BEYOND THE FIRST YEAR

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Aubrie Patrick
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1 Today we are going to do two experiments. BEYOND THE FIRST YEAR We will determine the rest-mass energy of an electron using gamma spectroscopy. We will calibrate a NaI(Tl) scintillation detector plus multi-channel analyzer (MCA) system to observe the spectrum from some common radioisotopes. A plot of the Compton edges will be used to determine the rest mass of the electron. This experiment is an excellent test of the idea of relativistic mass-energy. First, we will calibrate the system as above and use the multi-channel scaling (MCS) feature of the MCA to measure the half-life of the 1 st excited state of 137m Ba and to measure the statistical nature of the decay process. Due to the length of time needed to run this experiment, we will start it at the beginning of class. MULTICHANNEL SCALING AND 137m Ba HALF LIFE: The spectrometer memory locations (channel numbers) used to store the gamma data can be use in a different manner called Multichannel scaling (MCS). Using the MCS option will result in the computer storing the total number of gammas counted in the selected time period (called the dwell time), starting with channel 0 and proceeding through all the channels in order. This will allow you to record the gamma intensity as a function of time. This will be used to determine the half-life of 137m Ba. 137m Ba has a half-life of only a few minutes and therefore is very appropriate for use in a class lab. The 137m Ba is obtained from a 137 Cs/ 137 Ba mini-generator or cow. The mini-generator contains a small quantity of 137 Cs (<10 uci) bound on a special ion exchange medium. The 137 Cs parent beta decays with about a 30 year half-life to produce 137m Ba which in turn decays with a 2.55 minute half-life generating a 662 kev gamma ray emission. This gamma may be readily detected by a GM or scintillation detector. An eluting solution is used to selectively extract the 137m Ba isotope from the exchange medium leaving the parent 137 Cs isotope in place to regenerate more 137m Ba. While still in the PHA mode, obtain a 137 Cs spectrum. Place the lower and upper level discriminators (LLD and ULD) around the 662 kev peak using the convenient sliders at the bottom of the screen. Now switch the MCA to the MCS - Internal mode (under the Mode pull down menu) with a dwell time of 1 second. Obtain a 137m Ba sample in a planchet and place under the detector. (Facilitators will deposit about 10 drops of solution into a planchet when you are ready to acquire date.) Immediately, start the MCA and obtain a half-life spectra. Observe the spectrum in Log mode and determine the half-life by selecting a convenient count, noting the time that it took to acquire this count. Then, select a count that is approximately half of the original count. Note the time it took to acquire this second count. Subtract the first time from the second time and this value represents the half life. Spectrum Techniques, LLC Visit Page 1

2 Half life = time of second count - time of first count. What half life did you get? Ba137m Decay Spectra for Half-Life Determination, Log Display. If time allows, open up the discriminators to 100 kev and 750 kev. Do statistics improve? DETERMINING THE REST MASS OF AN ELECTRON We begin the second experiment by calibrating the spectroscopy system. To start, we will use the auto-calibrate feature of the MCA. Insert a 137 Cs source in the top shelf of the detector stand and select Autocalibrate from the drop-down SETTINGS menu. After a couple of minutes, the system will be calibrated to 1 channel equals 1 kev full scale. Since we use 137 Cs for auto-calibration, the photopeak should be in channel 662, corresponding with 662 kev. Erase and acquire a new 137 Cs spectrum for analysis. Set the live time for 5 minutes from the SETTINGS and Time drop down menus. Note that there is more structure in the spectra other than just the photopeak. You will notice a small peak, or more precisely an edge, at about 477 kev. This is marked as the Compton Edge in the handout. There is also a peak at 185 kev. Since there is only 1 gamma energy from 137 Cs, yet the observed spectrum has a photopeak at 662 kev, and the two Compton structures (an edge at 477 kev and a peak at 185 kev). How does this happen? Is it due to Compton scattering by electrons? What is the origin of the Compton edge at 477 kev? A gamma enters the detector crystal and Compton scatters off an electron at rest, through an angle Ө. The Compton scattered gammas leave the detector, so the amount of detected energy is the kinetic energy given to the electron. The maximum kinetic energy given to the electron, E max results from a head-on collision with the gamma, scattering the gamma photon backwards (Ө = 180º). Since cosine of 180º = -1, the maximum electron energy is 1/E c - 1/Eγ = (1- (1 - cos(180º))/m e c 2 = 2/m e c 2 for backscattered photons. and E max = E c = Eγ - E b = 2Eγ 2 /(2Eγ + m e c 2 ) Spectrum Techniques, LLC Visit Page 2

