STUDY OF LINEARITY OF GAMMA RAY SPECTROMETER

STUDY OF LINEARITY OF GAMMA RAY SPECTROMETER as per the updated details

1. STUDY OF LINEARITY OF GAMMA RAY SPECTROMETER Jesna George DoS in Physics Manasagangotri

2. CONTENTS Scintillation detectors Gamma ray spectrometer Calibration Experiment

3. SCINTILLATION DETECTORS Detects the light emitted by a crystal subjected to ionizing radiation Organic scintillation detector. Inorganic scintillation detector. It can be used for counting as well as for energy analysis.

4. Gamma ray Spectrometer Sodium iodide scintillation spectrometer is the most successful and popular method in gamma ray spectroscopy. Thallium activated sodium iodide crystals are used as scintillators. The scintillation detector is coupled to nuclear electronic system.

5. Photomultiplier Tubes Made up of glass or quartz Photocathode Dynodes Anode

6. Single Channel Analyser It is a pulse-height analyser. Analysis is made by taking the readings for one channel at a time. There are two discriminators.

7. Gamma spectrum for monoenergetic gamma ray

8. The observed spectrum is complex in nature. The various absorption processes involved are Photoelectric absorption. Compton effect absorption. Absorption by pair production.

9. The widening of line into a Gaussian shaped profile is due to the statistical fluctuations in the various processes which convert gamma rays into an output pulse. The processes contribute to statistical fluctuations are

10. Conversion of incident gamma rays into luminescence photons Light transmission and collection by photocathode Conversion of light into photoelectrons Focussing of photoelectrons on dynodes Multiplication of electrons

11. Gamma ray detector should have High efficiency of detection. Good energy resolution power. Linear response to electron. Good mechanical and electrical stability .

12. Characteristics of energy spectrometer Linearity :The amplitude of the output pulse must be directly proportional to the original number of scintillation and thus in turn to the energy deposited Stability :Gain of each dynode stage depends on the voltage difference ,any change in high voltage will cause a variation in the output pulse.

13. Calibration of gamma ray spectrometer We calibrate the spectrometer to 2 Mev using the standard source . The gain is set such that the peak count will appear on the scale. The count is noted for such channel and is checked for the peak. If there is any deviation we repeat the above steps so that peak is obtained at channel.  

14. The channel around photopeak are set and counts are noted. This is repeated for different sources. Resolution for each photopeak is determined. A plot of resolution versus energy is made which should yield a straight line.

15. Here we have two potentiometers with 10 turns each. One acts as ULD and other as LLD. ULD : 0 to 1V. LLD : 0 to 10V. For LLD-1 channel=20 keV

16. AIM To study the linearity of gamma ray spectrometer using SCA and determination of energy of unknown source.

17. APPARATUS Gamma ray spectrometer. -source -source -source -source -source Unknown source.  

18. Isotop Energy in keV 662 365 1170 122 511 Isotop Energy in keV 662 365 1170 122 511

19. PROCEDURE The energies of various gamma sources are provided. Calibration of gamma ray spectrometer to 2 MeV has been done. is treated as the standard source the counts per 30 second for the entire spectrum is noted down. Photopeak is obtained at channel no.33.  

20. Similarly photopeak for all other known sources are noted down. Then counts per 30 second for the entire spectrum for the unknown source is recorded. For all these sources a graph is plotted between counts per 30 second and channel number. Thus we obtain the photopeaks .

21. Now a plot of energy versus peak channel number is made which will yield a straight line. From this plot we can determine the energy of the given unknown source.

22. OBSERVATIONS   Channel Number Counts per 30 secs Channel Number Counts per 30 secs Channel Number Counts per 30 secs Channel Number Counts per 30 secs 3 3106 13 2822 23 1794 33 8318 4 2797 14 2645 24 1008 34 5194 5 2695 15 2429 25 571 35 1627 6 2817 16 2319 26 432 36 135 7 2872 17 2216 27 306 37 16 8 2975 18 2196 28 251 38 20 9 3636 19 2285 29 308 39 8 10 4039 20 2390 30 658 40 14 11 3795 21 2437 31 2438 12 3325 22 2353 32 6115

23.   Channel number Counts per 30 secs Channel number Counts per 30 secs 9 4477 17 8884 10 3249 18 11705 11 2031 19 6152 12 1671 20 2364 13 2539 21 749 14 4543 22 507 15 5745 23 391 16 4262

24.   Channel number Counts per 30 secs Ch51annel numb42er Counts per 30 secs 50 3141 59 50 51 2366 60 42 52 779 61 36 53 1333 62 37 54 2503 63 33 55 1319 64 25 56 251 65 21 57 55 66 26 58 42

25.   Channel number Counts per 30 secs Channel number Counts per 30 secs 1 22 6 140 2 28 7 47 3 59 8 25 4 67 9 21 5 78 10 14

26.   Channel number Counts per 30 secs Channel number Counts per 30 secs Channel number Counts per 30 secs Channel number Counts per 30 secs 22 100 33 42 44 95 55 12 23 153 34 54 45 109 56 7 24 550 35 51 46 97 57 7 25 1163 36 66 47 82 58 6 26 1263 37 71 48 41 59 5 27 497 38 72 49 35 60 5 28 122 39 78 50 90 61 1 29 67 40 81 51 53 62 30 44 41 88 52 150 31 51 42 82 53 201 32 62 43 87 54 62

27. Unknown source Channel no. Counts/30secs Channel no. Counts/30 secs Channel no. Counts/30 secs Channel no. Counts/30 secs Channel no. Counts/30 secs 1 1652 14 568 27 801 40 113 53 260 2 545 15 521 28 161 41 109 54 78 3 667 16 485 29 61 42 128 55 11 4 570 17 300 30 51 43 111 56 4 5 593 18 229 31 71 44 129 57 9 6 663 19 145 32 67 45 157 58 5 7 649 20 118 33 79 46 132 59 1 8 684 21 129 34 82 47 87 60 9 843 22 138 35 69 48 53 61 4 10 852 23 228 36 79 49 27 62 2 11 759 24 701 37 77 50 22 63 12 685 25 1609 38 86 51 64 64 13 581 26 1662 39 103 52 211

30.   Unknown source

31. Source Energy Peak channel 662 33 356 18 1170 54 1330 59 122 6 511 26 Unknown source 510 26 Source Energy Peak channel 662 33 356 18 1170 54 1330 59 122 6 511 26 Unknown source 510 26

32. RESULT The plot yield a straight line which shows the linearity of the spectrometer. The energy of the unknown source is 510 keV .

33. REFERENCES Singru R.M., Experimental Nuclear Physics ,Wiley Eastern Pvt Ltd,Delhi,1972. Kapoor S.S.;Ramamurthy V.S,Nuclear Radiation Detectors ,New Age International Publishers,Delhi,2008. Ghoshal S.N., Nuclear Physics ,S.Chand and Company,Delhi,1994.

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