuum system The test gases containing halogen are usually fluorocarbons which may be easily purchased on the market FREON 12 is the usual gas used in cooling machines refrigerators and air conditioners it has a boiling point of 30 C while FREON 112 is a liquid with a boiling point of 939C Both can be utilized for halogen leak detection The FREON liquid or gas gets easily into the vacuum chamber through the leaks in the wall and improves sealing of the vacuum vessel from the outside and therefore an in creased ion current will be detected by the electronics In order to achieve the highest possible sensitivity in the leak detection process it is advisable to place the halogen gauge
219. s The feedback trimmers are air insulated disk type the grid to cathode resistors are 50 W wire wound The current consumption of this oscil lator is about 200 400 mA at the maximum anode voltage The advantage of this oscillator is the easy and distortion free output power which can be changed 53 Fig 29 Push pull oscillator for HF ion sourceswith 200 W output power L 1 40 mm dia 5 5 windings 3 0 mm dia CuAg Lj 24 mm dia 4 x 20 windings 0 6 mm dia Cu L 37 20 mm dia 40 windings 0 7 mm dia Cu Vi V5 GS 90B or GU 6B or TL2 300 X Ua 1100V max Fig 30 Capacitively coupled HF oscillator with optical pulser L m 100 mm dia 2 windings 4 mm dia CuAg L7 8 mm dia 50 windings 0 8 mm dia Cu L3 amp mm dia 11 windings 1 0 mm dia Cu V QQE 06 40 54 by the anode voltage The diameter of the coil L4 fits that of the ion source balloon used This oscillator is utilized at the KFKI neutron generator A capacitively coupled oscillator of about 200 MHz frequency is utilized in the SAMES neutron generators This push pull oscillator is constructed on the basis of a double tetrode The output power is coupled to the ion source bottle by two metal rings The use of the double tetrode allows an easy pulsing possi bility by grounding the suppressor grid through a high voltage high frequen cy switching transistor which is driven by an optical cable For the DC mode neutron generator disconnect the wire between the suppresso
220. s is especially important if the machine is used as a charged particle accelerator for the determination of atomic and nuclear data with a high accuracy as well as for the improvement of various technological applications e g ion beam imaging ion microtomography particle induced X ray emission Rutherford backscattering ion implantation prompt radiation analysis Therefore improvement of the neutron generators with a beam energy analyzer and a beam profile detector is strongly recommended 25 4 TYPES OF NEUTRON GENERATORS The principle of the neutron generator is shown in Fig 10 This schematic applies for all particle accelerator A schematic drawing of a home made gener ator working in Debrecen is shown in Fig 11 From this figure it is easy to fol low the functions of each unit and its position The ion source the extraction the focusing and the gas supply units placed on the HV terminal need electric power cooling and insulated remote control systems The ion source is RF or Penning type for standard commercial or medium size neutron generators while it is a duoplasmatron duopigatron if high currents are required The deuterium gas flow is regulated into the ion source by needle thermomechanical or palladium leaks As the ion beam analyzer requires relatively high power the separation of the D beam is made usually after acceleration The pulsing of the ion source allows the production of a pulsed beam i e 14
221. sian coordinate system The rotating wire rotates along the y axis with radius R The thickness of the scanning wire is much less than the radius r of the beam Similarly the radius r of the beam is much less than the radius R of the rotating wire If the wire rotates with an angular velocity w the shape of the i t ion current in time will be described by i t 2 8 costwt 1 1 44 which is based on the usual geometric formula As the wire intercepts the beam twice during a single cycle as in Fig 127 the oscilloscope connected to the wire shows the shapes demonstrated in the positions I V described below 173 Beam line p Beam shutter Stainless steel housing Fig 123 Schematic diagram of an electromagnetically activated beam stop without water cooling Beam line 3 Water inlet Water ouflet Water cooled beam shutter Feedthrough Fig 124 Schematic diagram of a water cooled beam stop utilizing a vacuum feedthrough The beam Rotating wire Fig 125 Principle of a rotating beam scanner 174 Rotating wires CRANKED WIRE SYNCHRONOUS MOTOR Fig 127 Arrangement of a beam scanner If the beam is centered along the time intervals b and c between the up to down and the down to up the currents will be equal In case of a circular beam b c eccentric beam b c the diameter of the beam can be calculated from the following expression wa d 2 R sin E 45 For narrow beams when
222. ssure ie the air pressure prevailing at the respective location The vacuum ranges are given in Table 14 The relation ship between pressure p and concentration of molecules n for ideal gases is P nKT K 13807 x 10 J K Boltzmann constant T Thermodynamic temperature Vapour is a substance in gas phase which is either in thermodynamic equilib rium with its liquid or solid phase saturated vapour or can be brought to thermal equilibrium by compression condensed at the prevailing temperature un saturated vapour Note In vacuum technology the word gas has been loosely applied to both the noncondensable gas and the vapour if a distinction is not required The vapour pressure is the partial pressure of a vapour The volume flow rate is the volume of gas passing through the duct cross section in a given interval of time at a specified temperature and pressure di vided by that time The volume flow rate S is the average volume flow from a standardized test dome through the cross oe i the pump s intake port Units for the volume m h flow rate are m gl ls The water vapour capacity CWo is the maximum mass of water per unit of time which a vacuum pump can continuously take in and discharge in the form water va pour under ambient conditions of 20 C and 1013 mbar It is given in g h The relationship between water vapour capacity Du d and maximum tolerable water vapour inlet pressure Pwo is SPwo Cw 217
223. stations The requirements for voltage level current ratings and short or long term sta bility for every DC HV power supply may differ widely High voltage power sup plies are used in neutron generators for extraction focusing acceleration etc of the deuteron ions Rectification of the alternating current is the most usual method to obtain DC high voltages A typical circuit is shown in Fig 112 Most of the rectifier diodes nowadays adopt Si type and although the peak reverse voltage is limited to less than ca 2 5 kV rectifier stacks up to tens and hundreds of kV can be made by series connection of more diodes The use of the old selenium recti fiers has the advantage of serial connection without resistor chain but the voltage drop and the power consumption are high during the conducting period As the serial connection of the diodes for a high voltage over 100 kV causes more problems the high voltage power supplies over 100 kV use mainly voltage multi plier circuits The electrostatic high voltage generators manufactured by SAMES Societe Anonyme de Machines Electrostatiques Grenoble France or its successors AID Assistance Industrielle Dauphinoise Zirst Meylan France and ENERTECH France are electrostatic machines converting the mechanical energy into DC high volt ages Such electrostatic generators are called Felici generators after the in ventor as the insulating ribbon electrostatic HV machines are called Van de Graa
