2D Position-Sensitive Neutron Detector - DENEX-GmbH
Contents
1. lonisierendes mmm nn Photon j j a U ss StrommeRgerat Fig 1 Measurement of the ionisation current The applied voltage U and the distance between the electrodes d are chosen such that U is large enough and d is small enough to ensure that the charge carriers are collected quickly enough by the electrodes to avoid their recombination In the case of low energy photons the entrance window of the detector has to be very thin to allow transmission of the photons without significant energy losses and thus to ensure that they can give up almost their entire energy within the detection volume For the efficient detection of photons and neutrons it is therefore necessary to give careful consideration to detector design and to the selection of the window material and the detection gas 2 2 Secondary ionisation In contrast to photons neutrons can only interact with atom nuclei for example via absorption The absorption process involves incorporation of the neutron into the atom nucleus This generally leads to excitation of the atom nucleus and the subsequent emission of particles 3He 1n me He p T 0 77 MeV B Fig 2 Nuclear reaction between a neutron and He Fig 2 shows schematically such a reaction between a neutron and a He atom Excitation of the He atom by the incoming neutron leads to the emission of a proton and a tritium atom 0 77 MeV is released and the He atom is transformed to2. He The proton p and Tritium T released generate positive und negative charged particles via collisions with the gas molecules along their trajectories These are attracted to the correspondingly oppositely charged electrodes of the capacitor plate As illustrated in Fig 3 the two electrodes are normally configured in practise as a cylindrical capacitor or as tube detector or counting tube Zylinderkondensator StrommeRgerat Fig 3 Detector as cylinder capacitor 2 3 The proportional counting tube Whereas in a plate capacitor only the directly or indirectly generated charge carriers contribute to the signal the proportional detector makes use of an amplification effect that is based on the multiplication of these charge carriers in the gas volume The positive electrode anode of the cylinder capacitor illustrated in Fig 4 takes the form of a thin wire in the neighbourhood of which free electrons between two collisions with the gas atoms are so strongly accelerated that they can generate further charge carriers A primary electron thus produces an avalanche of secondary electrons Fig 5 so that ultimately a much greater number of charge carriers is generated than result from the primary ionisation of a single photon or neutron Zylinderkathode t Isolator LUN E m e U Z hidraht ww AA ii StrommeRgerat Fig 4 Principle of the proportional counting
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4. __3 ISOTOPIC CONTENT 299 914 ee FE Lee i conrent sa os III 4 CHEMICAL IMPURITIES 5 ANALYTICAL METHOD Chromatography ICP MS Responsable GAZ DATE F ROBERT 26 01 2006 oll TVA Intracommunautaire FR 56 380 479 055 SOCIETE ANONYME A DIRECTOIRE ET CONSEIL DE SURVEILLANCE AU CAPITAL DE 145 920 RCS CORBEIL ESSONNES B 380 479 055 SIRET 380 479 055 00010 CODE APE 241 G 28 5 2 Detector Specifications Comments n Neutron sensitive area 280mm x 280mm 2 Depth of detect volume Lem Limited by drift electrodes Detection gas He 5 bar Maximal total detection gas pressure 6 bar CF4 1 bar Position resolution FWHM 2 mm x 3 mm Horizontally FWHM 2 mm due to CF pressure Vertically FWHM 3 mm Gi to CF4 pressure and anode wire distance Efficiency for thermal neutrons QA 48 Yo i nominal neutron capture probability inside the detection volume s 76 Yo depth 20mm gt 94 Yo ii absorption in neutron window not taken into account gt 99 Yo iii electronic detection of every captured neutron assumed 16 A 100 iv dead volume losses between entrance window and first drift electrode can be neglected due to electrode design Window material AI Mg 4 5 Si Alloy Material and thickness is optimized Window thickness 10 mm Uncorrected differential non Measurement Starting from the centre of an uncorrected spectrum linearity of position detection and assuming a constant ratio change in x or
5. a lifetime which is practically unlimited Service and maintenance is therefore generally not required However leakage and ageing processes ca lead to changes in the detector characteristics The detector has been tested for gas tightness at DENEX If the gas valve remains well closed the gas pressure should not change significantly over a period up to a decade Should leakage occur contrary to expectations this could lead to damage of the anode In such a case the special measures described in the following should be taken a Check that the electrical connections are consistent with the diagrams in Figs 11 and 13 It is especially important to ensure that the anode and drift electrode are supplied with a positive and a negative voltage respectively b Carry out the procedure described in section 4 4 Recording the characteristic anode curve Check that characteristic anode curve is similar to that in Fig 24 If this is not the case contact DENEX immediately 27 5 0 5 1 General specifications Gas certificate eur ISO CE 3 j ey A PARC DES ALGORITHMES B TIMENT HOM RE ROUTE DE L ORME 91194 SAINT AUBIN CEDEX FRANCE TELEPHONE DIRECTION 01 69 41 97 98 TELEPHONE COMMERCIAL 01 69 41 95 96 TELECOPIE 0169 41 93 52 E MAIL eurisotop eurisotop fr Website http www eurisotop fr ANALYSIS CERTIFICATE PRODUCT HELIUM 3 REFERENCE GHE32L BATCH 26 010601 1 ISOTOPE 2 CHEMICAL FORM S lt
