STSN 6614PRACTICAL 1

EXPERIMENT 6

Name: Maimanah Bt. MuhamadMatric No.: P62840

Title: Calibration of Gamma Scintillator of Gamma

INTRODUCTION

NaI(Tl) CRYSTAL SPECTROMETER FOR MEASUREMENT OF GAMMA-RAYS

This spectrometer will be used to detect and measure the energy of gamma-rays in the range of 50KeV – 2MeV in the following experiments.

Gamma-rays passing through a NaI-Crystal produce electrons by photo electric effect, Compton Effect and pair production. The energy of these electrons is quickly dissipated in the crystal ionization and excitation. The phosphor (Tl) converts a fraction of this dissipated energy into a flash of light (call scintillation). The flash is picked up by the photocathode of a photomultiplier and an electrical pulse is produced.

The height of the pulse is proportional to the intensity of the light. If the gamma-ray energy completely transfer to the electron (like in photoelectric effect) and if the electron stopped within the crystal, then the intensity of light flash proportional to the gamma ray energy. Thus by measuring the height of the pulses produced, we can measure the energy of the gamma-rays.

A small amount of Thallium (0.1 – 0.2 %) is sufficient for a good conversion of electron energy into light flash. The light flash has duration of less than 1 μ sec and is confine to a wavelength region of 3200 – 5500 A. So the photo-cathode of photomultiplier tube should be sensitive throughout this region of wavelengths.

SCINTILLATION DETECTOR

The scintillation counter is a solid radiation detector which uses a scintillation crystal (phosphor) to detect radiation and produce light pulses

They are three classes of solid scintillation phosphor:

1. Organic crystals2. Inorganic crystals3. Plastic phosphor

They are 5 important components of Scintillation Counter:

1. PM tube (PMT)

A PM tube consists of light sensitive photocathode at one end, a series of metallic electrodes known as dynodes in the middle, and an anode at the other end. A high voltage of ~1000V is applied between the photocathode and the anode of the PMT in steps of 50 – 150 V between dynodes. Approximately one to three photoelectrons are produced from the

photocathode per 7 to 10 light photons. Each of these photoelectrons is accelerated to the second dynode and emits two to four electrons upon impingement. Process of multiplication continues until the last dynode is reached, where pulse of 105 to 108 electrons is produced. The pulse is then attracted to the anode and finally delivered to the amplifier.

2. Preamplifier

The pulse from the Pm Tube is small in amplitude and is initially amplified by preamplifier.

3. Linear amplifier

A linear amplifier amplifies further the signal from the preamplifier and delivers it to the PHA for analysis of its amplitude. The amplification of the pulse is given by the ratio of the amplitude of the outgoing pulse to that of the initial pulse from the PMT. The amplifier gains are given in the range of 1 to 1000 by gain control knob on the amplifier.

4. Pulse-height analyzer (PHA)

A PHA is a device that selects for counting only those pulses falling within preselected voltage intervals and rejects all others. Pulses corresponding to gamma-ray energies of interest are selected by energy discriminator knobs, known as lower level or upper level, or the window and baseline, provided on the PHA. The selected pulse will be delivered to recording such as computers, scalers etc. In the experiment, single channel analyser (SCA) can be used to determine the amplifier output pulse height. The Lower level (V volts) and upper level (V + dV volts), thus the dV is called the window width. The output of discriminator then is fed to an anti-coincidence circuit, which will not produce a pulse output if it receives signals from both discriminators. Thus the anti coincident circuit gives an output only when the amplifier pulses have a height between V and V + dV volts. The Output of the coincidence circuit is then shape into a rectangular pulse (duration a few μsec height 2 -5 V), which can be later registered by the counter/timer.

5. Recording devices

Pulses processed by the PHA can be displayed on the cathode ray tube (CRT) as images or can be counted for a preset count or time by a scaler-time device. A rate-meter can be used to display the pulses in terms of counts per minute (cpm) or count per second (cps).

