Gamma Spectrometer (Single Channel Analyzer)

Indian Institute of Science Education and Research, Kolkata 23rd Feb – 2nd Mar 2010

1 | S i n g l e - c h a n n e l A n a l y z e r

Gamma Spectrometer (Single – channel Analyzer)

Harsh Purwar (07MS – 76) 3rd Year, Integrated M.S.

Indian Institute of Science Education and Research, Kolkata Experiment No. – 3 Nuclear & Particle Physics Laboratory (PH – 325)

Objective of the experiment: To study the gamma spectra of the given radioactive source (Cs-137 in this case) using a single-

channel analyzer.

Introduction and Theory: Single – channel analyzer is one of the conventional methods used for the determination or

mapping of the gamma energy spectrum of a radioactive (gamma active) material. It uses a scintillation detector with an inbuilt photo-multiplier tube (PMT). The gamma particles (photons having energy of the order of a few MeVs) emitted by the radioactive materials (like Cs -137, Na – 22, etc) are converted to the visible photons (or scintillations) by the scintillator (as discussed below) and these visible photons are then used to knock out the electrons from the atoms, which are then multiplied using a photo-multiplier tube to give a detectable pulse/signal. Each such pulse or signal is counted as one gamma.

A scintillator is, in general, a material which is capable of producing scintillations when hit be energetic particles like gamma photons. Most commonly used scintillator in the detection of gamma radiations is that of sodium iodide activated with traces of thallium. The gamma radiations originating from radioactive source interact with scintillator or matter in general in any one of the following ways:

Photo-electric Effect: The photoelectric effect is a phenomenon in which electrons are emitted from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Emitted electrons are referred as ‘photo-electrons’. Electrons emitted via this process have almost same kinetic energy as that of the incident gamma particles since the binding energy of an electron is of the order of a few electron volts whereas that of the incident gamma radiation is in kilo-electron volts.

Compton Effect: Compton scattering is a type of scattering that X-rays and 𝛾-rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy (increase in wavelength) of an X-ray or gamma photon, called the Compton effect. Part of the energy of the X/gamma ray is transferred to a scattering electron, which recoils and is ejected from its atom, and the rest of the energy is taken by the scattered photon.

Pair Production: Pair production refers to the creation of an elementary particle and its anti-particle, usually from a photon (or another neutral boson). This is allowed, provided there is enough energy available to create the pair – at least the total rest mass energy of the two particles – and that the situation allows both energy and momentum to be conserved. All other conserved quantum numbers of the produced particles must sum to zero, thus the created particles shall have opposite values of each quantum number.



Modes of Energy Transfer: When a high speed charged particle passes through the crystal lattice, it may transfer a small fraction of its energy to an electron in the valence band as a result of which the latter will be raised to the conduction band which is normally empty in an insulator due to large energy gap or forbidden gap. This is about 6 to 8 eV in case of halogen halide crystals. The electron now moves through the crystal till it comes across an electron trap or a crystal imperfection due either to a structural defect in the crystal or due to an added impurity like thallium in our case. The hole left in the valence band (which behaves like a positively charged particle) also moves through the lattice.

Figure 1: Mechanism of working of an inorganic halide scintillation detector.

The electron trapped in the conduction band may subsequently return to the lower valance band. In this process light is emitted in the form of fluorescence radiation. The whole process occurs in times of the order of 10−8 sec or less. The light so emitted however, has to high an energy (6 to 8 eV) to lie in the visible region. However the presence of an activator (like thallium) shifts the emission spectrum to the visible region in the de-excitation process. Some impurity levels now appear in the forbidden zone through which the electron can de-excite to the valence band. As a result the photon has much lower energy and lies in the visible region.

For sodium iodide activated with thallium scintillator, the emission spectrum is shifted to longer wavelengths (visible region) for which the crystal is usually transparent. Thus the emitted fluorescent light escapes absorption in the crystal and can be used to form electrical pulse. Photo-multiplier tube: In all the above described processes electrons with different energies were produced. These electrons collide with the atoms of the crystal and excite them. These atoms emit photons with energy in the visible range during their de-excitation. These photons reach the photo-cathode of the photo-multiplier tube, the next part of the detector.

