- Nucleonix
Contents
1. Distance where Dose rate is in MR milli Roentgen Source Activity is in mCi milli Curies Distance is in cm Centimeters Gamma constant for Cs 137 is 3300 and gamma constant for Co 60 is 13200 14 B EXPERIMENTS ILLUSTRATING THE PRINCIPLES OF NUCLEAR PHYSICS Exp 1 STUDY OF THE CHARACTERISTICS OF A GM TUBE 1 1 1 2 1 3 PURPOSE To study the variations of countrate with applied voltage and thereby determine the plateau the operating voltage and the slope of the plateau EQUIPMENT ACCESSORIES REQUIRED e G M Counting System GC601A GC602A with A C main chord e G M Detector End window stand or G M Detector source holder bench optical bench e Detector in PVC cylindrical enclosure with connecting cable PROCEDURE e Make the connection between counting system to G M Detector by MHV to UHF co axial cable Also connect the mains chord from the counting system to 230V A C Power refer to Fig 1 e Place a Gamma or Beta source facing the end window of the detector in the source holder of G M stand or optical bench at about 2 cms for Gamma source or 4 cms for Beta source approximately from the end window of the detector For Beta source ensure that countrate is less than 200 CPS at 500V e Now power up the unit and select menu options to PROGRAM on the keypad of the G M Counting System and select 30sec preset time typically It can be in the range of 30 to 602. 6 180 30 710 21836 538 2400 3000 10750 196 6 3300 11593 192 38 50 12x 1874 Background 92 5 min 18 4 min HALF LIFE OF INDIUM FOIL 116 In Fig 22 Half life of Indium Foil 44 8 4 ANALYSIS AND COMPUTATIONS 8 5 8 5 Intensity of radioactive source changes with time in accordance with relation 1 is the decay constant Lis the intensity at any time t and is initial intensity The by definition is the time required for the intensity to fall to one half of its initial value 1 2 Hence from equation 1 we have In II In 0 5 0 693 Where dus is half life 7 The above equation can be written as T 0 693 1 2 Given the value of T one can calculate the value of 1 2 HALF LIFE DETERMINATION The Log is actually natural Log and should be denoted by In EXERCISE e Subtract the background countrate from each measured countrate Plot a graph of In N vs elapsed time min This should give a straight line graph From the plotted graph extrapolate and obtain T Substitute T in the above equation to calculate the decay constant 7 45 9 1 9 2 9 3 EXPERIMENTS ILLUSTRATING APPLICATIONS OF RADIONUCLIDES DEMONSTRATION OF NUCELONIC LEVEL GAUGE PRINCIPLE USING G M COUNTING SYSTEM amp DETECTOR This experiment has been designed suggested for engineering streams Mechanical Metallurgic
3. EXPERIMENTS WITH G M COUNTER G M Counting System with G M End window stand and G M Detector source holder bench Edited by J Narender Reddy B E M Tech Dr M S R Murty M Sc Ph D Published by NUCLEONIX SYSTEMS PRIVATE LIMITED Plot No 162 A amp B PHASE II I D A Cherlapally Hyderabad 500 051 Website www nucleonix com Ph 91 040 27263701 FAX 040 27262146 e mail infoGnucleon com PREFACE This is the second revised experimental manual for a product which is manufactured and fully supported in all respects Nucleonix Systems supplied instrumentation for studies in Physics provides an experimental manual of this nature to enable university faculty to utilize the equipment both for teaching and research labs Considerable efforts have been put in preparing the manuscript for this experimental manual Editors have gone through this and reviewed the manuscript thoroughly and made corrections However if there are any errors or omissions you are requested to write to us There may be still scope to add a few experiments to this manual We welcome feedback on new additions to this from professors and others from scientific community We may include such additional experiments in future editions if found suitable This manual on Experiments with G M counter has been written to include the important information such as basic definitions on radiation units and fundamentals of Nuclear amp
4. 91 1 5491 2 0094 logd2 logd1 0 6989 0 4770 2 07 Gradient 21 Exp 3 STUDY OF NUCLEAR COUNTING STATISTICS 3 1 3 2 3 3 INTRODUCTION Systematic errors control the accuracy of a measurement Thus if the systematic errors are small or if you can mathematically correct for them then you will obtain an accurate estimate of the true value The precision of the experiment on the other hand is related to random errors The precision of a measurementis directly related to the uncertainty in the measurement Random errors are the statistical fluctuations during a measurement If these values are too close to each other then the random errors are small But if the values are not too close then random errors are large Thus random errors are related to the reproducibility of a measurement STATISTICAL ANALYSIS OF DATA To minimize these errors one should have good understanding on Statistical analysis of data DEFINITIONS e Mean Mean is the average value of a set of n measurements in an experiment Mathematically it is defined as Mean is also called as average value e Deviation Deviation is the difference between the actual measured values and the average value Deviation from the mean di is simply the difference between any data point Ni and the mean We define this by When we try to look at the error or average deviation the value probably will become zero because we may have both positive and
5. 10 ves 27 281 o9 10 11 793 3 281 010 32 118911 1 8 13 773 17 281 060 05 IPSIS 1 9 15 8310 20 2831 070 05 17 796 6 21 021 18 796 6 281 020 0 20 786 4 281 0o14 90 21 0 5 22 71 49 2314 174 2 23 762 23 281 099 __ C25 r4 6 21 0o92 27 founded 26 773 17 281 060 05 28 792 2 281 007 0 20 818 28 281 099 1 0 5 791 1 28 1 0 823 35 z763 277 231 09 1 37 807 17 281 060 o5 39 7909 o 2314 o 0 41 762 23 281 099 1 44 20 45 46 2331 0o23 0 47 48 z 2314 02 4 0o 49 5 ER NI 28 1 0 17 784 50 785 The average count rate for n independent measurements is given by 1 2 n DM M LE PN nsus The deviation of an individual count from the mean is N N From the definition N it is clear that n X N N 0 i 1 The Standard Deviation o NN 28 3 9 EXERCISE Make a plot of the frequency of rounded off events Ni N Vs the rounded off values Fig 14 Below shows ideal situation which is a Gaussian or Normal Distribution STUDY OF NUCLEAR COUNTING STATISTICS OCCURANCE ps o 2 e ue ROUNDED OF VALUES Fig 14 Plot of Frequency of Occurrence Vs R
6. Radiation Physics general information on G M Tubes and their characteristics working principle and a list of G M Detector experiments which illustrate some of the important fundamentals of Nuclear Radiation amp its characteristics There is also condensed information provided on various G M Counting System models along with accessories which will help in having better understanding while going through this experimental manual Of course for more detailed information one can go through counting system user manual for operation and commands description Additionally basic calculation procedure on activity and dose rate as on a given date by knowing the activity on the date of manufacture of source are also given in this manual Apart from understanding the Physics principles by doing these experiments one will also know that there are quite a few Engineering and Industrial applications where nuclear techniques are employed using nuclear radiation detectors such as G M Detectors and Nal Scintillation detectors Typically applications are given below i Nucleonix level gauging in steel and cement Industries using G M detector and a radiation source ii Gamma column scanning in petrochemical industries using Nal Scintillation detector based system and other applications include detection of Liquid fill height for beverages soups pharmaceutical products baby foods Match boxes yogurt cartons etc for sorting or counting item