The advantages of scintillation counter that high efficiency, high precision and counting rate. Scintillation counter can be used to determine the energy as well as the number of the exciting particles (or gamma photons). The PMT output is very useful in radiation spectrometry (determination of incident radiation energy level). Besides, the energy absorbed in the scintillator is proportional to the number of light photons produced.

In the experiment, the scintillation detector used consist the detector of NaI(Tl) crystal mounted on the glass window of a PMT. The outer surfaces of the crystal is coated with Al2O3 or MgO powder so that light from the crystal will be reflected back into the crystal. Since the NaI(Tl) is hygroscopic, it is sealed in a metallic container.

Figure 1: Scintillation detector

OBJECTIVE

1. To determine the process of NaI(Tl) scintillation detector2. To determine the ability of NaI(Tl) scintillation detector to detect and measure

the energy of gamma ray in the range of 50KeV – 2MeV.3. To measure the resolution for each sources used (137Cs, 60 Co and unknown

source)

MATERIALS

1. 137Cs 2. 60 Co3. Unknown source4. Spectrometer

METHOD

1. 137Cs (E = 661.6 KeV) source was placed at 2 cm from the front face of NaI(Tl) crystal.

2. The amplifier output on the CRO set at 1V/cm and 1μs/cm horizontal sensitivity was observed.

3. Voltage was set to be 1000V.4. The window setting at the spectrometer at 0.1V , Lower Level (LL) at 0.2 V

And the time was 30sec. The number of gamma ray counted was recorded at this time

5. These procedures were repeated by increasing the LL in step 0.1V and keeping the same window width as before.

6. The procedures then continue with the 60 Co source and the unknown source.7. The graph of the number observed counts against the discriminator setting for

each sources, 137Cs, 60 Co and unknown source.8. The graph of E versus descriminator were drawed to find the gamma ray of

unknown source by measuring its spectrum under the same condition.

RESULT

DATA SHEET 1Gamma Scintillation Detector

Counter: Source: 137Cs (1μCi)Voltage: 1000V

Channel (LL) Counts Channel Counts0.2 1954 3.4 230.3 2354 3.5 130.4 19520.5 16030.6 16810.7 17430.8 22700.9 26141.0 23231.1 19371.2 15791.3 14591.4 13631.5 13051.6 12561.7 12701.8 13321.9 14172.0 11822.1 8292.2 4482.3 2702.4 2202.5 2072.6 2712.7 8082.8 39602.9 80973.0 58373.1 14083.2 1633.3 31

0 0.5 1 1.5 2 2.5 3 3.5 40

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t/sa

mpl

e

Figure 2: NaI(Tl) spectrum of 137Cs

Channel number = 2.90

Backscatter radiation

Compton edge

DATA SHEET 2Gamma Scintillation Detector

Counter: Source: 60Co Voltage: 1000V

Channel (LL) Counts Channel Counts0.2 536 3.4 4210.3 803 3.5 4600.4 658 3.6 4530.5 549 3.7 4890.6 549 3.8 4750.7 568 3.9 4820.8 592 4.0 4420.9 729 4.1 4231.0 888 4.2 4151.1 844 4.3 3541.2 676 4.4 3231.3 643 4.5 3211.4 563 4.6 3201.5 513 4.7 2901.6 473 4.8 3031.7 429 4.9 5311.8 422 5.0 9391.9 423 5.1 11592.0 389 5.2 7422.1 369 5.3 3082.2 407 5.4 1292.3 413 5.5 2122.4 427 5.6 4912.5 373 5.7 8072.6 372 5.8 8832.7 419 5.9 5112.8 387 6.0 2162.9 368 6.1 853.0 424 6.2 333.1 364 6.3 243.2 3943.3 426

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Figure 2 : NaI(Tl) spectrum of 60Co