224. supply The two transformers supply the two full wave rectifiers and the two Zener diodes regulating circuit with AC voltage The 34 V DC voltage is the in put voltage of the voltage regulators 7805 while the 24 V and the 5 6 V DC sources are needed for the operational amplifiers Fig 108 The regulator of the quadrupole lenses is a voltage regulator based on the three terminal 7805 circuit which gives adjustable output voltage through the first 741 operational amplifier The center tap of the series connected coils is powered by the electronic potentiometer built from an emitter follower The center tap of the two coils and the currents flowing through the coils can be changed by the symmetry potentiometer The basic voltage regulator is shown in Fig 106 A quadrupole doublet requires four regulator units The quadrupole lenses can be mounted onto the beam line by removing the top lid of the quadratic shaped lens The beam line of the neutron generator should 152 220 V 50Hz eo 380 C 5000 10000 uF 63V OV 34V 24V 5 6V Fig 108 Rectifier circuit of the biased quadrupole lens pair be placed between the pole pieces and returned to the upper part of the lens The pole pieces are vertical and horizontal respectively A quartz or Pyrex glass target 5 8 mm thick can be used to observe the fo cusing properties of the lenses As the beam of a commercial neutron generator with about 100 200 W power can heat up the gl
225. sure The abbreviation is NTP Normal Temperature and Pressure Standard temperature T 273 15 K 9 0C Standard pressure Pu 101325 Pa 1013 25 mbar The throughput is the quantity of gas in pressure volume units passing through a cross section in a given interval of time at the prevailing tempera ture divided by that time The throughput of a pump is the throughput of gas pumped The total pressure is the sum of all partial pressures present This term is used in contexts where the shorter term pressure might not clearly distinguish between the individual partial pressures and their sum The ultimate pressure is the value which the pressure in a standardized test dome approaches asymptotically with normal operation of the vacuum pump and without gas inlet Distinction can be made between the ultimate pressure which is due to noncondensable gases and the ultimate pressure which is due to gases and vapours ultimate total pressure Vacuum is the state of a gas with the concentration of molecules being less than that of the atmosphere at the earth s surface Vacuum is the state of a gas 95 Table 14 Vacuum ranges mbar Molecule concentration Low vacuum GV 1000 25x10 2 5x1022 Medium vacuum FV 1 10 2 5x10 2 5x1019 m High vacuum HV 10 197 25x10 25x10 9g Ultra high vacuum UHV 107 lt 25x10 Pm The molecule concentrations refer to a temperature of 20 C with a pressure below atmospheric pre
226. t forces the electrons between the anode and the cathode onto a long helical path and there fore the efficiency of ionization and the sensitivity of the pressure measure ment increase Penning principle like the ion sources of the same name The ionization vacuum gauges with cold cathode can generally be used within the range from 10 down to 107 mbar having a direct meter readout however this range can be extended by the use of current amplifier down to as low as 10 12 or even 10 2 mbar In spite of vacuum gauges with hot cathode the cold cathode ionization gaug es have the advantages of a much higher lifetime and needing simpler electronics for operation and readout 117 On the other hand there is the disadvantage that from time to time the gauge needs cleaning This is because the cold emission depends strongly on the cleanliness of the cathode surface Meticulous care should be taken especially in vacuum systems with an oil diffusion pump Oil deposits and any other contami nation on the surface of a cathode decrease emission capability and therefore higher and higher vacuum will be measured by the contaminated gauge It is worth while to clean the dismantled gauge head by washing it with hot water and household detergent distilled water alcohol and any other organic solvent 9 4 PRESSURE MONITORING AND LEAK DETECTION 9 4 1 Leak rate measurement The pressure in a vacuum chamber that is to be evacuated by any pump sys t
227. t disc itself Both lead to a fast pressure increase in the vacuum system i e the destruction of the high vacuum which can be fatal for the high voltage power supply or for the whole neutron generator 188 Target Room x d Thermal Image j Analyzer Digital Processor Color Monitor TV Mee eect Met aD RUNE ER RUD T UNE Telescope P a Sapphire Sapphire JdUC J Window Aperture Vel ega eet Rd Target f Vos DHlicror eat PRET Em Beam Target Assembly n Fig 139 The FNS JAERI target surface temperature and beam profile monitor Bad electrical contact between the target and the target current meter can cause an electrical breakdown between the target and the ground In addition to the thermocouple or thermistor the video equipment with the necessary electronics is sometimes useful equipment for monitoring the tempera ture of the target However a CCD device or similar radiation sensitive semicon ductor must not be placed in close contact with the target assembly in order to avoid radiation damage to these components It is therefore essential that a suitable distance is ensured by an optical connection between the target obser ving mirror and the infrared sensitive camera A monitoring system complying with this requirement is shown in Fig 139 This system is utilized at the 80 beam line of the FNS at JAERI 109 This equipment is basically a sort of scanning infrared telescope camera The infrare
228. taneously A test circuit for determination of the operational characteristics of the vacuum tubes is recommended the lack of oscillation in the case of a push pull oscillator may have been due to the poor characteristics emission of one or both tubes For the repair of HF oscillators always use the same quality components Silver coated electromechanical component should not be used without a reason e g HF coil anode grid connections capacitor holder 57 The wire wound resistors should be changed to the same size and value in ohms and watts components The condensers used in the HF oscillators should be inductance free mostly mica and high voltage capacitors The trimmer condensers are usually air insulated The actual size and details related to the mechanical mounting can be copied from the original solution The some 100 W power of the HF oscillator is high therefore for the measure ments on an operating HF oscillator do not use electronic i e digital multi meters The simple ANALOG MULTIMETERS are recommended 5 5 PENNING ION SOURCES The cold cathode arc discharge ion sources with oscillating electrons in the magnetic field are known as PIG type sources The operating principle of this source is demonstrated in Fig 34 The K and K plane electrodes are at the same negative potential to the ring shaped A anode of R radius The electric field drives the electrons towards the anode If a magnetic field B w