7. e Alpha radiation Radiation that consists of high energy helium 4He nuclei emitted during alpha disintegration of atomic nuclei Alpha particles possess discrete initial energies line spectra which are characteristic of the emitting nuclide e Becquerel Bq Name of the derived SI unit of activity Number of radioactive transformations or isomeric transitions per second 5 1 Bq 27pCi 1 KBq 27nCi 1 MBq 27 Ci 1 GBq 27 mCi 1 TBq 27Ci e Beta decay Radioactive conversion accompanied by the emission of a beta particle i e a negatively charged electron decay or a positively charged electron decay When a negatively charged electron is emitted a neutron in the atomic nucleus is converted to a proton with the simultaneous emission of an antineutrino so that the proton number Z is increased by 1 When a positively charged electron positron is emitted a proton in the nucleus is converted to a neutron with simultaneous emission of a neutrino so that the proton number Z is decreased by 1 e BetaRadiation Radiation that consists of negative or positive electrons which are emitted from nuclei undergoing decay Since the decay energy or if it is followed by gamma radiation the decay energy less that photons energy is statistically divided between beta particles and neutrinos or antineutrinos the energy spectrum of beta radiation is continuous extending from zero to a maximum value characteristic of t
8. G M Tube will produce the maximum number of counting pulses per second limited by its dead time and the circuit in which it is incorporated However due to the characteristics of a specific circuit the indicated counting rate may fall appreciably even to zero If dose rates exceeding 10 times the recommended maximum for window tubes or 100 times for cylinder tubes are likely to be encountered it is advisable to use a circuit that continuously indicates saturation Dead Time Losses After every pulse the tube is temporarily insensitive during a period known as the dead time Tq Consequently the pulses that occur during this period are not counted At a counting rate of N count s the tube will be dead during NxT g of the time so that approximately of the counts will be lost In an experiment if the inaccuracy in counts due to dead time must be 196 N should be less than 1 100 Td counts Example If 20m sec an inaccuracy of 1 is reached ata counting rate of approximately 500 counts sec Background The most important sources of background count are a Gamma radiation from the environment and from cosmic radiation b Mesons from cosmic radiation c Beta particles from contamination and impurities of the materials from which the detector itself is made d Spontaneous discharge or pulses in the detector and the counting circuit that do not originate from radiation Electronic noise From published exp
9. becomes equal to or less than half the count rate with zero absorber thickness e Data is to be collected for the standard source and the second source e Here in this case the standard source is TI 204 and the second source is Sr 90 e Tabulate the data as shown in table e Density of Aluminium 2 71g cm g cm cube e X The below data is taken with Thallium TI 204 33 Table 1 Counting Time 180 sec Absorber Aluminium Background 146 counts Source TI 204 4 KBq Thickness in mm in mg cm counts BG 0 25 67 75 81 30 se so 94 85 597 451 540 49 xs os i285 m 30 The below data is taken with Strontium Sr Table 2 Counting Time 100 sec Absorber Aluminium Background 79 counts Source Sr 90 Thickness in mm in mg cm counts BG a ee UNDER 0 35 3110 060 1 16260 2582 2503 34 Figure 16 176 15 Figure 17 35 5 4 ANALYSIS amp COMPUTATIONS 5 4 1 PRINCIPLE The range of Beta particles is given by R 0 52 E 0 09 g cm 1 Where E is the end point energy of of Beta rays from the radioactive source in MeV We have the ratio of thickness required to reduce the counts of Beta rays from one source to half to the thickness required for the other source is given by Range of Beta rays from first source tz Range of Beta rays from second source Lb R 1 72 E 1 zd 2
10. it can be concluded that the counts due to Back scattering increases upto certain thickness of the scattering material and almost remains constant beyond that thickness The thickness of the scatterer where the counts reach their maximum is called the Saturation thickness 40 Exp 7 PRODUCTION AND ATTENUATION OF BREMSSTRAHLUNG 7 1 7 2 INTRODUCTION Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged partice typically an electron by an atomic nucleus The moving particle loses kinetic energy which is converted into a photon be cause energy is conserved The term is also used to refer to the process of producing the radiation Bremsstrahlung has a continuous spectrum which becomes more intense and whose intensity shifts toward higher frequencies as the change of the energy of the accel erated particles increases Beta particle emitting substances sometimes exhibit a weak radiation with continuous spectrum that is due to Bremsstrahlung In this context Bremsstrahlung is a type of sec ondary radiation in that it is produced as a result of stopping or slowing the primary radiation Beta particles It is very similar to x rays produced by bombarding metal tar gets with electrons in X ray machines The amount of Bremsstrahlung increases as the atomic number density of the absorbing material goes up If the mass per unit area thickness X density of th
11. CPS Fig 15 Detector source arrangement for efficiency calculation for a gamma source 30 Gamma source emits radiation isotropically in all directions 411 geometry However only fraction of itis received by the end window detector This fraction is given by 4 2 4 1602 The present activity of the gamma source used for this experiment is 111 This gamma source is radiating isotropically in all directions A fraction of this only is entering the G M Tube which is given by 2 d R A pue 111000 x 0 001406 156 066 This is the fractional radiation entering the detector Hence efficiency of the detector for the gamma source at a distance D 10 cm PS N Efficiency E 7 E DPS R 61 oe 0 0231 2 31 156 066 Note CPS Counts per Second DPS Disintegrations per Second falling on the window of the ditector B EXPERIMENT TO ESTIMATE EFFICIENCY FOR A BETA SOURCE INTRODUCTION Equipment required amp procedure remains the same as detailed under 5 285 3 The only difference is here we place Beta source about 2 cm close to the end window amp calculate Intrinsic efficiency which do not take geometry factor into consideration PROCEDURE e Make standard arrangement amp interconnections for G M counting system detector G M stand e Place Beta source close to End Window approx 2cm from end window Record counts for a minute with and without source Take three rea
12. Half thickness The thickness of material layer that reduces the intensity of initial radiation by a factor of two lonising radiation Radiation that consists of particles capable of ionising a gas Isotopes Nuclides with the same atomic number but different atomic weights Mass numbers Mass per unit area Product of the density of a material and its thickness Nuclide Generic term for neutral atoms that are characterized by a specific number of neutrons N and protons Z in the nucleus Quality factor A factor which in radiation protection allows for the effects of different types of radiations and energies on people Rad Name for a unit of absorbed dose 1 rad 1072 J kg 10 2 Gy Radioactivity The property which certain nuclides have of emitting radiation as a result of spontaneous transitions in their nuclei Rem rem Roentgen equivalent man Name for a derived unit of dose equivalent a measure of the biological effect of radiation 1 10 2 J 10 mSv Roentgen R Name for a derived unit of exposure dose 1R 2 58 x 10 4 Sievert Sv The SI unit of dose equivalent 1 Sv 1 J DESCRIPTION ON G M COUNTING SYSTEM GC601A GC602A Nucleonix systems offers two models of G M Counting Systems One an economy model GC601A with optimal features and the other GC602A with more advance features The following paragraphs illustrate important features of both models with front
13. IMATE EFFICIENCY FOR A GAMMA SOURCE 4 1 INTRODUCTION By knowing the activity of a gamma source it is possible to record counts with the source for a known preset time amp estimate the efficiency of the G M detector 4 2 EQUIPMENT ACCESSORIES REQUIRED e GM Counting System GC 601A GC602A G M Detector source holder stand 56200 or bench 58201 Radioactive source kit SK210 G M detector in cylindrical enclosure GM120 Necessary connecting cables 4 3 PROCEDURE e Make interconnections such as mains power cord to GC601A 602A unit and connection between G M detector holder mount to rear panel of GC601 602 through HV cable e Place a gamma source in the source holder facing the end window detector Typically the distance between the source to end window of G M tube can be 10 cm e Now record counts for about 100 sec both background and counts with source and make the following calculations and analysis 4 4 ANALYSIS AND COMPUTATIONS e Let D be the distance from source to the end window e Let d is the diameter of the end window e Lt Ns Counts recorded with source Counts recorded due to background e Now make the following measurements Background counts in 100 sec 71 Average of three readings Distance from source holder to end window D 10cm Diameter of end window d 1 5cm No of counts recorded in 100sec with the source Ng 432 From the above data the net count rate recorded N Ng Np 100 cps 3 61