6. as shown in Fig 23 Connect the A outputs of the main amplifier TFA 111 to the inputs of the Ortec discriminator using short LEMO coaxial cables of equal length as shown in Fig 23 Set the zero crossing voltage of the individual channels with a small screwdriver see Fig 23 4 4 From XO preamplifier From X1 preamplifier From YO preamplifier From Y1 preamplifier From anode preamplifier a DENEX 5 a TFA 111 SI ir X0 output Dh xX output jr YO Output To MCA Sh pwe Yi output EE Anode output Zero crossing voltage for d x Ei 1 gt P a lt 4 MAINAMPLIFIER a W P Fig 23 Connecting the preamplifier to the CFD 8000 Recording the characteristic anode curve Equipment required An oscilloscope and a pre selection impulse counter a b d e f g Connect a good oscilloscope e g TEKTRONIX TDS 3054B to the B outputs of the anode channel main amplifier TFA 111 Turn the zero crossing of the anode CF discriminators to a minimum ca 7 mV Leave the drift high voltage supply unit switched off Open up the neutron beam Now switch on the anode high voltage supply unit and slowly increase the anode voltage from zero volts until the neutron signals just appear on the oscilloscope Connect a B output of the ORTEC CFD of the anode channel to the input of the pre selection impulse counter with a LEMO coaxia
7. in Fig 14 The externally applied voltages generate strong and almost homogeneous electric fields red arrows between the electrode planes When a neutron undergoes absorption in this detection volume the free electrons released via the ionisation process are accelerated towards the anode blue arrow The strong electric fields in the neighbourhood of the anode wires accelerate the electrons such that charge multiplication occurs by an avalanche process The resulting charge avalanche is registered as an electrical impulse green arrow by both read out cathodes X and Y cathodes and fed into the delay line which divides the incoming impulse into two equally strong pulses These flow to the two ends of the delay line The delay lines consist of a series of LC Elements that result in a delay of 2 7 ns between the pulses from two adjacent cathode wires The longer the distance of the event from the end of the delay line the greater is the time needed before it is registered at this end of the delay line The X and Y coordinates of the position of the event can thus be determined from the arrival times of the two impulses Driftelektrode Nachweisvolumen va K X Kathode l 4 Einfallendes Neuti l g Fig 14 Principle of position decoding in a 2D multi wire proportional detector with a delay line 3 1 Mechanical construction of the DENEX 300TN detector The DENEX 300TN detector consists of a housing u
8. in front of the detector Fig 26 Make sure that this counter is black light tight Ensure that the counter captures the entire neutron beam and then measure the maximum count rate Then with the same neutron beam measure the maximum count rate with the Denex detector using an arbitrary anode voltage in the range 2400 V to 2900 V e g 2700 V Differences in the count rates can be compensated by adjusting the anode high voltage In this particular example the DENEX detector has an operating anode voltage of exactly 2800 V Fig 26 Setup for measuring the efficiency 4 7 The Constant Fraction Discriminator CFD General A constant fraction discriminator CFD is an electronic signal processing device designed to mimic the mathematical operation of finding a maximum of a pulse by finding the zero of its slope Some signals do not have a sharp maximum but short rise times t see Fig 27 The input signals to the CFD are pulses from the cathode and anode of the DENEX detector However the detector pulses have identical rise times that are significantly longer than the desired temporal resolution in the nanosecond regime This forbids simple threshold triggering with a TDC Time Digital Converter which introduces a dependence of pulse height on trigger time the so called Time Walk Effect Fig 27 left Identical rise times and pulse shapes permit triggering not on a fixed threshold but on a constant fraction of the to