229. te at the same frequency as the oscillator For the condition of ignition of the ring HF discharge it was found that the maximum value H o of the applied alternating magnetic field H Hy sinwt must satisfy the equation nnt w m e E amp where r is the radius of the tube w is the angular velocity of the high frequen cy field and m is the mass of the electron The collisions between electrons and gas particles lead to ionization only if E Bc eFA QUSE where is the P mean free path of electrons of energy E and E is the ionization energy 43 Cooling fin Al Pyrex glass Quartz m un tube rex glass YN P SNC U To oscillator 100 MHz a Cement polyvinyl acetate Magnet coil AB IN lt Gas inlet _ eK Y Extracting electrode At Fig 18 Capacitively coupled high frequency ion source 37 The ignition voltage Us of the gas discharge depends on A and o 2zf The dependence of Ug on f for different gases at the same pressure is shown in Fig 20 In Fig 21 the curves show the Ug Ug f function for argon gas at differ ent gas pressures It is obvious that the minimum value of Up and that of the associated frequency increases with increasing pressure 44 Experiments show that the ignition voltage at pressures below 10 independent of the nature and pressure of the gas in the discharge tube and that its value is primarily
230. ted by the change of the deflection current needed to achieve the maximum beam current 11 4 VACUUM CHAMBERS OF DEFLECTING MAGNETS The vacuum chambers of deflection magnets should fit the vacuum system the beam line and the target holder of the neutron generator If the the monoatomic aero t 3 Target 1 Deformation Plasma welded Deformation Target 2 Fig 98 Vacuum chamber of deflection magnet using oval tubes beam component is selected then the molecular and three atomic components usu ally thermally load the wall of this chamber The corresponding chamber surface should be cooled properly The deflection magnet vacuum chamber is usually made of stainless steel but other suitable metals e g copper can also be used The vacuum chambers of the analyzing magnets are usually connected to the beam line of the neutron generators by elastic joints like bellows which make alignment of the beam line easy A very simple vacuum chamber construction is shown in Fig 98 The chamber was manufactured of two stainless steel tubes with their original circular shape pressed into an oval form and welded to each other and fitted to the 30 deflec tion magnet The undeformed ends of the circular tubes can be fitted easily to the beam line components of the neutron generator Vacuum chambers for different magnets can be manufactured in a similar way 11 5 PROBLEMS WITH ANALYZING MAGNETS For a correct analyzing magnet the shape of the
231. tem 114 is con trolled by a microprocessor controller The timer controller can be made from a parallel input output card of a personal computer or other independent industri al timer 199 17 NANOSECOND PULSED NEUTRON GENERATORS The nanosecond bunching of steady deuteron beams for the production of short 14 MeV pulses can be done before or after the acceleration Two typical systems will be described here a pre and a post acceleration bunched neutron generator A description of the principles of their operation is far beyond the scope of this Manual 17 1 PRE ACCELERATION NANOSECOND BUNCHED ION BEAM NEUTRON GENERATOR The block diagram of a pre acceleration bunched nanosecond neutron generator is shown in Fig 149 115 This neutron generator has the following characteris tics Average neutron output 10 n s Neutron yield in pulses 4 x 1019 n s Average target current 10 30 A Beam diameter 8 mm Beam current during the pulses gt 1 mA Acceleration voltage maximum 300 kV Target on the acceleration high voltage HV power supply single wave Cockcroft Walton circuit Ion source RF 200 W push pull oscillator Extraction voltage 0 15 kV Focus voltage 0 20 kV Gas consumption of the ion source 4 5 ml hour NTP D Vacuum system 1200 l s oil diffusion pump with booster 20 m h mechanical duplex pump liquid nitrogen trap Final vacuum lt 2 x 10 mBar with liquid nitrogen trap Pulsing twin gap klystron
232. ters components quadrupole mass spectrometers for leak testing accelerator components acceleration tubes lenses magnets etc Contact person Mr S Bohatka Mr E Koltay TUNGSRAM H 1125 Budapest P O Box 7 Szilagy u 26 HUNGARY Tel 36 1 169 2800 or 36 1 169 3800 Telex 225 058 or 225 458 Fax 36 1 169 2868 or 36 1 169 1779 Products mechanical pumps components UHV components vacuum materials NATIONAL ELECTROSTATIC CORPORATION Graber Rd P O Box 310 Middleton Wisconsin 53562 USA Phone 608 831 7600 Telex 26 5430 Fax 415 783 7245 Products accelerators accelerator components RF and duoplasmatron ion PME acceleration tubes beam line components related equipment like gas eaks 235 KFKI Central Research Institute for Physics Dept Material Sciences Budapest 114 P O Box 49 H 1525 HUNGARY Phone 36 1 1166 540 Telex 224722 Fax 36 1 155 3894 Products neutron generators Component neutron generator laboratories Contact person Dr Istvan Krafcsi IRELEC formerly AID and previously SAMES 20 rue du Tour de l Eau Postal adress BP 316 38407 ST MARTIN D HERES Cedex FRANCE Tel 76 44 12 96 Fax 76 63 19 68 Telex 980167 Contact person Mr Rechaten Products electrostatic HV Felici generators neutron generators neutron generator components accessories AMERSHAM INTERNATIONAL Amersham Laboratories White Lion Road Amersham Buckingshamshire HP7 9LL ENGLAND
233. the electrode is fully utilized while the ion current on the channel wall is diminished Therefore the magnification h of the immersion lens defined by M of 12 should be chosen to have a value close to unity K 2h For other values of this magnification the transmitted ion current decreases and for its low values the angle of divergence of the ion beam increases on leaving the channel Under 49 Fig 26 Diaphragm type ion extraction and its focusing characteristics these conditions the h and D parameters of the probe type extraction system are restricted to the values in the range of 0 6D lt h lt 0 9D 13 The actual characteristics for J ion current versus extraction voltage U ext should be determined experimentally The diaphragm type extracting system is applied in Penning and duoplasma tron ion sources This system can ensure a precise geometry with a well defined emitter plasma surface and it can also be used for HF ion sources In the Pierce type optical systems the electrodes are shaped to prevent the divergence due to the space charge effect in intense ion beams This ion optical system is based on the principle that the direction of a charged beam between the two surfaces of two concentric spheres with different radii is not affected by the space charge see Fig 26 The relation between the extracted ion current J i and the extraction voltage t is given by Ue J KUZ 14 The most important parameters ar
