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 PCA-II card to function as a 1024-Channel analyzer, a NaI(Tl) crystal scintillation detector, a high voltage source, a pulse shaping amplifier, 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 up the computer, and open the file pcaii.exe. Erase any data which may be on the graphics image display by holding down the CTRL key and pressing F2 once.

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. Press the F1 key to start the data acquisition. You should see a spectrum growing on the screen. Pressing the F1 key will toggle the acquisition of data between ON and OFF.

When "Acquire: On" is indicated at the top of the parameters display (left column), the voltage pulses from the detector are collected, sorted by energy, and displayed in a channel with a number proportional to their energy. When the "Acquire: Off" is indicated, the number of counts (Ctn: ) stored in each channel (Chn: ) is shown at the bottom of the parameters display. Any channel number, 0 - 1024, 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.

With "Acquire: Off", the UP and DOWN arrow keys provide a fast entry to the Vertical Scale selection window. (Alternately, ALT S for the Setup Window could also be used.) Using the arrow keys select the Log scale for now and press the <ENTER> key.

(3)      Acquire the Cesium-137 data.

Again press CRTL F2 to clear the screen, and then press F1 to acquire data. Keep collecting data for one minute. Then press F1 to stop the data collection and observe the Cesium-137 energy spectrum as displayed. Use the UP and DOWN arrow keys to select a scale where the tallest peaks are on scale without spilling over at the top.

Note the main features of the spectrum:

• the main photopeak on the high energy side,
• the Compton edge just to the left of that photopeak,
• the Compton continuum with some small broad peaks, and
• the sharp photopeak on the left produced by characteristic x rays.

(4)      Perform an energy calibration.

Key in ALT-C to display the the "CALC" menu. Select "Calibrate" and follow instructions in the upper left of the display:

• "Enter Units":    Key in the three characters "keV".
• "Move the Marker to peak 1":

Place the marker precisely at the top of the first peak on the low energy side (left) utilizing the mouse or more accurately utilizing the LEFT and RIGHT arrow keys.

• "Calibration data":    Enter the text "30.97"
• "Move the Marker to peak 2":    Place the marker precisely at the top of the large peak on the high energy side.
• "Calibration data":    Enter the text "661.6"
• "Move the Marker to peak 3":    Do not move the marker, but press <Enter> twice to complete the calibration process. 

(5)      Record the characteristic spectrum energies.

Note that now as the marker is moved around, the corresponding energy in keV is displayed on the parameters list (instead of the channel number). Place the marker at the Compton edge (about half way along the slope) 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:

E_I = the incident photon energy
E_S = the scattered photon energy
E_o =  the rest energy of the electron
KE_e = kinetic energy of the scattered electron
q = the Compton scattering angle

then Compton Scattering equation can be written:

Also,  from conservation of energy considerations, KE_e = E_I - E_S

(6)      Print the spectrum.

• With the spectrum displayed, press the Print Screen key to save this spectrum to the clipboard.
• Open a blank Word document and paste in the spectrum.
• Save the spectrum in your user account for printing.
• Repeat the process to obtain a copy for each student lab report.

(7)      Save the Spectrum File Data

• Press ALT F to select the File window.
• Highlight the "Save Binary File" with the arrow keys and hit <Enter>.
• When requested, type in a file name (extension .spm will be added).
• Press <Enter> to save the data file on the disk.

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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. Using the UP and DOWN arrow keys select the Log vertical scale and press the <ENTER> key.

Again press CRTL F2 to clear the screen.  We will still use the previous energy calibration. Press F1 to acquire data. Keep collecting data for one minute. Then press F1 to stop the data collection and observe the Cobalt-57 energy spectrum as displayed. Use the UP and DOWN arrow keys to select a scale where the tallest peaks are on scale without spilling over at the top.

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.

• Print this spectrum as before.
• Save this spectrum as before.

Perform a new energy calibration.

Now reload the Cesium-137 file which was saved earlier by performing the following steps:

    press ALT F, select DIR, specify a file mask of *.spm if necessary,   select file with the name you chose.

Without erasing the original data, superimpose the Cobalt-57 spectrum by collecting Co-57 counts on top of the Cesium spectrum.  Collect enough counts until the main energy peak is tall enough to use as another energy calibration.

Key in ALT-C to display the the "CALC" menu. Select "Calibrate" and follow instructions in the upper left of the display:

• "Enter Units":    Key in the three characters "keV".
• "Move the Marker to peak 1":

Place the marker precisely at the top of the first peak on the low energy side (left) utilizing the mouse or more accurately utilizing the LEFT and RIGHT arrow keys.

• "Calibration data":    Enter the text "30.97"
• "Move the Marker to peak 2":    Place the marker precisely at the top of the large Cobalt-57 peak.

• "Calibration data":    Enter the text "122.0"
• "Move the Marker to peak 3":    Place the marker precisely at the top of the large peak on the high energy side.

• "Calibration data":    Enter the text "661.6"
• "Move the Marker to peak 4":    Do not move the marker, but press <Enter> twice to complete the calibration process. 

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

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 HIGH 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

Using the UP and DOWN arrow keys select the Log vertical scale and press the <ENTER> key.  Again press CRTL F2 to clear the screen.  This will still use the present energy calibration. Press F1 to acquire data.  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. Use the UP and DOWN arrow keys to select a scale where the tallest peaks are on scale without spilling over at the top.  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?

Save this file as before: ALT F, Highlight Save Binary File, name the file and press <Enter>.

Print this spectrum as before.

(4) Search for another higher energy peak.

 Halve the COARSE GAIN again (select 80). Use UP ARROW to change the Vertical Scale to Log. Acquire data until the spectrum shows a small new peak far to the right of the main photopeaks. Re-calibrate this scale with the same 1173 and 1332 keV peaks in order to read of the energy of the new peak. What is the origin of the new peak?

(5)    Perform a new energy calibration.

Now return the GAIN  to 160. Load the Cobalt-60 spectrum from part (3).  Replace the source with 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.

Key in ALT-C to display the the "CALC" menu. Select "Calibrate" and follow instructions in the upper left of the display:

• "Enter Units":    Key in the three characters "keV".
• "Move the Marker to peak 1":  Place the marker precisely at the top of the large Cesium-137 photopeak.
• "Calibration data":    Enter the text "661.6"
• "Move the Marker to peak 2":    Place the marker precisely at the top of the first large Cobalt-60  peak.
• "Calibration data":    Enter the text "1173"
• "Move the Marker to peak 3":    Place the marker precisely at the top of the second  large Cobalt-60  peak.  
• "Calibration data":    Enter the text "1332"
• "Move the Marker to peak 4".  Do not move the marker, but press <Enter> twice to complete the calibration process. 

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.

Press CTRL F2 to clear the original data. Employ the UP and /or DOWN arrows to set the vertical scale to Log.

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.

Press CTRL F2 to clear the original data. Employ the UP and /or DOWN arrows to set the vertical scale to Log.

Using this same Energy Calibration acquire a spectrum for each of the unknown sources in the group marked B. 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.

(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.  Select the Search.com program (found at Energy|Programs/AppsPY/ScienceWorkshop/Phy320) and follow the instructions. It will probably be helpful to print out the list of possibilities for each unknown. The final determination will probably require the use of information from the RADIOLOGICAL HANDBOOK (or similar reference material). Helpful data may be found on:

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

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