9. tube The amplitudes of the charge impulses at the counting wire are proportional to the primary ionisation via the gas amplification factor which increases with increasing anode voltage U The upper limit to the anode voltage is dependent on the diameter of the anode wire as well as on the composition and pressure of the detection gas Raising the anode voltage U beyond the corresponding limiting value results in an electrical discharge and consequently the destruction of the counting tube or detector e Einfallendes Photon NAN LO NO d Elektron os J a L F FE Wi e Zahldraht a SY L 4 jc ew ef e D AH bo po root ho Fig 5 Principle of gas amplification The signals measured therefore correspond to the short lived impulses caused by the highly mobile electrons generated by gas amplification Proportional detectors are therefore mainly used to count these impulses although the proportionality between impulse height and primary ionisation allows the nature and energy of the radiation to be differentiated Since very small dead times are an essential feature of proportional counters Fig 6 the maximum count rate depends solely on the time interval needed by the counter following registration of an impulse to be ready to register a further impulse This so called resolution time amounts to less than 1 Us in good quality detectors so that count rates in excess of 10 counts s are achievable Erholu
10. y position per channel the relative distortion will not exceed 3 ie less than 4 5 mm at the border Background rate without neutrons lt lt 1 s Depends on the surroundings the analogue electronics and its adjustment E 1 MeV 500 CN and DENEX 200CN detectors filled with 1 1 5 and 3 bar CF4 EI EI ee detector Awooated Tungsten Rhenum wire Cathode wire Au coated Tungsten Rhenium wire Drift electrodes enn Metallic coatings on isolating layers on window and bottom of the detector Connector for HV supply B0ASAV TI Connector for delay lines signal SOQSMA TI 15 Valve for pumping and gas filling e Manufacturer Swagelok Type 6L ELD8 DDXX 16 a quality The detector housing is heated to 60 C and evacuated for more 99 91 than 100h before the detection gas is introduced e 0 09 delay lines D number of steps e delay per step e total delay e Bandwidth 19 Drift capacity GO O EE A eu 78 cm max Pressure sensor Type DRUCK PTX660 Metallic INDIUM 30 Dimensions D height D width D depth D weight 31 EU Declaration of Conformity The directives covered by this declaration 97 23 EEC Pressure Equipment directive PED Section ECC No L 18158 1 from 09 07 1997 Test pressure 6 bar gauge pressure according to AD200 Bulletins 5D and W7 29 Detector Cross Section 5 3 ts bd XO OGS ODODOS Une 60 W DOUMISO Gas MW nidlosnonio DO ZO Z TUE AEDA X TES EMAS ODOLM MOSVIKOOOOD nn 4 DAY ii SE a te os LO
11. Gen 1 1 1 2 2D Position Sensitive Neutron Detector DENEX 300TN User Manual and Technical Specification Lueneburg February 2008 eral Guarantee declaration EU Declaration of conformity Working principles of Denex detectors Primary ionisation Secondary ionisation The proportional counting tube Detection gases The characteristic curve The position sensitive proportional counter The delay line readout method Time measurement for position decoding Design and construction of the DENEX 300TN detector 3 1 3 2 Construction of the detector Quality control during production Installation and initial operation of the detector 4 9 4 10 General precautionary measures during handling and operating the detector Detector orientation Connecting the electronics to the detector Recording the characteristic anode curve Recording the characteristic drift curve Adjusting the efficiency The constant fraction discriminator Gamma suppression Switching off the detector Detector maintenance Detector Specifications 5 1 DZ 5 3 5 4 5 5 He Gas Analysis Certificate Detector specifications Detector cross section Front and rear views Side view 1 General This manual contains important information on understanding the way in which the Denex 300 TN B detector functions as well as instructions for installation and operation of the detector We strongly recom
12. as supply HV Drift electrode bias supply Fig 12 Circuit diagram for position decoding using only the cathode signals The cathode method requires two additional delay units in order to obtain a well defined stop signal Setting the stop signal with the delay units allows the spectrum to be shifted on the display These delay units are not necessary in the anode method However other special requirements must be fulfilled The use of coaxial cables means that consideration must be given to the phase and temperature stabilities as well as mechanical loading in order to ensure that constant reproducible start signals are obtained Display X Start K X Stop Compact PCI Y Start 5 Y Stop 3 CAMAC or VME FAST ComTec MPA 3 7072T FAST ComTec Y TDC X TDC Cathode preamplifier CF discriminat Drift filter VAX Anode filter and preamplifier FAST ComTec HV Anode electrode bias supply HV Drift electrode bias supply Fig 13 Circuit diagram for position decoding using both the anode and cathode signals 3 Design and construction of the DENEX 300TN detector The DENEX 300TN detector is a 2 dimensional position sensitive multi wire proportional counter for the detection of thermal neutrons The principle of position decoding is illustrated