From the photo-cathode numbers of emitted electrons rush towards the first dynode, strike it and

generate more electrons which then accelerate towards the next dynode. The supplied external high voltage (PMT voltage) is divided approximately equally across the dynodes. So when the resulting shower of electrons plunge into the anode with higher potential gives rise to an electric pulse whose amplitude is

Latt

ice

Ener

gy

Conduction Band

Valance Band

Forbidden Band

Photon



proportional to the amount of collected photons at the photo-cathode and therefore the energy of the incoming gamma photon.

This electric signal is then amplified and is converted into digital signal using an Analog to Digital converter. By analyzing the height of the pulse, energy of the incoming gamma photon can be measured. Radioactive decay scheme for Cs-137 is as shown below:

Experimental Setup: The single channel analyzer consists of a Scintillation detector with an inbuilt photomultiplier tube. The high voltage required for the acceleration of electrons is provided to the PMT by amplifying the usual AC (220V, 50Hz) using proper circuitry. Whereas the output pulse/signal from the PMT is pre-amplified and passed through a discriminator and pulse shaper and then finally to an electronic counter. The electronic counter counts the incoming pulses and shows them on its display screen. As evident the signal from the PMT carries a lot of noise which is primarily removed by the discriminator circuit. The pulse shaper changes the incoming bell shaped pulse to a square pulse so that it can be counted properly by the electronic counter.

Figure 2: Block diagram of the circuitry for a Scintillation counter

Procedure: The following procedure was implemented in order for determination or mapping of the gamma energy spectrum of the photons emitted by the Cesium – 137 source.

Before starting the experiment it was made sure that all electronic devices (like scintillation detector, counter, etc) are properly connected and switched on.

Cesium – 137 5137𝐶𝑠 source was placed at the top of the scintillation detector and was covered

with a plastic lid.

The high voltage required for electron multiplication in PMT was set to 750 volts.

The signal from the PMT was viewed on an oscilloscope (Figure 3) and for proper counting the signal was further amplified 1.6 times by setting the 10 turn gain helipot to 6.

Height of the above pulse sets an upper bound on the baseline voltage which in this case was about 1.8 volts.

An appropriate window size was chosen. Considering the time constraint, we choose window size to be 40 mV. The number of runs was set to 3 with time of each run equal to 10 sec.

Starting from zero baseline voltage, in steps of 40 mV, number of counts in each run was recorded. As mentioned above this was done up to baseline voltage 1800 mV.

55137𝐶𝑠

𝛽− 𝑑𝑒𝑐𝑎𝑦 𝛾 𝑑𝑒𝑐𝑎𝑦

𝛽− 𝑑𝑒𝑐𝑎𝑦

56137𝐵𝑎



Figure 3: PMT pulse/signal as viewed on an oscilloscope.

The spectrum was further resolved close to the maxima / photo peak by reducing the window to 20 mV.

Results & Observations: Table 1: For recording no. of counts versus baseline voltage for Cs-137 source.

Obs. No. Baseline

(mV) Window

(mV) Count 1

(per 10 sec) Count 2


(per 10 sec) Mean Counts (per 10 sec)

Mean Counts (per sec)

