The Inorganic Scintillation Detector NaI(Tl)-Advanced Physics-Lab Report, Exercises for Advanced Physics. Alliance University
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The Inorganic Scintillation Detector NaI(Tl)-Advanced Physics-Lab Report, Exercises for Advanced Physics. Alliance University

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This is lab report for Advanced Physics Course. It was submitted to Prof. Dhirendra Kapoor at Alliance University. Its main points are: Count, Spectrum, Physics, Energy, Resolution, Collect, Plot, Peak, Gamma, Determine,...
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Abstract

In this experiment, the inorganic scintillation detector NaI(Tl) was used for gamma spectroscopy. The

integral and differential pulse height spectra of the gamma source Co 60

were obtained using single

channel analyzer (SCA) in the electronic setup. The differential pulse height spectra of the two sources

Co 60

and Cs 137

were also collected using the multichannel analyzer (MCA). For channel to energy scale

conversion on MCA, energy calibration was done by relating channel numbers corresponding to the

photo peaks in the two spectra and known energies of the gammas emitted by the sources. Then various

important parts of the spectra were analyzed and explained. An unknown source was also identified using

energy calibration on MCA. Finally, the energy resolution curve was plotted by finding the resolution of

different photo peaks appearing in the spectra using MCA.

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Experimental setup and results

Figure 1.Electronic Setup

Experimental setup was arranged as shown in figure 1, Co 60

, the gamma source, was placed besides the

detector and power was switched on. Gain of the spectroscopy amplifier was adjusted so that pulse on the

CRO screen was of about 3 volts. Window of SCA was set as maximum and lower level was increased in

steps of 0.1 volt and at each level, corresponding counts/20 sec were noted. This process was continued

until the count rate reduced to the background value. Data was tabulated and integral pulse height

spectrum for Co 60

was plotted as shown below:

Table 1.Data for Integral Pulse height Spectrum of Co60

LLD

(Volts)

Count

rate

LLD

(Volts)

Count

rate

LLD

(Volts)

Count

rate

LLD

(Volts)

Count

rate

LLD

(Volts)

Count

rate

0 26063 1 20861 2 16431 3 11872 4 7471

0.1 25759 1.1 20247 2.1 16016 3.1 11376 4.1 7059

0.2 25108 1.2 19607 2.2 15417 3.2 10782 4.2 6279

0.3 24674 1.3 19419 2.3 15141 3.3 10211 4.3 5096

0.4 23934 1.4 18529 2.4 14657 3.4 9986 4.4 4156

0.5 23313 1.5 18445 2.5 14126 3.5 9134 4.5 3825

0.6 23024 1.6 18016 2.6 13667 3.6 8959 4.6 3535

0.7 22543 1.7 17753 2.7 13336 3.7 8662 4.7 3290

0.8 21954 1.8 17174 2.8 12756 3.8 8262 4.8 2516

0.9 21523 1.9 16666 2.9 12485 3.9 7874 4.9 1778

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Then the SCA window was set at 0.1 volt and lower level was increased from its minimum in steps of 0.1

volt and at each value corresponding count rate was noted. The data was tabulated and differential pulse

height spectrum for Co 60

was plotted as shown below:

Table 2.Data for Differential Pulse Height Spectrum of Co60

LLD

(Volts)

Count

rate

LLD

(Volts)

Count

rate

LLD

(Volts)

Count

rate

LLD

(Volts)

Count

rate

LLD

(Volts)

Count

rate

0 436 1 343 2 242 3 370 4 226

0.1 355 1.1 345 2.1 295 3.1 322 4.1 336

0.2 467 1.2 307 2.2 273 3.2 376 4.2 571

0.3 284 1.3 265 2.3 243 3.3 365 4.3 762

0.4 299 1.4 280 2.4 288 3.4 326 4.4 537

0.5 291 1.5 263 2.5 265 3.5 345 4.5 186

0.6 317 1.6 288 2.6 292 3.6 247 4.6 126

0.7 380 1.7 294 2.7 308 3.7 235 4.7 227

0.8 495 1.8 273 2.8 311 3.8 215 4.8 436

0.9 369 1.9 304 2.9 301 3.9 209 4.9 619

0

5000

10000

15000

20000

25000

30000

0 1 2 3 4 5 6

C o

u n

t ra

te

Voltage

Figure 2.Integral Pulse Height Spectrum of Co60

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After that differential pulse height spectra of Co 60

and Cs 137

were collected on MCA and their important

parts were analyzed. The two spectra are shown below:

Figure 4.MCA Spectrum of Co60

0

2000

4000

6000

8000

10000

12000

14000

0 200 400 600 800 1000 1200

D if

fe re

n ti

a l co

u n

ts

Channel No.

compton

continuum

compton

edge

photo peak

photo peak

0

100

200

300

400

500

600

700

800

0 1 2 3 4 5 6

C o

u n

t ra

te

Voltage

Figure 3.Differential Pulse Height Spectrum of Co60

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Figure 5.MCA Spectrum of Cs137

Since in the above MCA spectra the photo peaks correspond to complete energy deposition by gamma

photons from the sources through photoelectric effect, by noting the channel numbers against the photo

peaks in the two spectra and knowing the energy of the gammas emitted by the sources, the energy vs.

channel number curve was plotted which is shown below:

Figure 6.Energy Calibration Curve

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

0 200 400 600 800 1000 1200

D if

fe re

n ti

a l co

u n

ts

Channel No.

compton

continuum

compton

edge

photo peak

y = 0.0024x - 0.0999

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 100 200 300 400 500 600 700

E (

M eV

)

Channel No.