14. M S R Murty Editors EQUIPMENTS REQUIRED FOR DOING THE EXPERIMENTS Most of these items mentioned below are manufactured and or supplied by NUCLEONIX SYSTEMS EQUIPMENT SYSTEM TYPE 1 Geiger Counting System GC601A or GC602A 2 EndWindow G M Tube Halogen Quenched GM120 amp GM125 in cylindrical PVC enclosure 3 GM Detector End window stand SG200 or 4 Sliding bench for G M experiments SB201 5 Radioactive Source Kit containing one Beta Source SK210 204 amp one Gamma Source 5 137 b Another Beta Source Sr 90 6 Lead Shielding Optional LS240 7 Aluminium Absorber Set AA270 8 Absorber Set for scattering of Beta particles experiments AS272 9 Absorber Set for production and attenuation Bremsstrahlung experiment AS273 10 Cs 137 Ba 137m isotope generator or Indium foil and Neutron Howitzer for generating short lived isotope Note Item No 10 is not offered by Nucleonix systems CONTENTS DESCRIPTION PAGE NO GENERAL i General Information on Geiger Muller Tubes 01 05 ii Important definitions 06 08 ili Description on G M counting system GC601A GC602A 09 12 iv Activity amp Dose rate calculations procedure 13 14 EXPERIMENTS ILLUSTRATING PRINCIPLES OF NUCLEAR PHYSICS Study of the characteristics of a GM tube and determination 15 17 of its operating voltage plateau length slope etc Verification of Inverse Square Law for y rays 18 21 Study of nuclear counting st
15. TIVITY FOR TWO BETA AND TWO GAMMA SOURCES BETA SOURCES Sr 90 3 7 KBq Oct 2006 Half life for Sr 90 is 1 2 28 5 Yrs Activity Ag 3 7 as on Oct 06 3700 Elapsed time till Sept 07 11months Present activity A Ao e 0 693 T 1 2 t as on Sept 07 T42 28 5yr t 11 12 0 9166yr 3700 e 0 693 28 5 0 9166 3618 6 TI 204 11 1 KBq Oct 2006 Half life for TI 204 is 4 2 4Yrs Activity Ag 11 1 KBq as on Oct 06 11100 Bq Elapsed time till Sept 07 11months Present activity A Ag e 0 693 T1 2 t as Sept 07 Mie Ayr t 11 12 0 9166 41100e 0 693 4 0 9166 946941 Ba 13 GAMMA SOURCES Cs 137 3 1uCi July 07 Half life for Cs 137 is T1 2 30Yrs Activity Ap 3 1uCi as on Oct 06 3 1 3 7 1010 10 6 114700 Elapsed time till Sept 07 2months Present activity A e 0 693 T t as on Sept 07 4 2 0 t 2 12 0 1666yr 444700 0 693 30 0 1666 114264 14 Co 60 3 7uCi July 07 Half life for Co 60 is T4 2 5 3Yrs Activity Ag 3 7 uCi as on Oct 06 3 7X3 7X1019x10 6 136900 Bq Elapsed time till Sept 07 2months Present activity A Ao e 0 693 1 2 t as on Sept 07 4 2 5 t 2 12 0 1666yr 436900 e 0 693 5 3 0 1666 183961 Bq b DOSE RATE CALCULATION Dose rate can be calculated by using the following formula Source Activity x gamma constant Dose rate
16. al Instrumentation Electronics amp Instrumentation Chemical Engineering streams for illustrating Nucleonic level gauge principle INTRODUCTION amp PRINCIPLE Nucleonic level gauging is a very popular technique used in petrochemical steel cement amp other process industries where liquid sludge solid material levels in the reactor vessels smelting furnaces are to be detected once it reaches certain levels by indirect methods This technique using G M Detector involves placing of a radioactive source at the required height where it is to be measured It is shielded all around amp opening is provided as a collimated beam in the required direction for measurement A G M detector is placed exactly on opposite side outside the reactor vessel such that it sees the collimated beam of radiation Once the liquid solid material level reaches this height there will be change in the count rate With material not being filled in the reactor vessel to the required level the count rate will be higher initially Once the material level reaches the level where it interrupts the source detector beam the count rate changes abruptly This change can be detected and an alarm or relay contact could be activated DEMONSTRATION OF THIS PRINCIPLE IN LAB ITEMS REQUIRED e G M Counting System with A C Mains cord e G M detector with holder amp long connecting cable of 2 metres e A 3 PVC pipe with one end closed and other end open e Gamma so
17. alculate plateau percentage slope and plateau length operating voltage etc Table 1 GM Characteristics Data Background Counts Corrected Counts Nc N N N b BE TON BN EE D S 3 _ 172 35 169 6 480 _ 1792 36 1756 8 540 1779 9 s7ow 181 1 _ 10 eo 207 76 293 ANAYSIS amp COMPUTATIONS Estimate from the tabulated readings e V4 Starting voltage of plateau 360V Just after rising edge of knee e 2 Upper threshold of the plateau 570 V Just before the start of discharge region e Plateau length VPL 2 V4 570 360 210 V V2 V1 5704360 e Operating voltage Vo 5 465V 16 e The slope ofthe plateau is given by N2 N1 100 Slope Percentage X X 100 N1 V2 V1 1781 1677 100 X X 100 2 95 1677 570 360 Where N1 and N2 are the count rates at the lower and the upper limits of the plateau and V1 and V2 are the corresponding voltages Slope less than 10 is desirable G M Characteristics 4000 3500 3000 M a 5 6 Counts T306sec 5 S 1000 Fig 6 Plot of counts Vs EHT 1 5 CONCLUSIONS e From the plateau it can be noticed that mid point of the characteristics of the GM tube is defined as operating voltage and is to be used for counting applications The tube is operated at this voltage when used in Radiation Mon
18. amp rear panel photographs Geiger Counting System type GC601A is an Advanced Technology based economy model designed around eight bit microcontroller chip This system with accessories is an ideal choice for teaching demonstrating various G M Experiments as a part of Experimental Physics lab to U G P G Science and U G Engineering students Other streams such as Radiation Physics Radiochemistry Radiation Biology and Agricultural Sciences can also use this system This counting system can be used for carrying out a number of Nuclear Physics experiments G M COUNTING SYSTEM AT Fig GM Counting System GC601A Front amp Rear panel view Geiger Counting system type GC602A is an Advanced Technology based versatile integral counting system designed around eight bit microcontroller chip This system is highly recommended for research work apart from its usefulness in the academic fields for teaching This system along with wide end window G M Tube Type GM125 and Lead Castle will serve as an excellent Beta Counting System useful for swipe sample counting by Health Physics Labs This counting system is useful for carrying out a number of Nuclear Physics experiments G M COUNTING SYSTEM AT Fig GM Counting System GC602A Front amp Rear panel view Important features in the two models are given below o High voltage output 0 to 1500V 2mA ripple less than 20mV o Visual display 16x2 LCD dotmatrix character di
19. atistics 22 29 Estimation of Efficiency of the G M detector for 30 32 a Gamma source amp b Beta Source To Study Beta Particle Range and Maximum Energy Feather Analysis 33 37 Backscattering of Beta particles 38 40 Production and Attenuation of Bremsstrahlung 41 42 Measurement of short half life 43 45 EXPERIMENTS ILLUSTRATING APPLICATIONS OF RADIO NUCLIDES Additional experiments recommended for Engineering stream B E B Tech for Physics Engineering Physics Instrumentation Labs General physics labs in 1st year Demonstration of Nucleonic level gauge principle using 46 47 G M Counting System amp Detector Beam interruption detection system to check packs for content level or 48 49 counting of individual items uoueg 103206190 WD pue puejs W S uejs s W 9 614 LV EPETCIGLIPEDIR E S 00295 pugs 10 99 0q W 9 614 10295 siueuiuedx3 W S 104 uoueq BurpiS z 614 suewpedxg 10 u ueg jeondo 2 J9pJoH uns0 2u9 uj 410190190 W S pug A GENERAL INFORMATION GENERAL INFORMATION ON GEIGER MULLER TUBES Geiger Muller radiation counter tubes G M Tubes are intended to detect alpha particles beta particles gamma or X radiation A G M tube is a gas filled device which reacts to individual ionizing events thus enabling them to be counted A G M Tub