234. the heating is direct electric current sometimes regulated see Fig 72 The temperature and therefore also the resistance of the filament depends on the thermal conduc tivity of the surrounding air The resistance change of the filament can be cal ibrated for pressure The thermocouple gauge operates on the basis of similar principles where the temperature of the filament is measured by a thermocouple 66 Both types of gauge have an accuracy of about 10 In the case of a neutron generator Pirani or thermocouple gauges as fore vacuum meters can be connected as follows Close to the accelerating tube for measurement of the forevacuum usually on the vacuum manifold of the neutron generator Between the high vacuum e g oil diffusion turbomolecular pump and the ro tary pump for controlling the forevacuum needed for the high vacuum pump Somewhere to the target chamber for measurement of the forevacuum after a target exchange while the the main vacuum system of the accelerator is running see Fig 67 Cathode lon collector Anode UK Uy 50V 200V Fig 73 Operation principle of thermionic vacuum gauge I ion current I electron current 9 3 2 Ionization gauges a Thermionic ionization gauges The construction of the thermionic ionization gauge is similar to a triode electron valve see Fig 73 The operation principle is the following Electrons emitted by the incandescent cathode are accelerated t
235. the measurement of flow rate of ion source leaks 73 shaped glass tube mounted vertically mm scale Silicon oil E Fig 43 The U shaped glass tube manometer used to determine the pumping speed and gas consumption of ion sources through the measurement of the flow rate of the ion sources a good gas adjusting valve the U shaped manometer and an ex tra gas container usually plastic balloon is needed Fig 42 The setup with the U shaped glass tube filled with silicon oil for the cal ibration of gas leaks and pumping speed measurements is shown in Fig 43 The valve connecting the upper part of the U shaped manometer serves to equalize the pressure in the two separate vertical legs of the manometer 6 3 1 Measurement Regulate the gas leak for the required value of the gas flow This can be detected for example by the optimal colour gas discharge in the HF ion source The valve connecting the two legs of the U tube is open The flow rate gas con sumption of the ion source can be measured in such a way that the volume of the gas entering from the plastic balloon into the gas leak valve into the ion source in a given time will be determined Close the valve which bypasses the legs of the U tube manometer and start a stop watch The gas entering into the leak valve will flow from the left leg of the U tube causing a pressure drop against the right leg of the U tube The lev el of the silicon oil will rise at the left h
236. the vacuum by at least half an order of magnitude The FREON 12 cooled traps are advantageous in laboratories where liquid ni trogen supply is either difficult or impossible to obtain The manufacturers of diffusion pumps usually stock this type of refrigerator operating cooled traps as well Fig 65 shows some simple forms of cooled vapour traps It is quite practi cal to put a vacuum trap in the inlet chamber of the diffusion pump this reduces the risk of getting diffusion oil into the accelerator tube and helps to limit the different vapours present in the vacuum system Further questions on the correct use of a vapour trap are discussed later Buffer chamber Sometimes it is worth while to insert a buffer chamber with a volume of 3 to 5 litres usually made of stainless steel between the oil diffusion and the rotary pumps see Fig 64 In the case of power cut off the buffer chamber will play an important role and take over the duty of the rotary pumps for a short time until the diffusion pump cools down Owing to its rel atively large volume it can pump out for a while air from the exhaust port of the diffusion pump that is still working and reduce the contact of its oxygen with the still hot oil vapours Isolation valves Diffusion oils in contact with air may absorb a lot of hu midity from the air and gaseous material which will be released again when the pump is started up and results in extra gas intake Therefore after s
237. them for a new or properly sealing one At the installation of a new neutron genera tor after assembling the vacuum system it is very important to determine which of these three cases exists This kind of pumping speed determination as a test may also be needed several times especially if some changes have been made on the vacuum system of the accelerator e g inserting a new cooled trap replacing valves dismantling cleaning and re filling by oil of a diffusion pump etc The best method to check the hermeticity of a vacuum system is by measuring the leak rate which can be done as follows Measure the pressure p at a moment t and then separate the pump from the evacuated vessel The pressure will increase slowly due to the air intake through the leak and later at moment t5 we may measure the pressure p The air intake f is by definition P gt P f Gee 27 where Ve is the volume of the evacuated system If the pressure difference is measured in mbars the time in seconds and the volume in litres then f will be obtained in mbar l s In practice utilizing a pump of usual power and a vacuum system with rubber seals and gaskets if f is equal to or lower in order of magnitude than 10 mbarl s then the hermeticity of the system ie the effectiveness of the sealing can be considered acceptable If the hermeticity of the system is not good enough further leak tests should be carried out 9 4 2 Pumping speed meas
238. time of flight spectrometry 208 Peak B nt d ni He ES 2 76 MeV Peak A 3 H He d p He Peak C Ej 2 8 Mey Held p He d p H Ep 13 9 MeV Ep 2 61 MeV A Fig 156 Typical pulse height distribution of the charged particles detected by surface barrier detector using APM target head 3 D d pI H REACTION 10 LT 3y PROTONS e TRITONS z a 500 25 wa E e aca or 50 100 350 400 N Fig 157 Charged particle spectrum measured by APM from the H dp PH reaction Fig 158 Block diagram of the electronics needed for studies with a simple associated particle target head 209 peaks of the alpha particles and protons from the 3H d n He 3He d p He and the H d p H He d p He reactions respectively The tritons originating from the D D reaction can be observed at low channel numbers 121 Observation of the peak A gives a possibility for correction of the counts detected in the peak B originating from the alphas The peak of the protons from the H d p H reaction gives a good possibility to monitor the amount of drive in deuterons in the tritium target The charged particle pulse height spectrum from D D reactions is shown in Fig 157 The electronics of the APM technique depends on the purpose of the measure ment For monitoring or self target buildup studies a simple linear amplifier line shown in Fig 158 can be utilized The MCA can be replaced by a single chan nel analyzer and a
239. tion pairs The horizontal deflection results in about 50 ns wide triangular pulses b These deuteron pulses are selected by the Y deflector plates c The de flected pulses hit the second slit in the beam line before the two gap klystron The klystron itself works at a frequency of 4 MHz d and bunches the chopped deuteron beam onto the target The deuteron pulses of 2 ns width e are detected by a capacitive pick off electrode in the vicinity of the target 118 The steering of the deuteron beam is solved by additional steering voltages on the deflector plates X and Y The control of the nanosecond mode of the neu tron generator is monitored by oscilloscopic observation of the M M and M slits The pulse forms give good information on the problems related to the oper ation of the system A block diagram of the nanosecond pulsing of this J 25 com mercial neutron generator is shown in Fig 152 The horizontal dashed line divides the block diagram into the accelerator hall and the control room The beam cur rent and beam monitor panel is the unit where the beam shapes and beam currents at the diaphragms and at the target are monitored 205 18 THE ASSOCIATED PARTICLE METHOD The associated particle method APM is widely used for determination of the absolute neutron emission rates and to ensure the electronic collimation of the neutron beam in TOF time of flight and other coincidence measurements The al pha particles emitted in the