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The curve was a straight line given by:

Where x is the channel no. and y is the energy in MeV.

Putting the channel numbers corresponding to the Compton edges of the MCA spectra of the two sources

in the above equation, it was found that the corresponding energies were 0.916 MeV and 0.443 MeV for

Co 60

and Cs 137

respectively, which ,when calculated by theoretical formula, were found to be 0.963 MeV

and 0.478 MeV for Co 60

and Cs 137

respectively. In the same manner, the other peaks appearing in the

spectra were also analyzed.

Then the same calibration process was performed on the MCA and the channel scale was replaced by

energy scale as shown below:

Figure 7.MCA Spectrum of Co60

Again the different peaks appearing in the spectra of Co 60

and Cs 137

were analyzed and the results were

compared to those obtained from the previous manual calibration method.

An unknown gamma source was placed besides the detector and its spectrum was collected on MCA

which is shown below:

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Figure 8.MCA Spectrum of Unknown Gamma Source

The photo peak energy was very close to 0.662 MeV so it was confirmed that the source was Cs 137

.

Finally, from MCA spectra of Co 60

and Cs 137

the following data shown in table 3 was collected and

energy resolution curve was plotted.

Table 3.Energy Resolution

Source E(keV) Centroid (H) FWHM R ln (E) ln (R)

Cs-137 661.6 266.7 20.62 0.0773153 6.494661 -2.55986

Co-60 1173 538.43 30.35 0.0563676 7.06732 -2.87586

1332 609.26 32.84 0.0539015 7.194437 -2.9206

Where H = height of the photo peak, FWHM = full width at half maximum and

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Discussion

Gamma spectra of the gamma source Co 60

were obtained using both SCA and MCA. By comparison of

the SCA and MCA results, it is clear that MCA is a better means of spectrum analysis because in MCA

thousands of channels work simultaneously so as to count all the pulses at the same time and a relatively

much shorter channel window can be established.

The gamma spectrum of Cs 137

was also collected on MCA and energy vs. channel curve was plotted using

photo peaks appearing in the two MCA spectra and known energies of the gammas emitted by the

sources. This was done in order to explain various important parts of the gamma spectra such as Compton

edge. The Compton edge energies determined by using this curve were found be 0.916 MeV and 0.443

MeV for Co 60

and Cs 137

respectively, which ,when calculated by theoretical formula, were found to be

0.963 MeV and 0.478 MeV for Co 60

and Cs 137

respectively. The experimental and theoretical results are

in good agreement. The small discrepancy in the experimental results can be due to the small voltage drop

across the resistance of the connecting cables used in the electronic setup.

No annihilation, single escape or double escape peaks were observed over Compton continuum in the

Cs 137

spectrum because its gamma photon energy 0.662 MeV is much less than the threshold energy

1.022 MeV of the pair production phenomenon. However, one pair annihilation peak was observed in the

Co 60

gamma spectrum because its lower energy gammas energy is greater than 1.022 MeV. This peak

appears in the spectrum due to pair production in the surrounding material of the detector. Photo peaks ,

X-ray peaks and backscatter peaks were commonly observed in both types of spectrum.

The energy resolution curve was plotted using spectra collected on MCA. Resolution of the photo peaks

was determined which is described by:

y = -0.5266x + 0.8581

-2.95

-2.9

-2.85

-2.8

-2.75

-2.7

-2.65

-2.6

-2.55

-2.5

6.4 6.6 6.8 7 7.2 7.4

ln R

ln E

Figure 9.Energy Resolution Curve

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Where H = height of the photo peak, FWHM = full width at half maximum and resolution increases when

R decreases and vice versa. A plot of ln(R) vs. ln(E) called the energy resolution curve was drawn which

was a straight line having slope equal to -0.5266 which is very close to theoretically predicted value -0.5.

Normally, the following factors contribute to the finite energy resolution of the detector:

i. Charge collection statistics

ii. Electronic contribution

iii. Detector response variation

iv. Drifts in operating parameters

v. PMT gain fluctuations

The small difference in calculated and theoretical values of the slope of the energy resolution curve may

be due to the contribution of some other factors.

Conclusion

Gamma spectroscopy was done using NaI(Tl) inorganic scintillation detector. Both the integral and

differential pulse height spectra of Co 60

were studied using SCA and differential pulse height spectra of

Co 60

and Cs 137

were also studied using MCA and it was concluded that MCA delivers better results as

compared to SCA. An unknown gamma source was indentified to be Cs 137

by making energy calibration

on MCA spectrum. It was concluded that NaI(Tl) plays very important role in gamma spectroscopy

because in collected spectra the energies corresponding to various peaks are in good agreement with their

theoretically calculated values. It was also found that the resolution of the detector decreases with

increasing gamma ray energy.

References

1. Knoll, G.F. ; Radiation Detection and Measurement, John Wiley & Sons (1999)

2. Nasir Ahmad ; Experimental Radiation Detection, CNS-20, (1987)

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