13. bserved when this product is taken into service to maintain compliance with the above directives If changes are made to the product without prior agreement with the manufacturer this declaration will become invalid le In KEE Authority 2 Working principles of Denex detectors 2 1 Primary ionisation The physical principle employed for the detection of photons or neutrons is based on their interaction with the gas present in the detector chamber Absorption of the incoming photon or neutron by the gas is accompanied by local ionisation of the gas and the production of electrons and positive ions A Denex detector is in principle constructed like an electrical plate capacitor Fig 1 The plates are designated as electrodes with the anode connected to the pole and the cathode to the pole of the voltage supply U The electrodes are located opposite and parallel to each other and are separated by a small distance d within the gas filled chamber The voltage U applied to the electrodes serves to generate a homogeneous electric field F between them When photons or neutrons enters the space between the electrodes collisions with the gas molecules along their trajectories lead to the creation of positive and negative charge carriers ions and electrons which are then attracted to the correspondingly oppositely charged electrodes The resulting charge impulses can be measured as electrical impulses i Plattenkondensator
14. c curve for example 2600 V b Switch on the drift high voltage supply and set it to O V c Set the preselection counter such that the anode pulses can be counted for a time of 20 s 21 d Start the measurement and note the count rate After each measurement raise the drift high voltage in steps until the anode count rate no longer increases The measured values in the table are used to plot the characteristic drift curve illustrated in Fig 25 U Drift Count 10s U Drift Count 10s 0 11041 700 19649 100 12065 800 19873 200 13614 900 20365 300 15330 1000 20547 400 17020 1100 20939 500 17887 1200 20843 600 18975 1300 20974 Driftkennlinie Detektor DENEX 300TN Ger No 0704 0015 3He 5 bar CF4 1 bar N Quelle Am 241 Be 23041 21041 041 17041 15041 18041 1041 0 200 400 600 800 1000 1200 400 1600 Driftspannung V Fig 25 Characteristic drift curve of the DENEX 300TN detector The operating regime of the detector is determined from the course of the characteristic anode and drift curves We recommend that the anode voltage be set to a value in the range 2400 V to 2900 V Note that the drift voltage should not exceed the limit of 1000 V and that it should be held constant for each anode voltage 22 4 6 Adjusting the efficiency Equipment required A black He counter with impulse counter A He counter tube is placed
15. e in s U Anode V Fig 7 Characteristic curve for a detector Fig 7 shows a schematic of such a curve determined e g with a discriminator threshold of O V A distinctive feature of the characteristic curve is the so called plateau or Geiger M ller Regime which for good quality detectors should extend over a voltage range of more than 100V and a have slope of a few The operating voltage of the detector is selected such that random voltage fluctuations do not lead to departures from the voltage range of this plateau regime Electron avalanches created outside this voltage regime will propagate along the entire length of the counting wire The resulting charge impulses are much larger than those generated in the proportional regime and their magnitudes are independent of the nature and energy of the photons or neutrons entering the detector Impulsrate in s Spannung in V Fig 8 Characteristic detector curves for various discriminator thresholds The discrimination required for the separate measurement of neutrons in the presence of photons is here more difficult or even impossible to achieve In this case the anode voltage should not be raised above the average plateau value Above and beyond the average plateau value the count rate starts to increase more rapidly as a result of interference effects until finally a continuous gas discharge takes place that leads to destruction of the detector Fig 9 shows a circuit diag
16. ed by the presence of the additional constituent of the gas mixture the CF quenching gas but does generate undesirable signals whose count rates and amplitudes depend on the magnitude of the anode high voltage interference effects see section 2 5 The quenching gas CF4 also serves the purpose of limiting the path of the protons and tritium produced by the interaction of the neutrons with the counting gas secondary ionisation see section 2 2 and thus enables the desired positional resolution to be achieved Interactions between the photons y generated in the detector shielding and the quenching gas CF4 produce a further contribution to the background This can be mitigated by reducing the amount of the CF gas in the detector but this is not always an acceptable option An alternative is to adjust the zero crossing potentiometer on the CFD Constant Fraction Discriminator until the output of the discriminator shows a y count rate of zero The best approach here is to connect an oscilloscope to the main amplifier of the detector and measure the output signal amplitudes in volts The corresponding zero crossing potentiometers on the CFD can then be set with the help of a voltmeter 4 9 Switching off the detector The detector is switched off by reducing the anode and drift voltages to ZERO Volt The order in which the voltages are reduced is unimportant 4 10 Detector maintenance The detector is a closed system filled with noble gas with