3 Compton Edge, 477 kev Where: E max = E c = Energy of the Compton Edge Eγ = Energy of the original gamma (662 kev for 137 Cs) m e = mass of electron c 2 = speed of light squared m e c 2 = rest mass of electron Simplifying, E c = Eγ - E b = = 477 kev (for 137 Cs), where E b is the backscatter peak at 185 kev. The Compton edge represents this maximum energy given to the electron. The electron may suffer a gentler collision and have less than the maximum energy after the collision. This is the origin of the broad distribution of events at energies less than the Compton edge. If you have a peak at 185 kev, it is due to gammas that interact with an electron outside the detector where they are detected by the photoeffect. Only a few angles near 180º result in scattering into the detector so that a peak results at (E b = Eγ - E c ). This is called the Compton backscattering peak. Place a thick lead absorber directly beneath the source to sandwich the source between the lead and the detector. Start acquiring additional data and watch the backscatter peak at 185 kev increase. Energy conservation requires that the sum of the Compton edge energy and the backscattering energy be equal to the original gamma energy (photopeak energy). (Eγ = E c + E b ; 662 kev = 477 kev kev) With No Lead, the 185 kev backscatter region contains 105,847 Gross Counts With Lead directly below the source, the 185 kev backscatter region contains 126,940 Gross Counts. There is more backscatter. Note also that the Lead X-Rays have increased. (Continued on page 4) Spectrum Techniques, LLC Visit Page 3

4 60 Co has the highest energies we will be looking at so we next expand the calibration to include these energies. Replace the 137 Cs source with the 60 Co source, erase the current spectra and start acquiring a new spectra. Change the fine gain with the slide bar at the top of the display until you see the two peaks of 60 Co rising in the display. Adjust gain until you get the peaks in the far right side of the spectrum. (You will need to erase the spectra several times while acquiring to see the two peaks clearly). Next, perform a 3-point calibration using the 60 Co peaks and the 137 Cs source (or another source with a lower photopeak less than 137 Cs) with the 60 Co source. Erase and obtain a 60 Co spectrum. Determine the Compton edges and backscatter peaks. Co60 and CS137 Spectra, Log Display & 3-Point Energy Calibration Co60 and CS137 Spectra, Linear Display & 3-Point Energy Calibration If time allows, use some other sources to obtain Compton edge and backscatter peaks. Plot the data on the graph sheet provide in the handout and determine the rest mass of the electron. (Everybody get 511 kev???) Co60 Spectra, Linear Display. Where are the Backscatter and Compton Edges? Spectrum Techniques, LLC Visit Page 4

5 Plot for determination of the rest mass of an electron Copies of this handout may be downloaded at as well as an MCS spectrum that you may use to find the half life of of 137m Ba. Remember, you can download the UCS30 software from the Spectrum Techniques web site and analyze spectra without having a UCS30 attached to your computer. Thank you for your participating in our experiments. If you learned something, that s great! Please tell us what we did right and where we could improve. Call or contact us if you have any questions or if we may be of further assistance. us at Sales@SpectrumTechniques.com Spectrum Technques, LLC 106 Union Valley Road Oak Ridge, TN USA Phone: FAX Sales@SpectrumTechniques.com Spectrum Techniques, LLC Visit Page 5

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