240. tion of turbomolecular pumps gives a longer lifetime than frequent switching on and off 114 It is important to know that structural changes may take place in the bear ing materials of a turbomolecular pump even if the pump is not used Therefore it is advisable to review bearings every three to five years 9 3 PRESSURE VACUUM MEASUREMENTS Neutron generators of various types use the following vacuum gauges 1 Thermal conductivity gauges Pirani and thermocouple gauges for forevacuum measurements 2 Ionization gauges thermionic ionization or Bayard Alpert type and cold cath ode or Penning type ionization gauges for high vacuum measurements The principles of these gauges are summarized in the following 9 3 1 Thermal conductivity gauges It is well known that for higher pressures greater than about 10 20 mb the thermal conductivity of a gas is independent of pressure in this case the mean free path of the molecules is much smaller than the dimensions of a typical vacuum vessel At lower pressures from 0 5 to 5x10 mb the thermal con ductivity of a gas is proportional to the gas pressure The Pirani and thermo couple gauges operate efficiently at such pressures W GAUGE HEAD R1 R2 R3 BRIDGE RESISTORS P1 P2 BALANCE POTENTIOMETERS M AMMETER Fig 72 Circuit diagram of a variable resistance thermal conductivity gauge Pirani vacuum gauge 115 The Pirani sensor consists of an electrically heated filament
241. topping the vacuum system of the generator it is worth while to isolate the diffusion pump 107 Vacuum Vacuum chamber chamber or e Fe ad oe diffusion Rotary Liquid pump pump Nitrogen Liquid Stainless steel Nitrogen vessel Glass vessel Rotary pump Intet for liquid x LL JL Nitrogen MR Nitrogen Inner cylindrical vessel Fig 65 Liquid N 2 cooled traps Fig 66 Different types of isolation valves 65 108 from the other parts of the vacuum system by valves see Fig 66 These valves are usually quarter swing or butterfly valves The acceleration tube and the tar get beam line are usually isolated by gate valves In this way air is admitted only into the small space just at the inlet of the rotary pump by the air admit tance valve while the other parts of the system keep the vacuum After switching off the vacuum system the backing valve prevents the vapours of the warming up trap from streaming back towards the diffusion pump The role of this valve is similar to that of the high vacuum isolation valve For the safe running of the vacuum system it is worth while to control the cooling water supply of the diffusion pump continuously by a pressure switch this switches off the pump heating automatically if the water supply cuts off A relay indicating the pressure level built into the electronic unit of the vacuum gauge vacuum gauge controller can also be used for a similar pur
242. tron component of the beam but this can be rebuilt near the target The upper limit of the energy spectra of the secondary electrons induced by the accelerated ions in the target does not exceed about 100 eV depending on the target material However the X rays produced in the target when it is bombarded by protons or deuterons can contribute significantly to the high energy tail of the electron spectrum The problems related to the secondary electrons can be overcome by obtaining reliable measurements of the beam current by the suppres sion of the high flux of the fast primary electrons as well as the emitted slow secondary electrons 179 Amplifiers Horizontal wires Computer Multiplexed ADC Vertical wires Fig 129 Electronic block diagram of a multiwire beam scanner 101 HV ZA OUTPUT GRID VACUUM JACKET ZZ INSULATOR NNMENE HIGH ATOMIC NUMBER METAL Fig 130 Schematic representation of a Faraday cup for measurement of the beam current Starting from the fact that the main electron component is slow and that the number of fast electrons reaching the current collectors target either from the residual gas or from the target is small it is possible to suppress the electrons by surrounding the target with a negatively biased mesh screenmaintaned at a voltage of 200 V In addition the collector itself can be biased positively and thus re collect all the slow secondary electrons The schematic representati
243. ts with serial optical links between the ground and the HV terminal 8 1 MECHANICAL CONTROL This is the simplest solution for the distance control of mechanical leaks and variacs at the HV terminal Insulating Perspex Bakelite rods are fastened to the shaft of the needle leak or variac and there is a suitable knob to turn the insulating rod at the ground potential Since the insulating rod especially in high humidity may conduct the ground side of the rod should be grounded between the the manually touched knob and the HV terminal to avoid an electric shock to the operator If the mechanical transmission is made of nylon fibers similar precautions should be taken to avoid an electric shock A schematic rep resentation of this simple distance is shown in Fig 49 HV TERMINAL GROUNDED 7 oc INSULATING A TURNING FPP SOR ISS ORI oe Oe OO OPO ST amp 2 ERR E E emo ed KNOB E I ESRI OSI OL ROD ZA L Fig 49 Insulating rod control of the regulation elements at the HV terminal 8 2 ELECTROMECHANICAL CONTROL The electromechanical control of the units at the HV terminal is similar to the mechanical control However instead of manual control the rods or fibers are driven electrically by servo motors DC motors with gear etc The insulation between the HV terminal and the ground is the same Since the servo motors and especially the DC motors with gear may produce a higher momentum the mechanical
244. ty of charge of inverse polarity to the input capacitor to discharge it In the circuit shown in Fig 131 the input capacitor is charged by the input current beam current and it is discharged by the reference voltage source In principle all of the voltage to frequency converters work similarly The voltage to frequency converter chips can be used in simple target cur rent integrators in the range of 10 nA to a couple of mA A simple target current integrator utilized at neutron generators with beam current of several tens of uA is shown in Fig 132 This integrator is a useful device for the observation of the burn off of the tritium targets 103 14 5 TARGET ASSEMBLIES The target assemblies of neutron generators are constructed to fulfil the following requirements target holding target cooling target current measure ment to suppress the secondary electrons to achieve the shortest distance be tween the target spot to the sample to be irradiated The usual target holders are cooled with water or air Water cooled target holders are used at beam current higher than 500 uA when more than 100 W target load is present in the case of 200 kV acceleration voltage Water cooled target holders are used mainly for activation analysis where distortion of the original 14 MeV neutron spectrum does not influence too much the accuracy of the elemental analysis using reference sample of similar composi tion Bombarding d beam Evaporated occl