17. htness and functionality When all these tests are successfully completed the detector is filled with the gas mixture He CF required by the customer and the detector as shown in Fig 20 is now ready for shipping 16 Fig 20 The finished DENEX 300TN B detector 4 Installation and initial operation of the detector 4 1 General precautionary measures for the handling and operation of the detector The DENEX 300TN detector is an extremely sensitive measuring instrument that should be treated with extreme care When the detector is put into operation or is running this means that voltage is being applied to the anode or the drift electrodes and it should be ensured that e the detector is not subjected to any mechanical forces such as jolts or vibrations however it may be mounted and operated on a goniometer stage e no sudden voltage changes are caused by removal of the SHV connectors from the anode or drift filter This could lead to damage of the anode and all the pre amplifiers It is recommended that the high voltage always be switched off via the high voltage supply unit which as a result of its very large time constant can avoid any unacceptably rapid voltage changes 4 2 Detector orientation Normally the detector is delivered with the geometry as illustrated in Fig 21 The resolution along the X axis is better than along the Y axis because the anode wires are oriented horizontally i e parallel to the X axis The detector
18. l cable Set the preselection counter such that the anode pulses can be counted for a time of ca 20 s Start the measurement and note the measured count rate After each measurement raise the high voltage in steps of e g 50 V until the anode count rate increases dramatically 20 The measured values in the table are used to plot the characteristic anode curve illustrated in Fig 24 U Anode Count 10s U Anode Count 10s U Anode Count 10s 1800 12 2350 24674 2900 29804 1850 14 2400 25712 2950 30428 1900 41 2450 26178 3000 31810 1950 311 2500 26413 3050 32745 2000 1536 2550 26938 3100 34370 2050 5084 2600 26956 2100 10347 2650 27019 2150 14460 2700 27433 2200 18709 2750 27984 2250 21954 2800 28600 2300 23405 2850 28878 Anodenkennlinie Detektor DENEX 300TN Ser No 0706 0015 Helium Isotop 3 5 bar CF4 1 bar Neutronenquelle Am 241 Be 40000 35000 30000 25000 20000 15000 Anodenz hirate 20s 10000 5000 1800 2000 2200 2400 2600 2800 3000 3200 Anodenspannung V Fig 24 Characteristic anode curve of the DENEX 300TN detector at a discriminator setting of 0 7 V 4 5 Recording the characteristic drift curve of the detector Equipment required Pre selection counter a Set the anode high voltage to an arbitrary value on the plateau of the characteristi
19. may be rotated by 90 to interchange the X and Y axes if e g a better resolution is desired in the vertical direction ZN 4 7 SS 8 Se an a Kat E pa l x E end mt JI e L D EV JN HV X S D Anode H F gt a 2 i GN AF gas EY lt Gasventill Drucksensor a Drift ay Fig 21 Front and rear views of the DENEX 300TN detector 4 3 Connecting the electronics to the detector First connect the anode and drift high voltages to the NIM high voltage supply using the two red cables supplied Take care that the anode is connected to the positive and the drift to the negative voltage respectively Connect the cathode preamplifier FTP 110 and the anode preamplifier FTP 111 with the NIM module N 145 using the LEMO cables supplied see Fig 22 Adjust the individual trimming potentiometers with a small screwdriver until the voltage of the corresponding channels reaches a value of approx 9 7 V To XO preamplifier To X1 preamplifier To YO preamplifier To Y2 preamplifier To anode preamplifier Fig 22 Connecting the detector preamplifier to the NIM module DENEX N 145 Connect the preamplifier to the NIM main amplifier TFA 111 using the SMA coaxial cable supplied
20. mend that you read carefully the instructions contained in this user manual in order to ensure that your detector functions optimally right from the start The detector as supplied is ready for operation It should be unpacked immediately following delivery even if the installation and start of operation is planned for a later date The transport container should be inspected for any signs of external damage Transport packaging should be completely removed The container and packaging material should be stored safely in case of any possible claims for damages 1 1 Guarantee declaration The detector you have purchased has been manufactured with great care under continuous quality control Should however any defects or faults be found over a period of 1 year from the date of purchase please inform Denex in writing either with information on the fault or a description of the problem at the address below Assistance will be given without charge This guarantee is void in the case of damage caused through accident e g mechanical shock improper operation e g the use of excessively high anode or drift voltages and acts of nature beyond control e g lightening flooding etc This guarantee will lapse if it transpires that in any way unauthorised changes are made to the detector In the case that the detector has to be shipped to Denex for repairs please ensure that the detector is free of contamination and appropriately packed as Denex ca
