PHYSICS 320 LABORATORY

 

GAMMA RAY SPECTRA

OBJECT: To acquaint the student with the operation of a multichannel analyzer utilized for pulse height analysis in conjunction with a scintillation detector. To gain experience in the interpretation of gamma ray spectra for the purpose of identifying gamma ray emitting isotopes.

EQUIPMENT: Computer utilizing a Canberra PCA3 card to function as a 2048-Channel, 256k peak-height resolution analyzer, a NaI(Tl) crystal scintillation detector, a high voltage source and several low activity gamma emitting sources.

PRELIMINARY OPERATION

  1. On the Scintillation Amplifier and Power Supply, set the gain controls to their maximum positions (Coarse Gain at 320 and Fine Gain at 1.5). Select a High Voltage setting of about 900 volts. Turn on the power supply and also the high voltage source.
  2. Boot the computer, and run the Gamma Acquisition program located in the Genie2000 folder of the Program Menu. 

CESIUM-137 SPECTRUM

(1)      Collect the spectrum.

Place a Cesium-137 gamma ray source on a shelf just below the face of the scintillation crystal. Under the File menu, choose Open Data Source.  Press the Detector radio button and select DET01.  Open the data acquisition program. Clear any data which may be on the graphics image display.  Press the Start button.  You should see a spectrum growing on the screen.

When acquiring data, the voltage pulses from the detector are collected, sorted by energy, and displayed in a channel with a number proportional to their energy.  The number of counts stored in a particular channel designated by the cursor is shown at the top of the graph.  The energy using the last known calibration is also displayed.  You will have to enter your own calibration shortly. Any channel number, 0 - 2048, can be selected by moving the cursor marker to that horizontal channel position with the mouse or with the LEFT and RIGHT arrow keys.

(2)      Select the vertical Scale.

Examine the function of the toolbar icons.  One of things you can do is select either the linear or log vertical scales.  The log scale brings out a greater contrast for identifying the peaks but you may have a harder time looking at relative peak heights.

(3)      Acquire the Cesium-137 data.

Observe the Cesium-137 energy spectrum as displayed. 

Note the main features of the spectrum:

(4)      Perform an energy calibration.

Under the Calibrate Menu, choose the Energy Only Calibration.  With the cursor, select the center of the x-ray peak.  Then either enter the number of the channel or click on the cursor button.  Enter 32.19 (keV) in the energy box and Accept the data.  Write in your lab book the values for the channel #, FWHM, and energy that you use for calibration.  You will need them later.  Put the cursor in the center of the main photopeak and enter 661.6 keV for this calibration point.  Click OK to end the calibration process.  Look to see that the energy readings are fairly well calibrated.  In the next section you will get a third calibration point at an intermediate energy using the photopeak of Co-57.  Store the spectrum on the desktop under the calibration menu as a .CAL file.  You can load the file under the calibration menu as a energy/shape file.

(5)      Record the characteristic spectrum energies.

*********
For this lab report, you will have a separate page in a Word document for each gamma emitter specimen.  On that page you should list the main peaks of each spectrum and identify the process which causes each peak. Use PrintKey to copy and paste the graph into a Word document. You will need to use the Chart of the Nuclides and other documentation and software that is available to understand each of the decay processes in the different samples.
*********

Place the marker at the Compton edge (on the linear scale, at the corner edge) and record the energy. Also, place the marker on and record the energy of other 'peaks' along the continuum.  For each of the peaks along the continuum, identify the probable origin of each.

In the lab report analysis, compare your Compton edge energy with the theoretical computed value.  Note, if:

EI = the incident photon energy
ES = the scattered photon energy
Eo =  the rest energy of the electron
KEe = kinetic energy of the scattered electron
q = the Compton scattering angle

then Compton Scattering equation can be written:

Also,  from conservation of energy considerations, KEe = EI - ES

Find a backscatter peak in this spectrum. It's energy plus the Compton edge energy equals the energy of the main photopeak.


COBALT-57 SPECTRUM

Remove the Cesium-137 source and store it away from the detector. Place a Cobalt-57 source on a shelf just below the face of the scintillation crystal. Leave the GAIN controls at the maximum values.

We will still use the previous energy calibration. 