20. cket fixed on to it GM Detector ip onines Sliding bench for G M Experiments SOURCE KIT 1 TYPE SK210 Source Kit 1 type SK 210 offered by NUCLEONIX contains one each of Beta and Gamma sources These are low active disc sources of the order of 0 2 to 3 micro curie for Beta amp Gamma Gamma source disc is evaporated and sealed inside 25mm dia Lai X 5mm thick plastic disc Whereas Beta source disc is evaporated amp sealed in a 2bmm X 10mm thick plastic disc and covered with 10mg sq cm aluminised mylar foil 11 ALUMINIMUM ABSORBER SET TYPE AA 270 Aluminimum Absorber Set Type AA 270 consists of absorber discs in different thicknesses ranging from 20 to 300 mg cm sq Each of these absorbers is mounted in an individual plastic frame which exactly fits into the absorber tray holder of the G M stand G M sliding bench The diameter of each disc is approximately 50 mm including the frame There is identification number for each disc printed on it All these discs are housed in this acrylic box This absorber set will be useful in studying the Beta absorption coefficient using G M Counting systems LEAD CASTLE TYPE LS240 The Lead Castle is designed to shield the G M Counters from counting background radiation Lead Castle type LS 240 can house G M counter mounted in a G M stand This shield is of 40 mm thickness and is built up of six interlocking rings The top and bottom are covered by simi
21. d disc bmm thick with 2mm central hole and 1 dia in size to cover the face of the detector 47 10 BEAM INTERRUPTION DETECTION SYSTEMS 10 1 INTRODUCTION amp PRINCIPLE With narrow beam of source G M detector and electronic counting system having preset alarm facility one can illustrate how changes in count rate observed when a bulk material passes and interrupts the beam There are a variety of applications in process manufacturing industry where this technique could be used Some of these include e Liquid fill height level for beverages soups pharmaceutical products baby foods Match boxes yogurt cartons etc e For sorting or counting items in a process or pharmaceutical industry etc 10 2 DEMONSTRATION OF THIS PRINCIPLE IN LAB 10 3 ITEMS REQUIRED G M counting system GC601A GC602 A with A C power cord G M detector with stand or bench Radioactive source beta A set of 4 5 coins fixed with adhesive and spaced equidistance on a thin Perspex sheet Fig 24 G M Counting System detector source amp perspex sheet fixed with coins arrangement for source beam interruption experiment 48 10 4 PROCEDURE e Make standard interconnections for the counting system s functionality e Switch on the system and operate in CPS mode e Place gamma source in the source holder and ensure to have a distance of atleast 2cm between the end window of the detector and gamma source holder e Now place this Perspex
22. dings take average of them and tabulate e Record distance of the source from end window e Calculate the present day activity in DPS of the source refer to procedure given at the end of the manual e Calculate net CPM CPS counted e Intrinsic efficiency can be calculated as the ratio between CPM DPM x 100 or CPS DPS x 100 This will be efficiency of the end window detector for the given Beta Source at that distance 31 DATA COMPUTATION amp ANALYSIS Beta source used Sr 90 Activity AO 5 55 KBq as on Aug 2006 Activity A 5 373 KBq as on Dec 2007 use procedure given on pages 13 amp 14 Background count rate 57 CPM Counts recorded with source Average 14427 CPM Corrected counts 14370 CPM Net count rate 239 5 CPS Efficiency E of the End window detector with Beta source Sr 90 at 2 0 cm distance CPS E 0 0446 4 46 DPS 4 5 EXERCISE By knowing the efficiency of the G M detector for a particular Gamma energy at a specified distance amp geometry one can further calculate the following parameters namely activity of the source as on the day of experimentation of course it is assumed that activity of the standard is known as on the date of manufacture and also the activity of the unknown source if any with the same energy It can be noticed that End Window detector will have much better efficiency for Beta Source compared to a gamma source By knowing efficiency for a Beta so
23. e consists of basically an electrode at a positive potential anode surrounded by a metal cylinder at a negative potential cathode The cathode forms part of the envelope or is enclosed in a glass envelope lonizing events are initiated by quanta or particles entering the tube either through the window or through the cathode and colliding with the gas molecules The gas filling consists of a mixture of one or more rare gases and a quenching agent Quenching is the termination of the ionization current pulse in a G M tube Effective quenching in G M Tube is determined by the combination of the quenching gas properties and the value of the anode resistor e The capacitance of a G M Tube is that between anode and cathode ignoring the capacitive effects of general connections e OPERATING CHARACTERISTICS e Starting Voltage Vg This is the lowest voltage applied to a G M Tube at which pulses just appear across the anode resistor see Fig 4 and unit starts counting e Plateau This is the section of the GM characteristic curve constructed with counting rate versus applied voltage With constant irradiation over which the counting rate is substantially independent of the applied voltage Unless otherwise stated the plateau is measured at a counting rate of a approximately 100 counts e Plateau threshold voltage V4 This is the lowest applied voltage which corresponds to the start of the plateau for the stated sensitivity of the measur
24. e plates used as absorbers is such that the beta particles are completely absorbed then for materials of higher atomic number density correspondingly higher bremsstrahlung count rates are obtained EQUIPMENT AND ACCESSORIES REQUIRED Electronic Unit GC 602A G M Detector GM125 GM Detector Holder Sliding Bench Source Holder Absorber Holder for Bremsstrahlung experiment Beta Source Sr 90 Al 0 7mm Cu 0 3mm amp Perspex 1 8mm absorber set Fig 20 Expermental setup 41 7 3 7 4 7 5 PROCEDURE Make standard setup by connecting G M Counting system GC602A with G M Detector GM125 placed in the optical bench as shown in Fig 20 above The GM Detector Absorber and the Source are mounted as shown in Fig 20 Switch ON the GC602A Electronic Unit and set the operating High Voltage at 500V An absorber consisting of two materials with widely different atomic numbers say Perspex 1 8mm thick and Aluminum 0 7 mm thick is used and the count rate is measured with the absorber and then with the absorber reversed The absorber thickness must be such that each sheet of absorbent material has about the same mass per unit area The experiment is conducted with following three combinations of materials i Al 0 7mm amp Perspex 1 8mm ii Perspex 1 8mm amp Cu 0 3mm iii Al 0 7mm amp Cu 0 3mm EXPERIMENTAL DATA amp RESULTS Source Sr 90 Distance between source and detector 6cms Act
25. ecreasing the value of the anode resistor not only decreases the dead time but also the plateau length A decrease in resistance below the limiting value may affect tube life and lead to its early destruction The anode resistor should be connected directly to the anode connector of the tube to ensure that parasitic capacitances of leads will not excessively increase the capacitive load on the tube An increase in capacitive load may increase the pulse amplitude the pulse duration the dead time and plateau slope In addition the plateau will be shortened appreciably Shunt capacitances as high as 20 pF may destroy the tube but lower values are also dangerous Vs Starting Voltage 140 V2 V1 Plateau length VO Operating Voltage 120 V1 Lower threshold voltage V2 Upper threshold voltage N N COUNTS SEC 858 1 550V VO 650V V2 750 V VOLTAGE FIG 4 TYPICAL G M CHARACTERISTICS TIME BASE Minimum Signal detected by circuit Specific percentage of max Signal G MPULSE OUT FIG 5 TYPICAL G M PULSE OUTPUT SEEN ON AN OSCILLOSCOPE Maximum Counting Rate The Maximum counting rate is approximately 1 T g dead time For continuous stable operation itis recommended that the counting rate is adjusted to a value in the linear part of the counting rate dose rate curve Tube sensitivity at extremely high dose rates At dose rates exceeding the recommended maximum a