21. ngszeit Aufl sungzeit Totzeit Zeit SA U d f _S x we Zeie F TA 8 Ze N Ge j o Schwelle Diskriminator J 6 gt an T RaxC A 3 BAS S S Fig 6 Impulse voltage as a function of time S Discriminator threshold 2 4 Detection gases Noble gases such as helium argon or xenon are employed as detection gases whereby special measures have to be taken with regard to the termination of the discharge process This requires the use of stopping or quenching gases in particular hydrocarbons such as methane butane ethane carbon dioxide or tetrafluormethane The operating lifetime of closed detectors filled with noble gases is practically unlimited However leakage and ageing processes can lead to changes in the detector characteristics Decomposition of the hydrocabon gases as a result of the ionisation events contributes to such ageing processes These hydrocarbon gases are thus preferably employed in open detectors in which the detection gas flows dynamically through the detection chamber Depending on the specific application there are various types of proportional detector Detectors for low energy photons for example are fitted with thin walled entrance windows On the other hand detectors for the measurement of high energy photons have to be filled with gases of sufficiently high absorption capacity to achieve satisfactory detection efficiency In detectors for neu
22. nit AI Mg3 that supports a series of electrodes and which is closed by a pressure resistant cover plate Al Mg4 7M Fig 15 The neutron entrance window is integrated into the cover plate The size of the window 300 mm x 300 mm corresponds to the area spanned by the electrode wires The detection volume is limited by two drift electrodes each located at a distance of 5 mm from the X and Y cathodes The distance between the first and second drift electrodes corresponds to 20 mm The first drift electrode consists of a metallic foil which has been vulcanised with insulating material directly onto the window in order to avoid having a dead volume between the drift electrode and the entrance window All materials are chosen such that neutron absorption or scattering effects can practically be neglected 13 Fig 15 Diagram showing the housing the electrodes and cover plate of the DENEX 300TN detector 3 2 Quality control during production During the production process DENEX tests the detector with Fe und Co photons in order to check the quality of the individual components and the performance of the entire detector system Tests with Fe involve the measurement of photons with low energy 5 9 keV employing flowing Ar CO as counting gas The experimental set
23. nnot assume responsibility for damage that may be incurred during transportation Guarantee claims should be addressed to DENEX Detektoren f r Neutronen und R ntgenstrahlung GmbH St teroggestrasse 71 D 21339 L neburg Germany Tel 494131 248932 Fax 494131 248933 E Mail denex t online de Following expiry of the guarantee period the detector may also be returned to the above address for repair In this case repair costs will be invoiced EU Declaration of Conformity The Manufacturer of the Product covered by this Declaration is DENEX Detectors for Neutrons and X Rays GmbH St teroggestrasse 71 21339 L neburg Federal Republic of Germany The Directives covered by this Declaration 97 23 EEC Pressure Equipment directive PED Section EEC No L18158 1 from 9 7 1997 Test pressure 6 bar Calculated according to AD 2000 Bulletins 5D and W7 89 336 EEC Electromagnetic Compatibility directive as amended The Product covered by this Declaration 2D position sensitive detector for the registration of neutrons Type DENEX 300TN Serial No 0706 0015 The manufacturer hereby declares under his sole responsibility that the product identified above by reason of its design and construction complies with the essential health and safety requirements of the EU directives Attention The attention of the specifier purchaser installer or user is drawn to special measures and limitations to use that must be o