Acquire the spectrum.  Note the special features. Particularly notice the absence of any significant Compton distribution. (Can you explain the lack of Compton Scatter?) Record the energy for the "peaks" displayed. Can you explain the origin of any peaks on the low energy side of the main photopeak.

Perform a new energy calibration.

In the Calibrate menu, select the Energy Full by Entry menu.  Check the Append to Existing Calibration box.  Put the cursor on the center of the main photopeak and enter that channel number.  It has an energy of 122.0 keV. Click the Add button and then the OK button.  Note the the FWHM for this entry is zero.  The software needs to measure this before you can proceed.  Highlight that entry and click on the Cursor button.  The FWHM should have changed.  Click the OK button to accept the new calibration.

Save this spectrum with some new name to the Desktop.  When you open this spectrum, the calibration will be loaded.  You can clear the spectrum and start looking at unknown samples emitting low energy gammas.


COBALT-60 SPECTRUM

(1)      Replace the source.

Remove the Cobalt-57 source and store it away from the detector. Place a Cobalt-60 source on a shelf just below the face of the scintillation crystal. Change the COARSE GAIN control to the 160 position. This should approximately double the energy per channel ratio and will allow photopeaks of higher energy to be observed.

(2)       Acquire the Spectrum

Acquire the spectrum  As the spectrum grows, the fact that 60Co has two gamma rays in  cascade will be apparent. Keep collecting data for one minute. When the data collection stops, observe the Cobalt-60 energy spectrum as displayed. With the spectrum displayed note these special features. The Compton scattering also yields a combination of distributions from the two separate gammas, making the experimental determination of the Compton Edge uncertain.

(3)  Re-Calibrate, Save, and Print

Calibrate this energy scale as before, but now use the two main photopeak energies: 1173 keV and 1332 keV which are characteristic of 60Co. Record the energy for other peaks which are observed. Can you explain the origin of any peaks on the low energy side of the main photopeaks?

(4) Search for another higher energy peak.

What is the origin of the new higher energy peak? 

(5) Perform a new energy calibration.

Replace the source with the Cesium-137 source.  Without erasing the original data, superimpose the Cesium-137 spectrum by collecting counts on top of the Cobalt-60 spectrum.  Collect only enough counts until the main Cesium peaks are tall enough to use for another energy calibration.

Save this spectrum with some new name so it can be retrieved to determine energies from unknowns which generally emit high energy photons.


IDENTIFYING UNKNOWN GAMMA RAY EMITTERS

Gamma ray emitters can be identified by measuring their emitted gamma ray energies and correlating them with standards. You will use the energy calibrations already established for the low energy and high energy groups.

(1)      High Energy Gamma Emitters -- (Group A)

Check that the high energy calibration is still loaded and that the COARSE GAIN is on 160.

Using this same Energy Calibration acquire a spectrum for each of the unknown sources in the group marked A. Print-Screen and paste one spectra per page into a Word document and use the rest of the page to identify the peaks of the spectra.  See (3) analysis section.

(2)      Low Energy Gamma Emitters -- (Group B)

Set  the amplifier COARSE GAIN to 320 and load the low energy calibration spectrum.

Using this same Energy Calibration acquire a spectrum for each of the unknown sources in the group marked B. Use PrintKey to paste one spectra per page into a Word document and use the rest of the page to identify the peaks of the spectra.  Write the decay scheme for the unknown nucleus.

(3)      The Analysis:

Attempt to match the energy spectrum properties of the unknowns to standards, and hence determine the unknown identities. You may begin the identification procedure by comparing your data to that of the isotopes catalogued in standard data.  Save the Search.com program to your Desktop.  Run it and follow the instructions. It will probably be helpful to write down a list of possibilities for each unknown.  The Chart of Nuclides will be helpful in determining the unknown source.  The final determination may also require the use of information from the Gamma Spectra and Nuclide Identification Catalog, Radiological Handbook and the Table of Characteristic X-rays found in the lab.  

(4)      Before leaving the lab, be sure to check that you have turned off:

High Voltage Supply, Amplifier Power, Computer, Monitor

Finally, be sure to return all gamma sources to their proper containers, and place them in the provided storage location.