26. erimental data the gamma contribution accounts for approximately 70 of the background and a further 25 approximately is due to cosmic mesons For the majority of G M tube applications the background may be reduced to an acceptable level by shielding the tube with lead or steel Thus most of the gamma contribution is eliminated The values given in the data in count per minute are derived from averages over a long duration LIFE Storage life If stored in a cool dry place free form continuous or severe vibration there is hardly any deterioration in the tube s characteristics A storage life of years is not unusual Warning Generally life end of a G M tube is indicated by an increasing slope and a shorter plateau For older tubes operation is recommended at the first third of the plateau Operational life The operational life of a G M Tube is expressed in counts discharge Theoretically the quenching gas ionized during a discharge should be re combined between discharges However minute quantities will be chemically bound no longer taking part in the quenching process This will lead to a gradual reduction of the plateau length and for a given working voltage to an increased counting rate This will culminate in a continuous state of discharge of the tube rendering it useless Apart from the accumulated number of counts registered the ambient temperature during operation is of prime importance to the life of the tube At tem
27. h a mean value of C 904 The observed net count rate as a function of distance is given by 904 d An alternative analysis method involves transforming the data so that the results lie on a straight line For this purpose Net Count Rate R Vs Reciprocal of the distance square 1 d are plotted refer to Fig 9 This will be a straight line passing through the origin 0 O as this point corresponds to a source detector distance of infinity Gradient can be estimated easily from the net count rate R corresponding to a value of 1 d of 400 m In this example c 886 which is in agreement with the results of the previous method at 5cm Rd 35 42 x 25 886 19 Fig 8 Plot of Net Count Rate R Vs Distance d INVERSE SQUARE LAW am COUNT RATI Fig 9 Plot of Net Count Rate R Vs Inverse Square of Distance d 20 INVERSE SQUARE LAW 25 Log E 05 Fig 10 Plot of Log R Vs Log d c Another way of data analysis is by plotting these values on alog log graph sheet or com pute log values 4 plot them on a linear graph sheet log R Vs log d as shown in fig 10 Table 2 b Table with Log R amp Log d values computed SNo d ems Lodd R 6 45 0 6532 45 833 16611 55 Draw a line through the data points If this is a straight line with a gradient of 2 then it proves that Inverse Square Law is obeyed log R d2 log
28. he nuclide concerned The maximum beta energy is generally termed the beta end point energy of the nuclide 6 Bremsstrahlung Radiation that results from the acceleration deceleration of charged particles in the Coulomb field of atoms Curie Ci Name for derived unit of activity One Curie corresponds to 3 7 x 1010 nuclear disintegrations or isomeric transitions per second 1 Ci 3 7 x 1010 s 1 1 Ci 37 GBq 1mCi 37 MBq 1uCi 37 kBq 1 nCi 37 Bq 1 pCi 37 mBq Dose See absorbed dose exposure value and dose equivalent Dose equivalent Aterm used in radiation protection for the radiation dose It is the product of absorbed dose times the quality factor Unit kg 1 Name of unit Sievert see also Rem Dose rate Dose absorbed per unit time Electron radiation Particle emission consisting of negatively or positively charged electrons Exposure dose The ratio of the amount of electric charge of the ions of one polarity that are formed in air by ionising radiation and the mass of the air Unit C see also Roentgen Gamma radiation Photon radiation emitted by an excited atomic nucleus decaying to a lower energy state Gamma radiation has a line spectrum with photon energies which are specific to the nuclide concerned Gamma and X rays are both electromagnetic radiations and they are distinguished only by their mode of generation Gray Name of the derived SI unit of absorbed dose 1 Gy 1J kg 1
29. ing circuit See Fig 4 e Plateau length This is the range of applied voltage over which the plateau region extends See Fig 4 Upper Threshold voltage V9 This is the higher voltage upto which plateau extends beyond which count rate increases with increase in applied voltage Plateau Slope This is the change in counting rate over the plateau length expressed in per volt See Fig 4 Recommended Supply Voltage Operating Voltage This is the supply voltage at which the G M Tube should preferably be used This voltage is normally chosen to be in the middle of the plateau See Fig 4 Background BG This is the counting rate measured in the absence of the radiation source The BG is due to cosmic rays and any active sources in the experimental room NOTES Dead Time This is the time interval after the initiation of a discharge resulting in a normal pulse during which the G M Tube is insensitive to further ionizing events See Fig 5 Resolution resolving time TR This is the minimum time interval between two distinct ionizing events which enables both to be counted independently or separately See Fig 5 Recovery Time Tre This is the minimum time interval between the initiation of a normal size pulse and the initiation of the next pulse of normal size See Fig 5 Anode resistor Normally the tube should be operated with an anode resistor of the value indicated in the measuring circuit or higher D
30. itors for measurements e Repeat the experiment with Beta source by keeping the source slightly away from the end window when compared to gamma source and tabulate the data Calculate slope plateau length etc From this one could notice that with Beta source the effi ciency of the detector increases Also one can notice that with higher count rates plateau slope increases 17 Exp 2 INVERSE SQUARE LAW Gamma Rays 2 1 2 2 2 3 PURPOSE The Inverse Square Law is an important concept to be understood It states that intensity of gamma radiation falls inversely as square of the distance EQUIPMENT ACCESSORIES REQUIRED e GM Counting System GC601A GC602A with A C main chord e GMbDetector End window stand or G M Detector source holder bench e G M Detector in PVC cylindrical enclosure with connecting cable e source Fig 7 Detector G M stand holder and source arrangement PROCEDURE e Make detector source arrangement as shown in the Fig 7 and powerup the unit e Without source make few about 5 readings background measurements and take an average of them for a preset time of say 60 sec Compute Average background counts in 60sec 61 62 63 64 65 5 Compute Background rate t 60sec e Place a gamma source in the source holder and adjust the distance d from the detector end window to be 2 cm away from the centre of the source holder e If you have an End window detect
31. ivity 0 1mCi Preset Time 300Sec BG 1065 counts For Al 0 7mm amp Perspex 1 8mm combination Absorber position Net Counts Perspex facing source Al facing source For Perspex 1 8mm amp Cu 0 3mm combination Absorber position Net Counts Cu facing source Perspex facing source Al facing source Cu facing source RESULT amp CONCLUSIONS The count rate for the bremsstrahlung produced depends on the order in which the absorbent materials are arranged If firstly the sheet of metal faces towards the source then a higher count rate is measured since bremsstraulung is generated in the aluminium but is absorbed to a very small extent in the sheet of Perspex which follows however the beta rays first strike the sheet of plastic then the bremsstrahlung generated is of low energy and a large proportion of it is absorbed in the sheet of metal which follows These conclusions can be extended to other combinations of materials also 42 Exp 8 MEASUREMENT OF SHORT HALF LIFE 8 1 8 2 8 3 PURPOSE To determine short half life of a given source which can be obtained from a mini generator or produced with a neutron source by activation EQUIPMENT ACCESSORIES REQUIRED G M Counting system Type GC 601A GC602A G M Stand Type SG 200 End window G M detector Type GM 120 Short Half life source Neutron activated Indium foil or Cs 137 Ba 137m isotope generator flask with eluting solution for gene