245. uding metal Ti Er Sc Backing Cu Mo Al Fig 133 The construction of a target 182 The tritium or deuterium target used at the neutron generators consists of a good heat conducting backing made of Cu Mo or Al etc covered with thin vac uum evaporated layer of hydrogen occluding metal usually Ti Er Sc etc see Fig 133 The heavy hydrogen isotopes tritium or deuterium form a quasichemical compound with the occluding metal In principle the tritium concentration can achieve the stoichiometric ratio Ti4T g This ratio depends on the temperature because the disintegration of Ti T molecules is significant over 400 C The loss of tritium or deuterium from the targets in neutron generators is propor tional to the energy dissipation of the accelerated D beam The tritium implan tation using a mixed pt p beam as is usual in sealed tube neutron generators can delay the burn up of the tritium targets by a factor of about 1 5 1 7 In low voltage generators the most common targets are deuterium or tritium absorbed in thin metal layers Besides titanium and zirconium Er Sc Y and U are also used to produce intermetallic compounds with deuterium or tritium Theo retically the ratio of tritium to titanium atoms is about 1 9 1 but in the case of commercial targets it is about 1 5 1 To produce a thin target a 02 to 2 5 mg cm thick layer of Ti or Zr is evaporated onto Cu Ag or W backing metal A 1 mg cm titanium
246. udo and Kinoshito 11 have developed a method based on the pulse height distribution of the recoil edge for He nuclei in a He proportional counter for the determination of the energy 13 Table 2 Coefficients of Legendre polynominals for calculation of thin target angular distributions of D D source yields in laboratory system keV 50 100 200 300 400 500 spread and the mean energy of the D T neutrons The detector is calibrated at the of 14 00 MeV obtained at energy spread of neutrons dE s 10 keV obtained at these angles seems to be too reference energy point small The following variables can strongly influence the neutron energy m m mnm om m 0 11787 0 01741 0 03149 0 10702 0 02546 0 10272 Ay A3 0 58355 0 11353 0 88746 0 22497 1 11225 0 38659 1 64553 0 63645 1 05439 0 21072 1 09948 0 29820 Table 3 equation 3 normalized to 90 Ay 0 04222 0 08183 0 26676 0 67655 0 81789 1 09435 0 16359 0 37225 0 11518 0 35367 0 59571 0 76159 a 90 mb sr Ref 4 0 32016 1 01828 1 95031 2 66479 3 32222 3 63084 Coefficients in equation 3 for the calculation of the D T thick target neutron yield vs emission angle keV 50 100 150 200 250 300 325 pm oe ee ee D T A1 Ay 0 03003 0 00035 0 04087 0 00062 0 04727 0 00083 0 05124 0 00096 0 05419 0 00110 0 05651 0 00119 0 05616 0 00119 tion 1 the types of target atoms 14 2 the
247. uit connected to one of the above current regula tors is shown in Fig 96 The circuit powered by its own twin 12V power supply has three voltage comparators three indicator LEDs and an acoustic alarm device 90 As the voltage drop across the LM317 or LM338K for an output current of about 3 4 A should be more than 10 V during normal operation the first comparator detects the 10 V voltages In this case the green LED lights up and the acous tic alarm gives a continuous alarm signal These warnings inform the neutron generator operator that the resistance of the deflecting magnet has increased due to the warming of the coil and that the voltage drop along the current regu lator is below 10 V As the output voltage of the magnet powering rectifier can be raised by turning up the variac in Fig 94 the operator should turn up the variac If the operator turns it up and the voltage drop on the current regula tor is higher than 10 V the alarm signal stops and the green LED goes out If the operator turns more than is needed and the voltage drop is over 27 V the alarm starts to sound again and the red LED comes on As the voltage drop should be between 10 and 27 V if the red light comes on the variac should be turned downwards When the voltage drop on the current regulator reaches the absolute maximum ratings of the regulator circuits 36 V the ZF36 and 2N3055 based protection circuit starts to shunt the current regulator the third com
248. ul in upgrading neutron generators and accelerators as well in setting up new experiments 10 2 PRINCIPLES OF OPERATION The low voltage a few 100 kV neutron generators produce neutrons by the following reactions H d n He Q 3268 MeV 1 5H d n He Q 17 588 MeV 2 The large cross section of the 3H d n He reaction permits high yields of fast neutrons to be obtained even at low deuteron energy 150 200 keV The 0 differential cross sections of reactions 1 and 2 are 2 6 mb sr and 400 mb sr respectively The total cross section of the H d n He reaction has a broad resonance with a maximum value of 5 barns at E dm 107 keV At this deuter on energy the total cross section of the 2H d n He reaction is about 20 mb therefore the contribution of the D D background neutrons to those emitted in the D T reaction can be neglected As the D T cross section has a peak at around 107 keV and the tritium targets are thick metal titrides the neutron yield shows an increasing trend up to about 400 keV bombarding energy see Fig 1 The D D reaction is used for neutron production mainly by electrostatic accelera tors or cyclotrons where the neutron energy is changed by changing the deuteron E a u 5 2 5x10 Oo E i4 2xio a e Ld o a o oO 3 15x10 t S x z n D o2 Oo uu t z o ui xe 5xio o 0 100 200 300 DEUTERON ENERGY keV Fig l Neutron production cross section and the total yield of neutrons for a
249. um gas especially the large tritium carrying chunks created by sur face erosion For an intense neutron source the amount of tritium released in a year is too great to exhaust into the atmosphere There are two capture techniques to retain the tritium released from the target 1 trapping in ion pumps and bulk sublimators or 2 using a tritium scrubber that converts the tritium to water and binds it on a molecular sieve bed It was found that tritium released from the gas in metal target with an out put value of 10 to 102 Bq h through the scrubber system is typically less than 3 x 107 Bq d The tritium evolution rate during storage depends on the type of carrier gas and has a lower value for argon than for air Special attention should be paid at neutron generators with titanium getter pumps The amount of tritium 101 Bq h will be captured mainly by the ion getter pump The estimated amount of tritium found in a gaseous state at any time during the operation of the neutron generator is about 107 Bq The ion getter pump goes through a heating stage when it is started and consequently tritium is released into the vacuum system 20 2 RADIOACTIVE MATERIAL STORAGE AND WASTE DISPOSAL HAZARD Neutron generators utilize tritium targets of 3 37 x 1010 Bq approx 1 10 curie activity Spare targets may also be stocked as replacements All of this radioactive material must be used or stored in an exclusion or storage area in accordance with the r