1 0 40 2562 2585 2484 2544 254

2 40 40 2565 2515 2471 2517 252

3 80 40 2568 2413 2552 2511 251

4 120 40 2571 2520 2503 2531 253

5 160 40 2506 2427 2459 2464 246

6 200 40 2474 2409 2439 2441 244

7 240 40 2572 2513 2421 2502 250

8 280 40 2596 2607 2665 2623 262

9 320 40 2433 2564 2605 2534 253

10 360 40 2422 2350 2294 2355 236

11 400 40 2212 2299 2250 2254 225

12 440 40 2061 2174 2073 2103 210

13 480 40 1995 1980 2005 1993 199

14 520 40 1965 1977 1954 1965 197

15 560 40 1791 1879 1890 1853 185

16 600 40 2016 1977 2063 2019 202

17 640 40 2140 1998 2003 2047 205

18 680 40 1970 1913 1951 1945 194

19 720 40 1804 1698 1767 1756 176

20 760 40 1891 1752 1591 1745 174

21 800 40 1691 1635 1669 1665 167

22 840 40 1625 1710 1659 1665 166

23 880 40 1723 1677 1572 1657 166

24 920 40 776 779 696 750 75



25 960 40 606 621 607 611 61

26 1000 40 1087 1065 1240 1131 113

27 1040 40 835 834 886 852 85

28 1080 40 680 643 689 671 67

29 1120 40 548 520 558 542 54

30 1160 40 489 522 489 500 50

31 1200 40 487 512 454 484 48

32 1240 40 542 554 550 549 55

33 1280 40 667 702 723 697 70

34 1320 40 924 996 904 941 94

35 1360 40 1269 1311 1237 1272 127

36 1400 40 1555 1302 1350 1402 140

37 1440 40 1957 2913 1886 2252 225

38 1480 40 8111 8355 8357 8274 827

39 1520 40 5895 6760 5479 6045 604

40 1560 40 4172 4734 4711 4539 454

41 1600 40 6080 6174 6316 6190 619

42 1640 40 6725 4309 4844 5293 529

43 1680 40 175 259 163 199 20

44 1720 40 178 233 221 211 21

45 1760 40 201 167 197 188 19

46 1800 40 175 158 140 158 16

0

100

200

300

400

500

600

700

800

900

Me

an C

ou

nts

pe

r se

c.

Baseline (mV) with window 40 mV

Gamma spectrum of Cs - 137



Table 2: Further resolution near maxima / photo peak (Baseline 1440 to 1520 mV) with a smaller sized window.

Obs. No. Baseline (mV) Window (mV) Count 1



(per 10 sec) Mean Counts (per 10 sec)

Mean Counts (per sec)

1 1440 20 1004 1849 1487 1447 145

2 1460 20 3911 3604 3303 3606 361

3 1480 20 2458 3183 2745 2795 280

4 1500 20 968 1060 941 990 99

5 1520 20 1268 1314 1334 1305 131

The above data in Table 2 was fitted with a Gaussian model,

𝑓 𝑥 = 𝑎1 exp − x − b1

c1

2

to give the following results. The fitted parameters were, 𝑎1 = 345.5, 𝑏1 = 1477, 𝑐1 = 32.7

Clearly from above we have, baseline corresponding to the maxima or photo peak is equal to 1477 mV. Standard deviation 𝜎 of the curve is 23.1 mV. So, full width at half maximum (FWHM) which is given by,

𝐹𝑊𝐻𝑀 = 2𝜎 2 ln 2 = 54.4 mV So, resolution which is,

𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =𝐹𝑊𝐻𝑀

𝐵𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝑐𝑜𝑟𝑟𝑒𝑠𝑝𝑜𝑛𝑑𝑖𝑛𝑔 𝑡𝑜 𝑝𝑒𝑎𝑘

=54.4

1477= 0.037

𝑦 = 345.5 exp − 𝑥 − 1477

32.7

2



Conclusions: The photo peak or maxima is obtained at baseline voltage equal to 1.47 volts or (1.46 – 1.48 V).

Mean counts per sec with a window size of 40 mV and baseline voltage between 0 to 0.95 V are significant (above 175 mV) and this may be due to the phenomenon of Compton Scattering (explained above).

The resolution of the detector is found to be 0.037 or 3.7%. NOTE: The baseline voltage can be calibrated in general, with the energy of the gamma photons using the known energy values corresponding to the photo peak(s) for some radioactive material (like Co-60), just as is done in the multi-channel analyzer systems.

References: Wikipedia.org (http://en.wikipedia.org)

Supplied supplementary reading material and User’s Manual.

Nuclear Physics by S. N. Ghoshal

Gamma Spectrometer (Single Channel Analyzer)

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