32. l density 10 STAND FOR G M DETECTOR TYPE SG200 Stand for G M tube type SG 200 has been designed to hold PVC enclosed End Window G M tube as shown in picture This stand can be housed inside the lead shielding if required It has both sample and absorber trays The position of these trays can be adjusted from the end window of the detector The stand made up of acrylic sheet is precisely milled for sliding in of sample and absorber trays Sample tray is designed to hold planchets or disc type radioactive standard source Beta or Gamma Aluminium absorber discs can be interposed between the source and the detector for attenuating the radiation as seen by the detector Captive screw holds the detector PVC tube to any height To increase the distance between end window amp source one can lift the PVC tube further up which can be held by captive screw SLIDING BENCH FOR G M EXPERIMENTS TYE SB201 This essentially consists of a bench with sliding groves with a graduated S S scale fixed on one side of it Scale has graduations both in cm amp inches upto 50cm 20 inches There are three vertical sliding mounts each for mounting of End Window G M detector horizontally facing the absorber amp source mounts Each of these mounts can be positioned along the slide scale to have required distance between the end window to the source with absorber mount interposed in between End Window detector is housed in PVC enclosure with MHV so
33. lar interlocking discs A door is fitted in the bottom ring with 150 degree opening to facilitate easy access to the sample holding tray of G M Stand The door is fitted with heavy duty hinges and the inside of the lead shield is lined with thin aluminium sheet to minimize scattering ALLUMINIUM ASSORSER SET ABSORBER SCATTERER SET TYPE AS 272 For Scattering of Beta Particles Experiment Description The absorber scatterer set consists of 15 Aluminium foils the thickness of each foil being 0 05 mm For increasing the thickness of the scatterer the required number of aluminium foils are to be stacked together and put in the frame provided ABSORBER SET TYPE AS 273 For Production and Attenuation of Bremsstrahlung Experiment Description The absorber set consists of the following combination of materials Aluminium 0 7 mm thickness amp Perspex 1 8 mm thickness Perspex 1 8 mm thickness amp Copper 0 3 mm thickness Aluminium 0 7 mm thickness amp Copper 0 3 mm thickness 12 ACTIVITY amp DOSERATE CALCULATION PROCEDURE a Activity calculation as on date Itis known that given the activity at any previous date and by knowing its half life we can calculate the present activity by using the following equation A e Age 0 693 T1 2 t Where A Present activity Activity as on previous date T42 Half life of source t Elapsed time Decay constant TYPICAL CALCULATION OF AC
34. n value of counts The square root of variance the standard deviation is a measure of the scatter of individual counts around the mean value As a thumb rule we can say that approximately 2 3 of the results are within one standard deviation of the mean value i e within the interval and N c wherec NN Conversely given the result from an individual measurement the unknown true count lies within the interval N VN N with a probability of approximately 2 3 The above measured results of mean variance and standard deviation follow Poisson distribution Results show thatthe mean value N is almost equal to the variance o which is characteristic of the Poisson distribution The variance in any measured number of counts is therefore equal to the mean value of counts 26 3 8 EXPERIMENT B To illustrate that for number of counts recorded being high Poisson distribution follows closely normal or Gaussian Distribution PROCEDURE e Make standard counting setup as shown in figure 1 e Place a Beta source about 2cm from the end window of the detector e Record counts typically for a preset time of 25sec and take 50 data readings e Compute Mean Deviation and Standard Deviation and tabulate the readings Also compute other values as indicated in the table 2 782 8 281 025 6 803 13 281 046 05 z so 10 231 0o35 841 51 1 81 o ee 12 281 042 05
35. negative values which get cancelled Yet an average value of the error will be desirable since it tells us how good the data is in a quantitative way Therefore we need a different way to obtain the measure of the scatter of the data 22 e Variance c amp Standard Deviation One way is to obtain standard deviation c which is defined as 1 From this o c we see no negative sign and indicates average error contribution We find that all the deviations make a contribution We call the term c as variance Standard deviation is a square root of the variance which is widely used to indicate about the spread of our data 4 n o gt di for large samples The definition of the standard deviation differs slightly for small samples It is defined as follows di for small samples 3 4 MEASURING BACKGROUND RADIATION In this section several basic experiments are described to demonstrate the statistical nature of radioactive processes The importance of statistical methods in analyzing data and estimating measurement uncertainties is also covered The G M detector registers pulses even when not exposed to radioactive sources These pulses are caused by natural and man made radioactive isotopes found in our environment and also by cosmic radiation The background radiation varies with time and depends on the local environment the building material shielding and the weather Hence the background count rate cou
36. nt of radiation The spread in measured values decreases as the number of pulses registered increases STATISTICAL ANALYSIS OF RESULTS We have already defined mean variance and standard deviation at the beginning of this chapter These parameters for the above set of tabulated background readings can be calculated as follows Mean Value DN 6 5 Variance 0o 6 53 Standard Deviation o 2 55 24 Measurement of the background in 10 s NO OF COUNTS IN 10 s Fig 11 Plot of no of pulses Vs Index Number No of Counts in T2100 s for 10 Measurements No of Counts in 100 sec 1 2 3 z 5 6 7 8 9 10 Index Fig 12 Plot of no of counts T 1005 for 10 measurements FREQUENCY DISTRIBUTION 5 0 3 4 5 6 7 B 9 10 13 1 2 11 12 N e o o 3 o o ee gt o 5 No of Counts in 10 s Fig 13 Frequency distribution for 100 measurements of the back ground with T210s 25 3 7 The sample variance is calculated with the divisor n 1 to give an unbiased estimated value for variance of the process INTERPRETATION OF THE RESULTS The results follow a Poisson distribution on which practically all radioactivity measurements are based The results show that the mean value N is equal to the variance o this is characteristic of the Poisson distribution The variance in any measured number of counts is therefore equal to the mea
37. nts per second should be recorded before and after carrying out measurements In the following discussion the total number of counts recorded for a counting period will be indicated by N for countrate N and background counts by B background rate B The net count rate is given by N B T where T is the counting period in seconds 23 3 5 e 3 6 EXPERIMENT A Make standard set up by connecting G M Counting System GC 601A 602A with G M Detector placed in the optical bench or G M stand as shown in figure 2 or 3 Remove the radioactive source from the source holder and set the preset time to 10 sec and take a set of 100 readings and tabulate them as shown in table no 3a Now plot a bar graph for number of counts registered versus the Index Number say for group no 1 as shown in fig 11 m xno ___ 1 2 4 5 6 7 s 9 10 ccomsmose e e 3 s 3 Table 3 a Background counts registered for 10 seconds Now repeat the experiment to have large data counts Store the data for 100 sec amp take a set often such measurements as shown in table 3 b Plot these no of counts Vs index no as shown in fig 12 ma xno fj 2 4 516 7 s o jio BG Counsoosecleo es ee ez es ex 66 70 oi lor Table 3 b Background counts registered for 100 seconds By comparing these two figures 11 amp 12 we can deduce one of the most important laws of the measureme
