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Experiment 4
Gamma Ray spectroscopy Using a Scintillation Detector
Objectives
:
1. To learn the basic working principles and the components of a scintillation detector.
2. To calibrate the scintillation detector system.
3. To observe the spectra of different gamma sources.
4. To find the resolution of the detector for different sources.
5. To study the full structure of 137Cs and 22Na spectra.
Theory:
1. Introduction to Scintillation detectors:
a. Description:
The basic function of a scintillation detector, much like other types of detectors, is to transform the energy of an incoming particle to a measurable electronic signal. A scintillation detector consists of two components:
1. Scintillator.
2. Photomultiplier tubes (PMT).
The scintillator converts a fraction of the energy of the incoming particle to light and the photomultiplier converts the light to a current signal, which can be manipulated further in an electronics system.
• Scintillators:
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A scintillator is a material that emits light, scintillates, when absorbing radiation. When a particle passes through the material it collides with atomic electrons, exciting them to higher energy levels. After a very short period of time the electrons fall back to their natural levels, causing
emission of light.
There are six different types of scintillators:
1. Organic crystals.
2. Organic liquids.
3. Inorganic crystals.
4. Plastics.
5. Gases.
6. Glasses.
In our experiment we will use Sodium Iodide (NaI ) crystal as our scintillator. NaI is an example of inorganic crystals.
• Photomultiplier tubes (PMT):
A photomultiplier is a device that converts photons to electric current. It consists of three parts:
1. Photocathode
2. Electron-Multiplier section (consisting of dynodes).
3. Anode.
In the photocathode an incident photon is absorbed by photoelectric effect, releasing an electron. In the Electron-Multiplier section this electron is accelerated through a series of secondary emission electrodes, dynodes, knocking out a number of new electrons from each of them, multiplying the number of electrons in each step. After multiplication, the total electron current is collected in the anode.
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Photomultiplier tube
b. Operation:
In order to clarify how a scintillation detector operates; the system can be divided into six consecutive stages involved in the detection of incident radiation. These are:
SCINTILLATION DETECTOR ASSEMBLY DIAGRAM
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1. Absorption of nuclear radiation in the scintillator.
2. The conversion of energy dissipated in the scintillator into emission of photons.
3. The transference of the light photons to the photocathode.
4. The absorption of the photons at the photocathode and the emission of the photoelectrons.
5. Electrons multiplication process within the photomultiplier.
6. The current pulse from the photomultiplier tube is amplified then analyzed by various electronic equipments.
c. Advantages:
1. It has high detection efficiency.
2. It has short dead time.
3. It has the ability to distinguish between types of radiations.
4. It can measure the energy of the radiation and hence can be used to identify different isotopes.
5. It has good energy resolution.
2. Calibrating the detector system:
The scintillation system gives the spectrum in terms of counts vs channel numbers. A channel number is an integer that corresponds to the voltage of the electron signal corresponding to each number of counts. Before we can use the detector we need to convert these channel numbers into actual energies. This process is called: energy calibration.
3. Energy resolution of a scintillation detector:
An important aspect of a Gamma ray spectrometer is the ability to
distinguish between Gamma rays with slightly different energy. This quality is characterized by the so called energy resolution. The resolution at a certain energy peak in the spectrum is the ratio of some measure of the
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width of the peak to the energy that corresponds to the center of the peak (the centroid). It is standard practice to use the Full-Energy Width at Half the Maximum peak count (FWHM) as the measure of the peak width.The energy resolution is then given by:
𝑅 = 𝐹𝑊𝐻𝑀
𝐸 × 100 Where: E is the energy of the centriod.
Gamma spectrum from the
decay of 137Cs.
4. Different structures in the gamma spectrum:
We will be mainly interested in 3 different structures each corresponding to one of the ways by which gamma radiation interacts with matter. Namely these ways are:
1. The photoelectric effect 2. Compton scattering 3. Pair production.
1. The Photopeaks:
These are peaks that correspond to the interaction of gamma rays by the photoelectric effect. Here the incident photon disappears and a photoelectron is produced with an energy that is very near to the original energy of the incident photon. Hence these peaks are the most
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important structures in the spectrum and the ones that are used to identify different isotopes.
2. Compton edge:
This is due to Compton scattering by electrons. The Compton effect is an elastic “collision” of the gamma photon with an atomic electron in which relativistic mass-energy and momentum are conserved. After the collision, the electron has a kinetic energy that is a fraction of the gamma’s original energy, The gamma loses this energy, becoming a lower energy wave with energy, E . The relation between the gamma’s energy before the collision compared to after the collision is
𝐸1
𝛾́− 1
𝐸𝛾 =1−cos 𝜃
𝑚𝑒𝑐2 (1)
Fig 1
where mec2 is the rest mass energy of the electron (511keV) and q is the angle through which the gamma is deflected. A depiction of the Compton scattering is shown in Figure 1. What is the origin of the Compton edge? A gamma enters the crystal of the detector and Compton scatters off an electron. The Compton- scattered gamma leaves the detector, so the amount of detected energy is the kinetic energy given to the electron. The maximum kinetic energy given to the electron, Emax, results from a “head-on” collision with the gamma, scattering the gamma photon backwards ( = 180o).
From equation (1) this maximum electron energy is:
𝐸𝑚𝑎𝑥 = 𝐸𝛾 − 𝐸𝛾́ = 2𝐸𝛾2
2𝐸𝛾+𝑚𝑒𝑐2 (2)
This Emax is the energy of the Compton edge. Of course the electron may suffer a gentler collision and have less than the maximum energy after the collision. This is
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the origin of the broad distribution of events at energies less than the Compton edge.
