Experiment 10 Spectrum analysis of Co-60 and Na-22files.uod-physics-edu.webnode.com › 200000108... · Experiment 10 Spectrum analysis of Co-60 and Na-22 Introduction In gamma ray

Experiment 10

Spectrum analysis of Co-60 and Na-22

Introduction

In gamma ray spectroscopy the intensity of gamma rays are measured as a function of the

gamma energy. The detector used must be able to measure the energy of each gamma

ray and it must have good detection efficiency. A NaI scintillation detector detects gamma

photons with good efficiency up to the MeV energy range and it also has adequate energy

resolution to distinguish gamma rays of different energy from each others. A gamma ray

spectrometer includes a detector with amplifers and a multi channel analyzer (MCA). The

detector produces analog output pulses with heights proportional to the energy of the

absorbed gamma rays. The MCA digitizes the pulses and classifies them according to

their heights and creates a histogram with pulse height (i.e., gamma energy) as the x-axis

and intensity (i.e., number of counts) as the y-axis. The histogram is called an energy

spectrum.

Gamma ray spectroscopy is applied for example in nuclear medicine, environmental

monitoring of radioactivity and in security screening to identify radioactive isotopes.

Theory

The unstable isotope Co-60 decays through β- emission. Figure 10.1 shows the decay

scheme of Co-60.

Figure 10.1. Decay scheme of Co-60

From the decay scheme we see that in 99.88 % of the decays an electron (β- particle) of E

= 0.31 MeV is emitted followed by two gamma rays of energies Eγ1 = 1.17 MeV and Eγ2 =

1.33 MeV.

The radioactive isotope Na-22 decays through β+ emission. Figure 10.2 shows the decay

scheme of Na-22.

Figure 10.2. Decay scheme of Na-22.

The decay scheme shows that the β+ emission is in 99.94 % of the decays followed by a

gamma ray with an energy Eγ1 = 1.27 MeV. The emitted positron will immediately

annihilate with an electron producing two additional gamma rays of an energy equal to the

electron rest mass energy Eγ2 = 0.511 MeV.

When the radiation emitted by Co-60 or Na-22 is measured and acquired by a NaI

scintillation detector the energy spectrum exhibits the theoretical decay energies shown in

figures 10.1 and 10.2. However, in addition to the decay energies the spectrum will contain

several other features that are related to the detection process. These features are mainly

due to

1. Background radiation

2. Characteristic X-rays

3. Compton scattering from the scintillator material

4. Compton back scattering from materials surrounding the detector.

Figure 10.3 shows a NaI scintillation detector receiving gamma radiation. The produced

energy spectrum reflects all the different detection processes and sources of radiation.

Figure 10.3. Detection of gamma radiation with a NaI scintillation detector.

In figure 10.3 point 1 shows a gamma photon that undergoes photo electric absorption in

the scintillator. This gamma photon releases its total energy in the detector and

disappears. The energy of the gamma rays absorbed through the photo electric process

appears in the energy spectrum as peaks at the specific decay energies. These peaks are

called photo peaks.

Point 2 shows a gamma photon that is Compton scattered from the scintillator atoms.

These gamma rays leaves only part of their energy in the detector and continues their path

in another direction. The Compton scattering process is a random process which means

that the energy absorbed in the detector can take any value between 0 and Emax. The

Compton scattered gamma rays produce a continues background in the energy spectrum

called the Compton continuum. If Eγ is the energy of the gamma ray entering the detector

and interacting with the scintillator then the energy of the scattered gamma ray is given by

1

2

3

4

5

Compton back scattered photon

Background radiation

)cos1(12

0

'

cm

E

EE (10.1)

where m0c2 is the electron rest mass and θ is the scattering angle. The smallest value of

Eγ’ corresponds to θ = 180°. Thus

2

0

'

min, 21

cm

E

EE

(10.2)

Since m0c2 ≈ 0.5 MeV we may rewrite equation 10.2 as

E

EE

41

'

min,

(10.3)

where the energies are given in units of MeV. The maximum energy absorbed in the

detector corresponding to θ = 180° is

E

E

E

EEEEEC

41

4

41

2

'

min,max, (10.4)

This energy is called the Compton edge and it is always less than the photo peak energy

(gamma decay energy) Eγ. Equation 10.4 gives Emax in MeV when Eγ is given in MeV.

Point 3 shows a gamma ray that scatters from materials surrounding the detector. The

energy of these gamma rays is given by equation 10.2. They may hit the detector and

contribute to the Compton continuum.

Point 4 shows a gamma ray that scatters from materials behind the detector with θ = 180°.

This gamma ray is called a back scattered photon and it may interact with the detector

through the photo electric process releasing all its energy which equals

E

EEE scatterbackC

41

'

min,, (10.5)

EC,back scatter appears in the energy spectrum as a peak which is called the Compton back

scatter peak. Equation 10.5 gives EC in MeV when Eγ is given in MeV.

Point 5 shows a background gamma ray absorbed by the detector. Background radiation

often appears in the energy spectrum resembling Thorium or Uranium decay energies.

The energy spectrum may also contain characteristic X-ray peaks of the decay products.

Figure 10.4 shows a typical energy spectrum acquired by a NaI scintillation detector. The

different contributions 1 – 5 are indicated in the figure.

Figure 10.4. Energy spectrum of Cs-137.