Channel number = 5.10

Channel number = 5.80Backscatter radiation

Compton edge

DATA SHEET 3Gamma Scintillation Detector

Counter: Source: 60Co Voltage: 1000V

Channel (LL) Counts Channel Counts0.2 4577 3.4 16580.3 4874 3.5 63230.4 4679 3.6 146390.5 4265 3.7 147130.6 4309 3.8 70620.7 3859 3.9 12970.8 4231 4.0 2270.9 5970 4.1 911.0 5643 4.2 781.1 4799 4.3 1021.2 3995 4.4 1031.3 3507 4.5 881.4 3339 4.6 731.5 3143 4.7 681.6 30591.7 29291.8 29271.9 28712.0 28572.1 28812.2 30612.3 30142.4 29562.5 31892.6 32462.7 26682.8 18572.9 11383.0 7263.1 5813.2 4923.3 679

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Figure 3: NaI(Tl) spectrum of Unknown source

Channel number = 3. 65

Backscatter radiation

Compton edge

Table 1: Photopeak of number of observed counts

Event E γ(KeV) Disc. Setting (LL)(V)Photopeak 661.6 2.9Photopeak 845 3.7Photopeak 1173.2 5.1Photopeak 1332.5 5.8

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gy (K

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Figure 4: Energy calibration curve for NaI(Tl)

Energy resolution of NaI(Tl) detector

The resolution of a spectrometer is its ability to resolve two peaks that are fairly close together in energy. The resolution R (%) defined as:

R= δEγEγ ×100

a) Cs-137 source

R =

b) Co-60 source

c) Unknown source

DISCUSSION

In NaI(Tl) detector energy spectrum, the shape of the pulse-height spectrum is dependent on the photon energies and the characteristics of the crystal detector. When a gamma photon deposits all of its energy in the crystal, the amplifier output is a single electrical pulse whose amplitude is, proportional to the energy of the original gamma photon. Ideally this conversion of gamma energy to electrical pulse would be identical for each photon, and a plot of these pulses would appear as a single narrow ‘spike’. However, for a variety of reasons, such as the physical variations and the minor imperfections in the process of collecting and converting light photons into electrical current, the plot of electrical pulses corresponding to the photon energy is only a statistically blurred version of the original spike (Figure 5).

Figure 5: Imperfections and in the crystal and circuitry cause blurring of the photopeak.

Photopeaks in the spectrum correspond to the principal energies of gamma rays from the radioactive source. Kiloelectronvolt (KeV) is representing the pulse-height spectrum. In many applications, it is desirable to discriminate against background radiation or other spurious events. In these instances, it is necessary to count only the full energy or photopeak events generated by a detector. There is no easy way to calculate the number of events that are expected in the photopeak. However, this includes the full energy peak, Compton edge, single and double escape peaks, backscattered and other Compton events.

0 1 2 3 4 5 6 70

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Figure 6: NaI(Tl) spectrum for 3 sources

Back Scatter radiation Compton edge radiation

Photopeak

Based in the Figure 6, Backscatter radiation, Compton edge and the photopeak be seen during the collection of photon in the NaI(Tl) Scintillation detector. Backscatter radiation could be detected due to interaction of gamma radiation to the lead shielding or other material. However, photons may exit the crystal without detection only to be deflected 180° off the lead back into the crystal. These photons account for the backscatter. Backscatter peaks only evident when the incident is great enough to contribute a significant degree of Compton scattering in lead (approximately 200keV).The energy of backscattered photon is equal to;

Ebackscattered photon= 256× Eincident photon(256+Eincident photon )

As a result the low energy backscatter was detected by the crystal. While, the Compton edge or Compton peak were resulted by the maximum energy of Compton electron. Basically, there are two types of Compton (Compton electron and Compton scaterring) could be detected and will equal to the incident photon and the event will register as photopeak. However, only the Compton electron detected and the Compton scattering often escape. These Compton electrons’s energy are always less than incident photon thus it is represent to the left of photopeak. Maximum energy of Compton electron can be calculated as follows:

Emax comptonelectron=E2incident photonEincident photon+0 .256

They are another 2 compton events could be seen in Figure 6 and it is happened before and after the Compton edge which the Compton plateau and Compton valley respectively. Compton plateau refers to electron energies that are less than the Compton peak while the Compton valley reflects the sum energy of multiple Compton electron generated by a single photon.