3. Annihilation peak:
When gamma rays that have energies higher than (1022 KeV) are incident on the detector, some of the gammas can interact with the material
surrounding the detector crystal via pair production. The photon disappears and an electron positron pair appears. The positron have a very short range and hence it will combine with another electron shortly and the pair will annihilate producing two gamma rays in opposite directions (to conserve momentum) each of which has an energy of 511 keV. Since these photons are in opposite directions only one will enter the detector. Those gammas will cause a peak to appear in the spectrum at 511 KeV.
Apparatus:
Scintillation detector with (NaI) crystal.
MCA (multi channel analyzer) connected to computer.
Different Sources of gamma radiation.
Safety precautions:
1. Handle the radioactive sources with respect. Don’t bend or try to break them.
2. Never touch the source using bare hands. Always use forceps to handle sources.
3. Do not eat or drink during the lab.
4. Practice ALARA by being as far away from the sources as possible and returning the source to your instructor as soon as you are finished with it and also by using appropriate shielding.
5. Lead is a potentially toxic material if it inters the body by swallowing or breathing its dust and if the exposure is long term. DO NOT touch your face or put your hands in your mouth after touching the lead bricks used for shielding. Always wash hands before leaving the lab.
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Procedure:
1. Connect the detector to the computer (make sure not to confuse the HV cable with the NBC cable)
2. Connect the plugs to the electric mains and switch the computer and monitor ON.
3. Go to start menu then programs then spechtech and click on ICW16 to open the software.
4. Set the high voltage to 700 V and to the on position. (A green bar will appear on lower part of the window, written on it 700V indicating that the high voltage is on).
5. Put the Na22 source on the second shelf under the detector.
6. Click the start button (green diamond) on the software window to start counting and acquiring the spectrum. (see figure on next page)
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7. After 10 minutes (when real time is at 600) click the stop button to stop counting. Hover with the mouse over the rightmost peak in the Na22
spectrum and right click on it. Choose “ROI >>> set ROI” (region of interest) and drag the pointer to shade the whole peak area. Write down the
centriod channel number.
8. Keep the spectrum and replace the Na22 source with a Ba133 source and repeat steps 5-7 highlighting this time the rightmost peak of the Ba133 spectrum.
9. Calibrating the detector: On the menu bar go to: settings >>> energy calibrate >>> two point calibrate. A window will pop up asking you which units you want to use. Type: keV and click OK.
10. The following window will pop up.
Enter the channel number of the centroid of the Ba133 that you wrote down in step 8 and enter the energy (356 keV) and click ok. Do the same for cal point 2 entering the channel number of the centroid of the Na22 peak and its known energy (1274.5keV). Click OK. Now your spectrum is
calibrated.
11. Go again to the ROIs that you shaded earlier and note for each the centriod energy and the FWHM in table 1.
12.Set the other peak in the Na22 spectrum as a ROI and take its readings for centroid and FWHM and write them down in table 1.
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13. Click the erase button to erase the spectrum and right click anywhere in the window. Choose : ROI >>> clear all.
14. Remove the Ba-133 source and put a Cs 137 source and acquire its
spectrum as you did with the previous two sources and the information of the photopeak (centriod and FWHM) and write them in table 1.
15. Compton edge: For the same spectrum, calculate the Compton edge energy. The Compton edge energy is the maximum energy (Emax) an electron can get in a Compton scattering with a gamma photon. It is the energy corresponding to about half the change in the count rate from the trough between the photopeak and edge, to the immediate maximum in the edge.
16. Remove the Cs-137 source and put a Co-60 source and acquire its spectrum for 10 minutes and write down the centroids and FWHM for both peaks in table 1.
17.Remove the Co-60 and put the unknown source and acquire its spectrum.
Write down the energies of the centroids of all its photopeaks.
18. Calculate the resolution of the detector for the various peaks of all the sources you used.
19. From equation 2 find the rest mass of the electron and find the percentage error in it.
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Data sheet
Experiment 4: Gamma Ray spectroscopy Using a Scintillation Detector
Sources information:
Data and calculations:
Calculating the energy resolution:
element
Channel Number
Energy Centroid
( )
Energy FWHM ( )
R
22Na Peak 1 Peak 2
133Ba
137Cs
60Co Peak 1 Peak 2
Calculating the Compton edge energy (Emax) and the rest energy of the electron.
- Counts corresponding to the trough between the photopeak and the Compton edge: C1= ……… (counts)
element activity (Ao) ( )
half-life t1/2
( )
Calibration date
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- Counts corresponding to the maximum in the Compton edge: C2
=……… (counts).
- Counts corresponding to the maximum energy (Emax):
𝐶 = 𝐶1+2𝐶2 = ……….. (counts).
- Emax =………. ( ).
- Calculation of rest mass of electron (mec2):
mec2 =………..….. ( )
- % error in the rest mass of electron:
Energies of centroids of photopeaks in the spectrum of the unknown source:
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Post lab questions:
1. For all the peaks in table 1 determine whether they are photopeaks (resulting from photoelectric effect), Compton edges (resulting from
Compton scattering or pair annihilation peak. (You can check appendix E for help if you need).
element Peak type
22Na Peak 1 Peak 2
133Ba
137Cs
60Co Peak 1 Peak 2
2. Use appendix E to determine the unknown isotope.