24. on independent of the signal amplitude this is followed by a CFD Constant Fraction Discriminator 2 8 Time measurement for position decoding Two principal methods for registering small time differences are available 1 The signals XO and X1 are transmitted via a Gate and Delay Generator for position specific detection One signal output of the delay line provides the start signal and the other the stop signal The measured time difference is transformed into a voltage by means of a TAC Time Analogue Converter and transmitted via an ADC Analogue Digital Converter to an MCA Multi Channel Analyser Alternatively as shown in Fig 12 the start stop signals can be transformed into digital addresses using a TDC Time Digital Converter and transmitted directly to the MCA 2 The anode signal provides the start signal and the cathode signal the stop signal The time difference is measured and registered as described in a and Fig 12 see Fig 13 Display X Start XStop XTAC Compact PCI Y Start Y ADC X ADC CF discriminator Y TAC Y Stop FAST ComTec MPA 3 7072T FAST ComTec CAMAC or VME Gate and delay generator Cathode preamplifier Drift filter VAX FN Ka Anode filter and preamplifier Test point HV Ariodeelectrod bi
25. ons are shown in Fig 19 Each channel corresponds to a distance of ca 0 3 mm FEB MPANT 1A x 18 5 File Window Region Options Action ABIES edel E Late Cell leste Teg Breze m eirzipleal Es re F7 ra eaipgn Zange 1024x1024 Real 68106 974 Run 68100 851 Status OFF 3000000 Singles D Coinc 294 202 106 SalRate 0 00 CoiRate 0 00 eS ADC1A 4 215824 Live 67950 798 E Dead 0 23 Total 288 912 811 Rate i ROI 70 543 705 Net 18100798 ADC1B GES Live 67951 472 4 S K Dead 0 23 Total 276 621 446 Rate 0 00 ROI 66 175 108 Net 9683695 ADC1C Live 0 000 Dead 100 00 Total 0 Rate ROI Net ADC1D Live Dead Total Rate 1247711 518928 89762 37332 15527 2686 ROI 0 Net o 100 20 30 w TotalSum 271332151 Cursor Counts Fig 18 Test of the positional resolution with a Fe source TEER Fig 19 Line scans in the X and Y directions of the spring and the cord After testing with Fe the detector is closed with a pressure resistant covering plate and subjected to pressure and leakage tests 5 bar overpressure with 10 mm bar s The detector is then filled with an Ar CO gas mixture at 1 bar and again tested for contamination using a Co source The detector is then heated and evacuated over a period of 5 6 days to remove hydrogen Subsequently the detector is tested once more with respect to gas tig
26. position sensitive detectors a Linear arrangement b Circular arrangement The DENEX solution to this problem is to combine each individual counting tube into the proportional counter The cathodes are connected via a delay line Gehause Non Gasventil a Delay Line Ausgang XO ea N Anodedraht A C rs Fig 11 Design principle of the DENEX 1D Detector Fig 11 shows the arrangement used A single anode wire is located perpendicular to the array of cathodes The signals are registered in the individual cathodes and are transmitted via the delay line through the signal outputs XO and X1 to the corresponding electronics The spatial resolution is simply given here by the distance between one cathode and the next one In a 2D detector an additional array of cathodes perpendicular to the first array is employed 2 7 The delay line readout method The advantages of the delay line readout method compared to other readout systems are as follows a parallel signal read out wire per wire b large frequency range c simple electronics for signal handling d signal amplitude independence A delay line consisting of L und C elements with a total delay time depending on detector type from ca 100 ns to 400 ns and an impedance of 50 Q is employed The first electronic element connected to 10 the output of the delay line is a linear low noise impulse pre amplifier with 56 dB amplification For correct timing signal informati
27. r both CFD channels to identical positions Note that the LEMO coaxial cable of the CFD output that provides the stop signal has to be at least 5 cm longer than the cable providing the start signal Open up the neutron beam to the detector By toggling the jumper between the two channels of the CFD measure the optimum delay t that generates the narrowest peak width Fig 30 Repeat this procedure for all the remaining channels X1 YO Y1 and the anode TIN ie pDENEX g TFA 111 1 N AA gt To theTDC OUT From XO preamplifier 75 Ma Stop From X1 preamplifier From YO preamplifier From Y1 preamplifier our From anode preamplifier J L B m MAINAMPLIFIER Z Fig 29 Adjustment of the internal CFD delay 25 Messung mit 2 ns interne CFD Fig 30 Programming the delay rin the CFD 4 8 Gamma y suppression An essential requirement for optimum background reduction is the effective shielding of the detector with polyethylene and boron carbide plates to prevent interference by stray neutrons from the reactor or from neighbouring experiments Successful elimination of these sources of interference will not 26 however result in zero background because of the contribution caused by ionisation of the unavoidable impurities in the counting gas of the detector as occurs e g in fluorescent lighting This process is halt