250. urces mostly HF ion sources with probe type ion extraction and in those utilizing the expanded plas ma surface the ion optical system is similar to an immersion lens objective Fig 24 Probe type ion extraction system quartz sleeve and extractor electrode of SAMES neutron generators Sizes are in mm 48 Virtual image Fig 25 Schematic diagram of electrostatic immersion lens type extraction and the relation between the parameters In ion sources mostly Penning ion sources with diaphragm type ion extraction where the electrodes are shaped so as to reduce the space charge effect in the ion beam the extracting system is similar to a quasi Pierce type ion optical system The scheme of a probe type extracting system is shown in Fig 24 An immersion lens objective consists of two electrodes both with diameter D One of them is of length h and closed at one end while the other electrode open at both ends is placed at some distance from the first The relations be tween the parameters of this simple immersion lens are shown in Fig 25 graph It can be seen from the figure that for h 0 785D the distance K of the image is negative It means we have a virtual image for h 0 785D the distance K o and for h 0 785D the image is real and the distance of the image decreases as h increases In the optimum case the parameters of the extracting system are chosen so that the total cross section of the ion extracting channel in
251. urement A very important part of the vacuum system design is determination of the pumping speed which must be known in order to attain the required ultimate pres sure This is a basic problem at the different accelerators where the ion source naturally consumes gas When a new neutron generator is put into operation and also later when a modification has been made the pumping speed of the system should be tested When connecting a pump to a vacuum system of volume V the pressure change with time during operation will be P dp _ dc where S is the pumping speed The negative sign corresponds to the fact that 28 119 the pressure decreases with time If constant pressure is maintained by a needle valve controlling the air intake from the atmosphere the pumping speed will be seo 29 In practice when the pumping speed is to be determined a definite change in atmospheric pressure air volume during the corresponding time interval has to be measured This can be done by using the equipment shown in Fig 76 This setup is also used for measuring ion source gas consumption 66 Manometer ext Glass tube Oil Fig 76 Equipment for the measurement of pumping speed The pressure p in the vessel under evacuation is adjusted by a needle valve in the ON position of the valve V In equilibrium the pump removes the amount of air inflows into the system through the needle valve Turning the valve V in the OFF position ai
252. urface of the isolator rings some metal or carbon tracks can be observed Clean the outer surface of the isolators with polish ing paper and organic solvents Observe the inner side of the acceleration tube When the ion source and the extraction system can be easily dismantled open the ion source end of the acceleration tube Light the inside of the acceleration tube If the elec trodes not only the accelerator electrodes but also the extraction and focus show some contamination such as oil vapour in the form of brownish black layers clean the electrode surfaces Polishing and washing with organic sol vents N hexane petroleum ether acetone etc depending on the materials used should be done carefully The form of the oil deposits on the electrodes can give some information about what is making the ion beam current decrease The eroded surfaces indicate the effects of the electron bombardments along the acceleration tube and give an indication on the poor vacuum or other con tamination sources Check for continuity between the inner electrodes and the outer contacts 11 PRINCIPLES OF BEAM FILTERS 11 4 ELECTROSTATIC AND MAGNETIC BEAM DEFLECTION The accelerated deuteron beam contains D D5 D3 ions and also other ion species N t O from the residual gases and vapour of the oil or grease This vapour will cover the surface of the target The target lifetime in a neutron generator depends very strongly on the quality of the vacuum The
253. utput will reach a maximum voltage of 2nV nax 161 2nV Ideal output voltage V max Output voltage ripple V T n 1 f frequency _ u loading current Voltage drop AV n gt n 3 Loaded HV output V 2nV I2n pus o max 3fC Fig 115 Circuit diagram of Cockcroft Walton HV generator 162 1zH V output I l l D aco Ds conducting h D4 Un conducting Fig 116 Waveforms of the potentials at the nodes of the Cockcroft Walton cascade circuit Loaded HV output I 0 If the generator supplies any load current I the output voltage will never reach the value 2nV nax 95 shown in Fig 116 There will also be a ripple on the voltage and therefore we have to deal with two quantities the voltage drop Vo and the peak to peak ripple of 20V The sketch in Fig 116 shows the shape of the output voltage and the definitions of V 6 and 20V The peak to peak ripple is giv en by 20V IT 1 C The total ripple will be 96 V t C 2 C 3 n C 41 42 For a given load V o may rise initially with the number of stages n but reaches an optimum value and even decays if n is too large For constant values 163 of I Nn f and C the optimum number of stages is obtained by dV differentiating the equation for V di with respect to n i e from f 96 3 max C Vma V mar LAB 3fC we have hop UE p 43 For a Cockcroft Walton generator with
254. uum When a new target is placed in the seat of the target holder switch on the forevacuum pump of the target tube The vacuum will hold the target the O ring on the vacuum side should seal properly This can be observed by the Pirani or thermo pair vacuum meter of the target tube The O ring of the vacuum side should be cleaned with organic solvent and greased slightly with high vacuum silicon grease The tissues used for cleaning the target assembly and the old O rings should be handled as radioactive litter and stored in the special bin for radio active litter When the target sealing on the vacuum side seats properly assemble the wa ter sealing O ring and the water cooling cup As the tritium target backing seats in the isolated seat between the two O rings the cup should be tightened care fully During the tightening of the water cooling cup observe the vacuum meter of the target tube The sealing both on the vacuum side and on the water side should be done properly When the cup of the target holder has been fixed con nect the water line Check the target assembly If the water drops from the tar get holder the cup should be tightened even more Attention When the target separates the water from the vacuum tritium side every operation on the target assembly should be carried out carefully Details of the radiation protection procedures related to neutron generators can be found in Refs 107 108 The electrical connection in
255. ve 4 to valve body 1 using screw 16 Screw actuating knob 2 with scale drum 3 clockwise onto sleeve 4 until groove of spindle 5 protrudes clearly over ball bearing 10 Attach locking washer 12 Close the actuating knob 2 with cap 9 71 d Adjusting the spindle clearance Take off the cover cap 9 from the actuating knob 2 Remove locking washer 12 guard ring 11 and ball bearing 10 Adjust clearance with threaded pin 21 Assemble in reverse sequence e Testing the needle valve after cleaning Open and close the valve about 10 times to compensate for any rough spots The leak tight valves should fulfill two requirements Leak tightness of the valve seat in closed position needle seating Leak tightness between valve interior and valve surrounding Tightness of the second type is less important because such leaks merely re sult in gas losses to the atmosphere if the needle valve is used to regulate the gas consumption of ion sources from gas tanks of more than 1 bar pressure If the valve operates correctly it must be possible to alter the gas flow smoothly without steps If the direction of the rotation is changed the valve must react immediately The values of the calibration curves for 0 01 or 0 1 mbar l s should be reproducible within about 10 f Leak testing the valve with vacuum gauge The leak tightness of a valve can be checked with the vacuum gauge which is normally install