38. or Isolation Source Detector Lead Block Schematic diagram Fig 18 Experimental setup 38 Lead Block Fig 19 Individual blocks of experiment setup 6 3 PROCEDURE Make standard setup by connecting G M Counting System GC602A with G M Detector GM125 In this experimental setup the detector Beta source and scatterer stand with scatterer are placed as shown in Fig 18 Alead block is placed in between the Beta source and Detector so that the detector does not receive any direct radiation from the Beta source Switch ON the GC602A Electronic Unit and set the operating High voltage at 500V To start with remove the scatterer stand and measure the counts for 200 secs Now place the scatterer stand and load Aluminum foil scatterer of thickness 0 05mm The apparatus is first set up to give maximum count rate by adjusting the source detector positions After doing this record the counts for 200 secs Then increase the thickness of the scatterer in steps of 0 05mm by adding one foil to the previous scatterer and observe the counts each time for 200 secs Tabulate the data 39 6 4 6 5 EXPERIMENTAL DATA Source Sr 90 Unit GC602A Activity 0 1mCi Detector GM125 Preset Time 200 secs Sliding Bench Thickness 1 f M Average Netcounts Al 252 285 5 382 465 627 5 759 5 867 932 1187 5 1320 5 1307 1364 1357 1 2 3 4 5 6 8 RESULTS amp CONCLUSIONS From the obtained results
39. or stand keep the source holder in the 1 slot amp raise the end window detector enclosed in a cylindrical shell by unscrewing the captive screw such that you get 2 cm distance from the end window to 1 slot as shown in Fig 7 e Set the HV to Operating Voltage say 500 V program preset time to 60 sec and record the data counts by pressing START button e Increase the Distance d in steps of 0 5 bmm and for each step record the observations and tabulate table 2 the data as given below till you reach a distance of 8 to 10 cms from the detector face e Subtract the background counts from the recorded counts which results in corrected counts N in 60sec From this obtain Net Count Rate R per sec 18 2 4 COMPUTATION amp ANALYSIS a Compute and tabulate Net count rate R Distance d product of C R d transformation 1 d etc as shown in table 2 Plot a graph of Net count rate R Vs distance d in cm Fig 8 Table 2 Data for Inverse Square Law Experiment Dist Corrected Net Count T i ii istance in Counts N Rate ransformation No om d 1 92 in 1 m ES MH in 60sec Rin 1 sec 6 45 2750 4583 60 1469 2483 EA EA If the count rate obeys the inverse square law it can be easily be shown that the product R d is a constant The results of the product R d are shown in the table above allowing for statistical fluctuations the results obey this law wit
40. ounded of Values Two important observations can be made at this point about gaussian distribution amp figure obtained above The distribution is symmetric about the mean value Because the mean value is large the adjacent values of the function are not greatly different from each other i e the distribution is slowly varying which is the expected behavior of a normal distribution 3 10 EXAMPLES If a measurement of 10s duration yields 3 pulses the result is correctly expressed as N 3 1 7in10sorZ 0 3 0 17 1 sas V3 1 7 In experiment 1 in the first 10 measurements i e after 100 s 30 pulses were counted The result would be N 30 5 5 in 100 s 2 0 30 0 055 1 5 After 100 measurements in Experiment 1 i e 1000 s 286 pulses were counted The result would be N 286 17 in 1000 s Z 0 286 0 017 1 s If you compare the errors indicated for the count rate Z in the examples 1 and 3 you can see that a counting period which is 100 times longer or 100 measurements leads to a result where the measurement uncertainty is 10 times smaller If the result is divided by the count time T N AN N N 1 VZ QM RM X 2 __ T T T T The uncertainty in the count rate Z is therefore proportional to one over the square root of the counting period T or equivalently to the square root of the number of readings taken 29 Exp 4 ESTIMATION OF EFFICIENCY OF THE G M DETECTOR A EXPERIMENT TO EST
41. perature above 50 C changes in the gas mixture may occur possibly reducing the total number of counts attainable Short periods of operation not exceeding 1h up to approximately 70 C should not prove harmful butlife will progressively decrease with increasing temperature Thus depending on application and circumstances the quenching gas could be exhausted in as little as a few hours or theoretically last for many years For these reasons G M Tubes cannot be guaranteed unconditionally for a specified period of time IMPORTANT DEFINITIONS e Absorbed dose The energy transferred to a material by ionising radiation per unit mass of the material Unit J kg 1 Name of unit Gray see also Rad e Activity Measurement of quantity of radioactive material It is the number of nuclear transformations or isomeric transitions per unit time Unit 1 Name of unit Becquerel see also Curie e Alpha decay Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus They are generally produced in the process of alpha decay but may also be produced in other ways Alpha particles are named after the first letter in the Greek alphabet o A radioactive conversion accompanied by the emission of an alpha particle In alpha decay the atomic number is reduced by 2 and the mass number by 4 Alpha decay occurs with a few exceptions only for nuclides with a proton number exceeding 82
42. rator PROCEDURE An Am Be neutron source of strength of about 5Ci is the Neutron Howitzer The maximum thermal neutron flux produced by this neutron source is about 4 x 10 n cm sec e Irradiate the given indium foil for about 12 hours by placing it in appropriate position in the Neutron Howitzer normally at the centre of the column e Apply the required operating voltage for the GM tube e Place the irradiated indium foil under the window of the GM tube at a convenient distance 1 cm in order to get a good number of counts per second e Collect the counts for every 5 minutes for at least one hour e Note down the back ground count rate for 5 minutes before and after the experiment in order to subtract from the observed counts and record your observations as shown in the Table below e Determine the count rate N for each interval of 300 seconds 5 minutes e Plot graph of log of the count rate log N versus time minutes e It will be a straight line as shown below dy decay constant T y cons Time t in 43 e Find the slope of the straight line graph using the least square fit methods use the formula m nXxy Xxy nx to determine the slope of the graph which gives the value of the decay constant Where n number of observations x time interval Log N OBSERVATIONS El Duration nts Corrected 2 60 10 2660 2476 550 3 90 15 93862 23906 547
43. rence of scattering by collisions of Beta particles with electrons in the material Such a collision changes the speed and direction of the Beta particles With increasing atomic number Z of the material the chance that a collision results in a scattering of the Beta particle increases too Back scattering occurs when the angle of deflection is greater than 90 The Back scattering rate is predominately dependent on the atomic number 2 of the back scattering material With an atom of high atomic number the scattering occurs at a large angle and with little loss of energy The back scattering factor is approximately proportional to the square root of atomic number The mass per unit area thickness x density or the thickness ofthe irradiated material only influence the back scattering factor up to a saturation value The maximum back scattering is practically attained at a mass per unit area which is smaller than half the range of the Beta particle in the material because large layer thicknesses lead to absorption of the scattered electrons The saturation value is less than 200 mg cm for all materials This corresponds to a saturation larger thickness of x 0 74 mm for Aluminum and 0 17 mm for Lead EQUIPMENT AND ACCESSORIES REQUIRED Scala i Electronic Unit 0 ii Wide end window GM Detector GM125 iii Absorber stand for Back scattering of Beta iv Absorber set Beta particle scattering experiment v Beta source Sr 90 vi Lead Block f
44. s in a process or pharmaceutical industry Two experiments SI No C 9 and 10 included in this manual illustrate engineering and industrial applications These experiments SI No 9 and 10 when demonstrated to Engineering students will illustrate the possible real time applications and scope of nuclear techniques for industrial and engineering applications These experiments will be of interest to Engineering stream students in their Engineering Physics Instrumentation labs It may be noted that some of NITs IITs and Technological universities have included G M counter experiments in Engineering physics labs Editorial board is of the opinion that when these physics experiments are done for Engineering streams emphasis should be with Engineering applications to the particular branch stream so that the student appreciates and understands the application well In this second revised edition two new experiments SI No 6 and 7 have been added which are primarily to cover Backscattering of Beta particles and Production and Attenuation of Bremsstrahlung We also thank all our staff at NUCLEONIX SYSTEMS who have helped us in preparing this manuscript for releasing to Press Finally Editors will be happy if this manual has served the purpose for which it is written Efforts will go on continuously to improve on this in the next edition Suggestions and feedback are welcome from all concerned with this subject J Narender Reddy Dr