Based in the Figure 6 also, both Cs-137 and the unknown source have the backscattered, Compton edge, Compton valley, Compton plateau and the photopeak. However, in the Co-60 spectrum is quite different maybe it is due to high energy of photopeak and it contains 2 photopeak which is at 1173.2 keV and 1332.5 keV. Because of it has the higher energy so that it may have the annihilation interaction occurred and produce the positron in the pair production process. The gamma photon may be absorbed near the nucleus of an atom, creating a positron and electron. The positron will undergo annihilation with an electron, producing two 511 keV photons. In the same reaction, new photons will be emitted with energy 1.02 MeV less than that of the incident photon. If the energy of all three photons is detected by the crystal , the total absorbed energy will be equal to the original energy of the incident photon and will contribute to the photopeak. This show in the second peak produced in the Co-60 spectrum. If however, one 511 keV photon escapes the detector, the sum will be reduced by 511 keV; If both photons escape, the sum will be reduced by 1.02 MeV. The resulting peaks are called the single escape and double escape annihilation peaks. However, these 2 peaks do not appear in the Co-60 spectrum in Figure 2. As a result the second peak produced.

Resolution is important to measure the performance of the crystal. Normally, the Full Width Half Maximum (FWHM) of NaI(Tl) is around 15 – 20 keV. The high resolution could represent by narrow line spread function and the low resolution is represent by the broad spread function. The window that we set up from beginning is at the window of PHA which it is beneficial to rejecting the scatter radiation. The narrow the window, more scatter radiation could be rejected. Based on the calculation done for Cs-137, Co-60 and the unknown source the resolution are x, x and x respectively.

There are few advantages of NaI(Tl) scintillation crystal ;

Good absorber of high energy x-ray, gamma ray (50 – 250 keV) due to high Z number ( Iodine, Z=53)

Less self absorption due to structure of crystal which is transparent. Inexpensive and it is even in large plate Match to the peak of scintillation light = 450 nm, where wavelength of PMT

photocathode = 400 nm (Sensitive to the PMT) Efficient scintillator – yield 1 visible photon per 30 eV

Disadvantage of NaI(Tl) scintillation crystal ;

Fragile / easily fractured by mechanical trauma or sudden change temperature(Fracture will create opacification – reduce amount of scintillation light)

Hygroscopic, when expose to the moisture will create yellow discolouration that will impair transmission of bluish violet scintillation light to PMT

Reduce spatial resolution, if too thick capture more gamma ray or high energy gamma ray (> 250 keV)

Large disc shape. High thickness of crystal will increase sensitivity and decrease the spatial

resolution.

The unknown source’s peak energy is845 keV and based on the table it is the Mn – 54 and its half life is 312.2

CONCLUSION

1. The process of NaI(Tl) scintillation detector when gamma-rays passing through a NaI-Crystal produce electrons by photo electric effect, Compton Effect and pair production. The energy of these electrons is quickly dissipated in the crystal ionization and excitation. The phosphor (Tl) converts a fraction of this dissipated energy into a flash of light (call scintillation). The flash is picked up by the photocathode of a photomultiplier and an electrical pulse is produced.

2. NaI(Tl) scintillation detector is able to detect and measure the energy of gamma ray in the range of 50KeV – 2MeV.

3. The resolution for each sources used (137Cs, 60 Co and unknown source) are x, x, and x respectively.

REFERENCES

1. Rachel A. et al.(2006). ‘’Essential Nuclear Medicine Physics’’. 2nd Edition. Blackwell Publishing.