Experiment

In this experiment a background energy spectrum and the energy spectra of Co-60 and

Na-22 are acquired. The spectra are energy calibrated and the different features of the

spectra are identified. The observed Compton edge and back scatter peak energies are

compared to theoretical values.

ATTENTION! The scintillation detector is a very sensitive and expensive instrument.

Handle it with extra care!

PREPARATION

1. Place the scintillation detector on the soft plastic cover and

connect the detector to the computer with the USB cable.

2. Start the MAESTRO program .

0

5000

10000

15000

20000

0 100 200 300 400 500 600 700 800

Co

un

ts

Energy/keV

1

Background radiation (thorium)

Compton continuum

Gamma photo peak

Compton edge

Compton back scatter peak

X-ray peak

EC,back scatter EC,max

3. Connect to the detector from the Detector/Buffer list.

4. Set the measurement time to 300 s: click Acquire and

choose MCB Properties. Click the Presets tab and enter the

time.

5. Check that the external amplifier gain is 1.0: click

Acquire and choose MCB Properties. Click the Amplifier

tab. Enter 1.0 and click Close.

6. Set the PMT HV to 700 V: click again Acquire and choose MCB Properties. Click

the High Voltage tab and enter the voltage. Click On. Click Close.

7. Make sure that the calibration is off: click Calculate and choose Calibration

. Click Destroy Calibration (if the button is

gray the calibration is already off).

DATA ACQUISITION

8. First a background spectrum is measured. Make sure no source is in front of the

detector. Click Go to start the data acquisition .

9. Adjust the horizontal and vertical scale to see the spectrum. Use the + and –

buttons and the arrow keys.

10. Wait until the measurement is completed (5 minutes).

11. Save the spectrum as a text file: click File and choose Save As .

Select the File type as ASCII and save the file to your flash memory or on the PC.

Name it “Background”.

12. Reset the data acquisition: click the Clear button .

13. Now place the Na-22 source in front of the

scintillator. Acquire a spectrum following steps 8 –

11. Name the spectrum “Na-22”.

14. Replace the Na-22 with the Co-60 source and

repeat step 13. Save the spectrum and name it

“Co-60”.

13

14

ENERGY CALIBRATION

15. From the Co-60 spectrum choose a region of

interest (ROI) around the peak furthest to the

right: click at the left side of the peak and

sweep over it. Then right click and choose

Mark ROI.

16. Double click the red ROI to find the channel

number of the center of the peak. This

channel number corresponds to 1.33 MeV.

17. Click Calculate and choose Calibration

.

18. In the window that appears enter the energy

value in keV: 1333 (this is the exact energy

of the Co-60 peak in units of keV).Click Ok.

19. Clear the ROI: Click ROI and then Clear All.

The Co-60 spectrum is now energy calibrated. You can now check

the energy in keV at each point of the spectrum by moving the

cursor to the desired point and reading the energy at the bottom left

of the program display.

20. Save the Co-60 and close it.

15

16

21. Open the Na-22 spectrum:

22. Destroy the calibration (see step 7).

23. Define ROIs around both photo peaks (see below).

24. Click Calculate and choose Calibration. Enter the first peak channel number and the

corresponding energy: 511 keV. Click Ok. Repeat for the second peak with the

energy 1275 keV. Click Ok. Clear the ROIs (see step 19).

25. Save the Na-22 spectrum and close it.

BACKGROUND SUBTRACTION

Before analysing a spectrum the background has to be subtracted.

26. Open the Co-60 spectrum

27. Click Calculate and choose Strip. Select ASCII, choose the Background file and

click Open.

511 keV

1275 keV

The background is now subtracted from the Co-60 spectrum

28. Save the subtracted Co-60 spectrum. Name it Co-60-b.

SPECTRUM ANALYSIS

29. Study the Co-60 spectrum. Identify the spectrum features as below, check their

energies with the cursor and write your findings in the table of results (see figure

10.4 for help).

3

5

1

2

4

6

E1 E2 E6 E4 E3

30. Close the Co-60 spectrum: .

31. Open the Na-22 spectrum: .

32. Subtract the background from the Na-22 spectrum following step 20.

33. Identify the spectrum features as below, check their energies with the cursor and

write your findings in the table of results (see figure 10.4 for help).

34. Save the subtracted Na-22 spectrum. Name it Na-22-b

35. Close the Na-22-b spectrum.

1

3

2

4

6

E1 E3

E6 E2 E4

Zoomed view

5

36. Switch off the high voltage: click Acquire and choose MCB Properties. Click the

High Voltage tab and click Off. Click Close.

37. Close the Maestro program.

PRINTING THE SPECTRA

38. Open the Co-60-b spectrum: double click on its icon . Click Options and

choose Plot . Change the vertical range to linear, deselect Auto

scale and click Range. . Enter 4000 and

click Ok. Click Ok again.

39. Print the spectrum: . Close the Co-60-b spectrum.

40. Open the Na-22-b spectrum, change the vertical scale and print it (repeat steps 38

and 39).

Co-60 Na-22

Feature name Energy/keV Feature name Energy/keV

1 E1 1 E1

2 E2 2 E2

3 E3 3 E3

4 E4 4 E4

5 E5 – 5 E5 –

6 E6 6 E6

Figure 10.5. The table of results.