256. ween the HV terminal and the control desk give the computer control and regulation of the whole neutron generator Most neutron generators have ion beam handling facilities magnetic or elec trostatic quadrupole lenses beam profile monitors ion beam analyzers simple electromagnet or Wien filter beam stops and scanners etc The size and the 26 HV terminal Beam handling Acceleration Ion source Lenses ion Gas supply d d5 d3 Beam analyzers Cooling Ion beam Pulsing etc Power supplies Accel HV Control Insul transf console Vacuum and cooling systems Fig 10 Block diagram of neutron generator HIGH VOLTAGE TERMINAL LIQUD N TRAP DIRECT ANALYZER VALVE VALVE NEEDLE VALVE doesent oe MAGNET t mih VOLTAGE owvoeR FE as os 0 07 aan 1 Lu72 L eene eee 3 22 TS IMP anc EU ee E ee ZA N z H i TAR ET C sl i A l ACCELERATOR TUBE SUPPRESSOR A j Ss regm 300V D BEAM m VALVE HIGH VOLTAGE R QUADRUPOLE DOUBLET F CZ 00M amp DIFF ANALYZEO STACK INSULATED BEAM DRIVING __ SHAFT__ CURRENT ve E MECHANICAL PUMP EXHAUST GENERATOR 5nF 5nF 300ka poy sooko 90kV 90kV REGULATED MAINS Fig 11 Diagram of a working neutron generator 27 technical solution of the target holder depends on the purpose of the neutron generator A neutron generator utilized mai
257. x depends on the flux distortion and self absorption caused by the sample while for a point source of fast neu trons the average fluence is determined by the source sample geometry In addi tion to the geometry the neutron attenuation in a thick sample can influence the activity distribution demanding a careful evaluation of the measured gamma line intensities Therefore during irradiation and measurement the cylindrical sam ples are rotated around two axes perpendicular to each other This solution is used at the KAMAN twin tube pneumatic system Unfortunately this method requires complicated sample holders at the irradiation and measuring sites In the case of periodical irradiation and measurement required for short half life isotopes the positions can change randomly resulting in an average value for the activ ity A proper pneumatic transfer system for activation analysis with neutron generators should be a twin tube with 197 b c d Sample rotating system for cylindrical samples or Rectangular tubes for disc shaped samples irradiated in the direction of the axis of the sample which do not need rotation of the sample Neutron production e g ion source switch on and off facility Accurate time controller for the irradiation and measurement Sample position detectors at the irradiation neutron generator and measurement gamma detector side and a Loading ejecting port for the samples ION SOURCE Cooli
258. y an external bypass branch If the serial connected coils and an external potentiometer are connected parallel and the central tap of the potentiometer is connected to the center of the seri ally connected coils the balanced bias of the lens can be altered by sliding the potentiometer This is the principle of the biased quadrupole lens The cir cuit diagram of a biased magnetic quadrupole lens is shown in Fig 104 148 Fig 105 Circuit diagram of a quadrupole doublet The biased quadrupole lens both magnetic and electrostatic requires in practice split power supplies We now discuss a magnetic quadrupole doublet lens for neutron generators up to an accelerating voltage of 150 200 keV The focusing distance of the quadrupole doublet is about 60 80 cm and the beam can be steer ed at this distance both horizontally and vertically in a range of a few centi meters The power supply of the coils is a voltage source it can be a current source as well and instead of a center tapped power potentiometer an electronic equivalent potentiometer and emitter follower is used 93 The circuit dia gram of the quadrupole doublet is shown in Fig 105 and that of one with split power supplies is shown in Fig 106 149 BD 242A CURRENT SYMMETRY Fig 106 Split power supply 1 4 of the quadrupole doublet shown in Fig 105 The focus of the beam can be changed by the current potentiometers control ling the output voltages of the
259. y the operator and service personnel The functions of the subunits are de scribed and proposals for troubleshooting in case of malfunction of a component are outlined The Manual contains descriptions of some symptoms and proposals for their repair The proposed repair and maintenance methods have been chosen bear ing in mind that some laboratories may have poor infrastructure The Manual contains a short description of the commercially available neu tron generators including the sealed tube and intense models The description of the operation of a specific component is usually followed by a general guide to troubleshooting and repair As this is NOT A SERVICE MANUAL the guidelines for troubleshooting and maintenance are not given for all types of neutron generator although the information herein can be used for any type of low voltage accelera tor The detailed description of some methods such as pulsing or the associated particle technique is intended to help in improving and widening the applica tions of the original machine The list of manufacturers and other sources of components in Annex A is intended to assist laboratories in developing countries to find suppliers for spare parts or improved components The short review of the hazards related to accelerators is particularly directed towards operators who have less experience in these matters and should be read with care by them The detailed schematic and workshop drawings should be usef
260. ypically up to about 25 stages can be used every stage being modular constructed as indicated in Fig 120 These modules are quite small they can be 166 Termination t HV d c output Further stages ait L H Voltage multipliers ee Ge Fig 121 Circuit diagram of the Dynamitron HV cascade stacked in a cylindrical unit sometimes under SF pressure The output voltage is usually controlled by regulation of the primary side The stages of the mul tipliers usually have an HV voltage divider for feedback As the supply frequency ranges from several kHz up to 100 kHz the voltage divider for regulation of the output is a combined R C voltage divider The modular power supplies of the WALLIS HIVOLT United Kingdom GLASSMANN Highvolt USA and the Technical Uni versity Buadapest Hungary have a similar structure These HV generators are used as acceleration extraction and focus power supplies in neutron generators as well as for the supply of electrostatic quadrupole lenses 167 e The Dynamitron HV power supply The Dynamitron power supply is a parallel fed cascade circuit using high frequency of a few MHz The MHz range has the advantage of using the parallel charging capacitors C in Fig 121 constructed as simple spray capacities be tween the nodes of n and n of the original Cockcroft Walton voltage multiplier The loadability of the Dynamitron is several tens of mA with a Va output voltage of MV range The volta

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