4- Locate the channel number of the peak centroid with the software cursor and note the corresponding energy. Record these values.

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1 EXPERIMENT 2.1 GAMMA ENERGY CALIBRATION 1- Turn the power supply on to 900 V. Turn the NIM crate on to power the amplifiers. Turn the Oscilloscope on to check the gamma pulses. The main amplifier should be set to x8. 2- Place both calibration sources Cs137, and Co60 in the holder about 7 cm in from of the NaI crystal on the lift table. You can take these spectra together. Check for pulses on the oscilloscope. 3- Obtain spectra on the MCA (Multi-Channel Analyzer). 4- Locate the channel number of the peak centroid with the software cursor and note the corresponding energy. Record these values. Calibration Energy Channel # Source Cs Co Co Sources NaI Exercise-A Plot the Energy vs C# and fit this to a straight line of the form E() = A + B C# When A and B are known you can find the energy of an unknown peak by identifying the channel number of it s centroid. Record your fit values for A and B. E A B Hypothetically a source gives an unknown peak in channel # 300. What is the energy of that peak? Energy = Crudely Graph your Measurements here. Label axis please Exercise-B The MCA program has a calibration (mode-energy Calibration Only). Enter the three calibration peaks in to the software calibrator. The energy will appear on the screen for each channel number. Record the channel # and energies on a DATA PLOT. 1

2 EXPERIMENT 2.2 IDENTIFICATION OF UNKNOWN GAMMA EMITTERS 1- In the source cabinet you will find 2 unknown gamma emitters. U1 and U2. 2- Take spectra of the unknowns U1 and U2 and find the peak energies. DATA PLOT. Exercise- Determine the unknown isotope(s) by consulting Appendix A and NaI gamma spectrum in your Labook. Record energies on the DATA PLOT and identify unknowns. Record energies measured in identifying the peaks below and the standard values found in the laboratory manual. Unknown-1 Energy-1 Energy-2 Energy-3 Energy-4 Energy-5 Energy-6 measured standard Unknown-2 Energy-1 Energy-2 Energy-3 Energy-4 Energy-5 Energy-6 measured standard EXPERIMENT 2.3 GAMMA ANALYSIS OF Cs Follow experiment 2.3 for Cs 137. Place the Cs137 source next to the Pb absorbers to enhancc the backscatter peak. See page 5 for analysis. Exercise A- Complete the Table 2.4 but for Cs137. DATA PLOT. Photopeak Compton Edge Backscatter Pb X-ray Channel # Energy () Measured Energy () Theory Pb Source NaI 2

3 EXPERIMENT 2.4 ENERGY RESOLUTION of Cs137 Scintillators do not measure the precise gamma energy deposition due to the random nature of the energy release, PMT function, and associated electronics. Simplistically the number of electrons N e or charge measured by the MCA has a corresponding error ΔN = N. If we assume that the gamma energy is proportional to N, then E = a N ΔE = a ΔN = a N ΔE/E = 1/ N = a / E ΔE/E = b/ E (1) We can measure the fractional resolution R by measuring the width ΔE of the Cs137 photo-peak and comparing it to it s central energy Eo. Or equivalently ΔC and Co R(%) = ΔE/Eo x 100 = ΔC/Co x 100 (2) This is equivalent to measuring the number of channels across the peak at half maximum (FWHM) and dividing by the channel number of the centroid. Exercise A: Use equation 2 to calculate the resolution of the detector for two gamma energies, using the Cs137 and Co60 sources. Photo-peak ΔE or ΔC# E or C# R(%) Cs137 Co According to ( 1) the ratio of ΔE 1 /ΔE 2 ~ E 1 /E 2. Is this true?? EXPERIMENT 2.5 ACTIVITY OF A GAMMA SOURCE by RELATIVE METHOD The activity of a source is measured in Curie (3.7e10dps) or Bq (1dps). In determining activity with the relative method the activity of a known source As is compared to the activity of an unknown source of the same species. Rs and Rx are the number of counts accumulated in the photo-peaks. Ax = (Rx/Rs) As (3) (1) Find the present day activity of the standard Cs137 source ( 4/13 5µCi) with As = Aso exp(-t/τ ). T1/2 = 30 yrs for Cs137. As = (2) Measure the counting rates for the standard and unknown by taking 200s recordings and integrating the photo-peaks. Use the MCA left and right cursors and scroll to the ROI window. Determine Ax. Rs(cps) Rx (cps) Ax (Ci) 3

4 EXPERIMENT 2.6 ACTIVITY OF A GAMMA SOURCE by ABSOLUTE METHOD In the absolute method the counting rate for the unknown Rx is measured at a distance such that the detector radius r is small compared to the detector-source distance R. We will have to integrate the number of counts in the Cs137 photo-peak again with the source-detector distance at 20cm. Cs137 r R=20cm r = 9.3 cm Three correction factors must be applied. (1) f g is the probability that if a gamma enters the NaI(Tl) detector that it will end up in the photo-peak. for the 3 ½ NaI(Tl) f g = 0.40 at 20cm. (2) εa = 0.92 is the fraction of the time that a 662 kev gamma comes from a Cs137 decay. (3) G = [ (π r 2 )/ (4π R 2 ) ] is the fraction of the time a gamma enters the detector relative full area 4π steradian area. The measured rate is then Rx = G f g εa A and solving for A, A = Rx / G f g εa (4) (1) Place the source about R=20cm from the detector. Record the gamma spectrum. (2) Find Rx with the cursors. And compute the activity from equation (4). Ax = Ci (3) Compare this with the relative method in Exercise 2.5. Explain any significant differences. Activity (Relative Method) Activity (Absolute Method) 4

5 1) Full energy peak- The gamma from the source interacts in the NaI(Tl) crystal and its energy Eγ is fully absorbed. 2) Energy Resolution- Due to statistical processes in the crystal, PMT, and electronics the spectrum on the MCA has a centroid at E γ and a ΔE spread. The fractional resolution is given by ΔE/E ~a/e 4 for a NaI detector. 3) Compton scatter- A gamma interacts with an orbital electron in the shield or NaI(Tl) detector. The Compton scattering formula for the final gamma energy Eγ () in terms of the initial gamma energy Eγ(), scattering angle θ, and electron mass m e can be written as E! ' = E! 1 + (1 " cos# )(E! / m e c 2 ) (5) The kinetic energy of the electron is given by conservation of energy, T e = E γ - E γ (6) 4) Backscatter- A source gamma enters the lead shield and a Compton scatter occurs at θ ~ 180 o. The gamma which is emitted has energy distributed about Eγ () from equation (5) E! ' = E! E! (7) 5) Pb X-ray- The gamma excites a K-α x-ray (75KeV) in lead absorber which enters the NaI(Tl) crystal. 6) Compton Edge- The Compton edge occurs when the electron obtains it s maximum energy in the NaI(Tl) crystal upon Compton scatter or θ = 180 o. This electron s energy is absorbed in the crystal and results in an energy absorption just below the full energy peak. Example if Eγ =1 then from equation (7) Eγ = 0.2 and the T e = Full Energy Peak Pb X-ray 75 KeV Backscatter 0.2 Compton Edge 0.8 5