45. sec For all command button functions refer to G M Counting System GC601A GC602A user manual e Now press START button to record the counts and gradually increase the HV by rotating the HV knob till such time the unit just starts counting Now press STOP button e Now take a fresh reading at this point STARTING VOLTAGE and record the observations in the format as given in Table 1 e Also record for each HV setting corresponding background counts without keeping the source e Continue to take these readings in steps of 30V and for the same preset time keep observing counts amp tabulate the data with and without source e Initially within 2 to 3 readings counts will steeply increase and thereafter remain constant with marginal increase may be within 1096 After few readings one will find a steep increase as one enters the discharge region Take just one or two readings in this region and reduce the HV bias to 0 volts It is important to note that operating the G M detector in discharge region for longer time can reduce the life of tube or can result into permanent damage of the detector 15 e Now tabulate the readings and plot a graph of voltage against counts corrected counts This graph should look as shown in Fig 6 e Identify from the graph tabulated data i Starting Voltage ii Lower threshold voltage V4 iii Upper threshold voltage V2 Itis called Breakdown threshold voltage iv Discharge region e C
46. sheet affixed with coins to interrupt the source detector beam e On each interruption by a coin and without coin interruption i e with only Perspex sheet record the count rate changes and tabulate them e One can pre set alarm level if available as a feature in the counting system such that on each change over of count rate one can see annunciation of aural visual alarm on the electronic counting unit 10 5 CONCLUSIONS Using this principle number of real time applications in a process industry could be developed or implemented for inspection checking the fill of the items and counting number of items under process 49 NUCLEONIX SYSTEMS PRIVATE LIMITED Plot No 162 A amp B PHASE II I D A Cherlapally Hyderabad 500 051 Ph 91 040 27263701 FAX 040 27262146 e mail info nucleonix com Website www nucleonix com
47. splay indicates HV preset time Time counts and other parameters O Counts capacity 999999 counts Preset time 1 9999 sec o User interface Through front panel keypad Command buttons provided are START STOP PROG STORE INC A DEC o GM pulse output G M detector output is provided on the rear panel BNC Socket Inverted output o Power Unit works 220V 1096 A C 50Hz o Memory storage Built in memory to store data readings upto 1000 ADDITIONAL FEATURES AVAILABLE ON GC602A ONLY o Printer port centronics For data printing is built in o Paralysis time Variable paralysis time OFF 250 350 amp 550 sec o RS232C Serial port for data communication to PC is built in ACCESSORIES FOR GEIGER COUNTING SYSTEM GM 120 is a Halogen Quenched End Window GM Detector supplied by NUCLEONIX It is suitable for general purpose GM Counting applications amp all G M Experiments Its operating voltage is approximately 500V It has got a very wide plateau length and plateau slope is better than 696 per 100V This detector is supplied in a cylindrical PVC enclosure with MHV socket arrangement for applying HV bias voltage Application Suitable for Beta amp Gamma Counting Operating Voltage Range 450 600V Tube Dimensions Max over all length 2 125 inches PVC Enclosure dimensions 25mm dia x 77mm Ht Max Diameter 0 59 inches Gas filled Ne Hal End Window mica 2 0 mg cm sq Area
48. t 72 R 5 4 2 EXERCISE Subtract the background count rate from each measured count rate Plot a graph of Net countrate CPS Vs absorber thickness mg cm for both sources Draw the curve through these points as shown in Figures 16 amp 17 From the plotted graph extrapolate and obtain thickness of aluminium absorber required to reduce the countrate of Thallium and Strontium Beta rays by half t 72 and t 2 Substitute 1 7 and t 2 in the above equation 2 and calculate the range of prays from Sr90 source Once we know the R we can find out the energy of Sr from equation 1 36 5 4 3 For Thallium 204 End point energy of TI 204 0 764 MeV Range of TI R 0 52E 0 09 g cm 0 52 x 0 764 Mev 0 09 g cm 0 30728 g cm e Thickness of Al absorber required to reduce the count rate of 2 by half 57 34mg cm square e Thickness of Al absorber required to reduce the count rate of Sr 90 by half t 72 121mg cm From Equation 2 tA a R t7 0 30728 X 121 X 10 gt R R X asc CER t7 34 X 10 30728 X 121 R AMT 1 09355 gm cm 34 End point energy of Sr Y _ R2 009 _ 1 09355 0 09 WE pod 0 52 0 52 E 2276Mev 5 4 4 RESULT End point energy of from Sr 2 28 MeV 37 Exp 6 BACK SCATTERING OF BETA PARTICLES 6 1 6 2 INTRODUCTION When Beta Particles collide with matter absorption may occur Another possible result is the occur
49. urce if an unknown activity Beta source is kept for counting one can calculate and find out its activity 32 Exp 5 TO STUDY DETERMINATION OF BETA PARTICLE RANGE AND MAXIMUM ENERGY BY HALF THICKNESS METHOD 5 1 5 2 5 3 PURPOSE To carry out the absorption studies on f rays with the aid of a GM Counter and hence to determine the end point energy of B rays emitted from a radioactive source EQUIPMENT ACCESSORIES REQUIRED e G M Counting System 601A 602A with A C main cord e GM Detector End window stand or G M Detector source holder bench e Radioactive source kit e Aluminium absorber set PROCEDURE e Make standard connections and arrangement between G M Counting system detector absorber and source e Setthe GM voltage at the operating voltage of the GM tube e Without source make a few about 5 readings background measurements and take an average of them for a preset time of say 60 sec e Compute Average background counts in 60sec Ba b b b b b 5 e Compute Background rate t 60sec e Place a Beta source in the source tray at about 3 cm from the end window of the GM tube e Take the Aluminium absorber set e Place an aluminium absorber of zero thickness in the absorber holder at about 2 cm from the end window of the GM tube and record the counts e The absorber thickness is increased in steps of 0 05mm and every time counts recorded e This process is repeated until the count rate
50. urce 2 uCi Approx Fig 23 G M Counting system with sand filled PVC column detector amp source arrangement 46 9 4 PROCEDURE Typical arrangement is shown in the Fig 23 e Fill the PVC tube with sand to half height approx e Mount the detector in the special enclosure given and connect it with cable to the counting system as shown in Fig 23 e Switch on the G M Counting System and select acquisition mode to be CPS CPM mode e Now place a gamma source along the length ht of the PVC tube which is kept vertically in standing position and exactly diagonally opposite side place the detector horizontally window facing the PVC pipe e Now observe the count rate on the instrument and record the observations for each ht in steps of 1cm or 15mm or as required e Ht vs CPS data may be recorded till sand filled level is crossed e Atthe level of crossing the sand level you will find noticeable change in count rate e From the above observation it can be confirmed that this is the ht level to which material is filled Electronic circuits can be modified in the G M Counting System to detect this transition in count rate and alarm or relay contact can be activated at this level e The above experiment illustrates the principle of Nucleonic level gauge e To predict correct accurate level one can open a small narrow window of 2mm 3mm on the detector amp cover the rest of the window of the detector by a lea
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