28. ram of a proportional counting tube with its associated electronics Start Stop Reset J l kk A Teimer Impulsverst rker CH Z hler e Il gt Diskriminator J Ck gi Koinzidens Gate Ra Il pa Cp l anan Rb IH Ci we Fig 9 Circuit diagram of a proportional counting tube with electronics C Coupling capacitor Cp Pump capacitor C Integrator capacitor Ra Work resistor Rp Current limiting resistor 2 6 The position sensitive proportional counter The position sensitive proportional counter is designed to ascertain the specific position in a radiation field from which the incoming photon or neutron originates As shown in Fig 10 a 1 D position sensitive detector for the detection of scattered photon or neutrons can be constructed by arranging counting tubes next to each other in arrays The positional resolution of such devices corresponds to the diameter of the counting tubes employed Between the individual counting tubes there is a dead zone which is created by the walls of the counting tubes Such arrangements are very expensive and difficult to operate since each counting tube needs its own electronics and these have to be exactly trimmed to match each other and should show the same temperature drift Quelle Elektronik Elektronik S b Fig 10 Arrangements of proportional counting tubes as
29. tal pulse height Fig 27 right yielding trigger times Zero Crossing independent of pulse height 23 EN a 4 Ni ANA No A Vi VA VA N L E Z N Walk t Fig 27 Comparison of threshold triggering left and constant fraction triggering right Principle of operation The incoming signal is split into two components One of the components is delayed by a time t where 0 t lt t and is inverted while the other component is multiplied by a factor c where 0 lt c lt 1 The two components are added together again and the first zero crossing with a positive first derivative is taken as the trigger time Fig 28 Threshold 1 shot Out l Zero Crossing x D Q Fig 28 Function diagram of a CFD 24 Programming the delay t on the CFD Each channel in the CFD 8000 from Ortec has an integrated delay unit which can be programmed in a stepwise manner between 2 ns and 12 ns by plugging in a jumper In order to programme the correct delay the following procedure is recommended method devised by Dr Thomas Wilpert HMI Berlin Connect the A and B outputs of any of the channels e g the X channel of the main amplifier TFA 111 to the CFD using LEMO coaxial cables of equal length as shown schematically in Fig 29 Set the jumper of the delay unit fo
30. trons special gas mixtures are employed in which especially strongly ionising secondary particles are generated The large charge impulses that result can easily be distinguished from the relatively small impulses generated by photons This special feature of proportional counters therefore makes them especially suitable for the separate measurement of neutrons in the presence of photons 2 5 The characteristic curve Following the arrival of an impulse in a proportional detector no further impulse can be detected during the so called dead time see Fig 6 After the dead time has expired impulses first reattain their original magnitude when the so called recovery time has expired T R x C where R anode resistance Ra and C detector capacity For registration in a counting device the resolution time is decisive The time period after resolution of an impulse is determined by the discriminator threshold S resolution time The amplitude of the impulses depends only on the detection gas the thickness of the anode wire and the applied voltage U although the amplitude of the impulses generally increases with increasing anode voltage U The appropriate operating voltage for the detector is established by determining the so called characteristic curve which illustrates the dependence of the count rate on the applied voltage U Plateau RQ Geiger M ller Dauergasentladung Proportionalbereich Impulsrat
31. up is shown in Fig 16 Fig 16 Testing the DENEX 300TN detector with a Fe source Initially experiments are carried out to test the homogeneity of the signal over the entire area of the detector Effects due to defective wires or entrapped dust particles are easily identified and can be rectified immediately by replacement or removal The next experiment involves determination of the anode characteristic curve Fig 17 which allows the quality of the detector to be assessed In this case the count rate vs applied voltage curve is characterised by an excellent plateau regime between Ua 2670 V and Ua 2740 V Kennlinie DENEX 300TN 55Fe und Ar Co2 60000 50000 40000 30000 Reihe1 Count 10s 20000 10000 0 2450 2500 2550 2600 2650 2700 2750 2800 Ua kV Fig 17 Anode characteristic curve of the DENEX 300TN B detector determined with a Fe source Further tests are performed with the Fe source in order to check the positional resolution of the detector The image of the pliers so obtained Fig 18 allows specific details such as for example the spring between the handles or the suspending cord with diameters of ca 0 6 mm to be resolved Although this test object is very simple it enables the spatial resolution of the detector to be tested effectively The image in Fig 18 contains in total 1024 x 1024 channels Line scans across the image in the X and Y directi
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