Purpose
This lab should familiarize you with the operating characteristics of a High-Purity Germanium (HPGe) Detector system and with the Genie-2000 spectroscopy software.Relevant Reading
Tsoulfanidis: Chapter 7, pages 235-262; special attention: sec. 7-5, Chapter 12- sec. 12-7, pages 400- 418
Knoll: Chapter 10, 11, 12.
Equipment
HPGe detector setup is very similar to that of NaI(Tl) lab. However when available please make use of the Bias-Shutdown facility of the HVPS for the HPGe detector.
Lab Objectives
1. Familiarize yourself with the workings of the Genie software package
2. Learn the operating characteristics of an HPGe counting system
3. Analyze an unknown spectrum to determine the isotopic composition
4. Determine the energy resolution and efficiency curve for the HPGe detector and compare with NaI detector
Procedure
1. The detector system should be set up when you arrive. Ask for any pertinent information to develop your lab report.
2. Calibrate you system and make sure that it compares well with NaI calibration. You will have access to the data file spectrum generated for Cs-137 source using NaI detector. Load that spectrum for reference.
3. Set the software to count for 15 minutes of live time. Run a count with no source present. This will form your background spectrum. Save this spectrum to a file, as well as save the image to be included in your lab report. Also save the counts information as was done in previous lab work.
4. Set the software to count for 10 minutes of live time. Place a 137Cs source in front of the detector and run a count. When finished, save the spectrum to a file, as well as save the image to be included in your lab report.
5. For each of the photopeaks (ROI’s) record/determine:
a. Centroid channel and energy
b. ROI range (both channel and energy)
c. FWHM
d. Peak Areas (net +/- error) and Integral area
e. Resolution (in %)
Compare Cs-137 spectrum spectrum obtained using HPGe detector with the one from last week, obtained using NaI detector. In particular compare FWHM, efficiency, peak to total ratio and peak to Compton ratio. Also compare the two spectra by plotting them on the same graph.
6. Obtain a mixed gamma standard and place it in front of the detector. Count this sample for ten minutes. After counting is complete, save the data file. This is your unknown spectrum. Visually identify the peaks in the spectrum. How many can you see? Be sure to identify Compton edges, escape peaks (if any) and backscatter peaks. Save the image to a file to include in your report.
7. Find all significant peaks. For each of the photopeaks (ROI’s) record/determine:
a. Centroid channel and energy
b. ROI range (both channel and energy)
c. FWHM
d. Peak Areas (net +/- error)
e. Resolution (in %)
f. Peak ID
8. Plot the energy as a function of channel number. This should be equivalent to your energy calibration curve. Generate a trendline or best-fit approximation and estimate the channel number for a peak with energy as 2505.74 keV.
9. Open up the background file you saved for the unknown sample in parts 3 and 4. For each of the ROI’s determined, find the total counts. Subtract these from the peak areas found in this lab to determine net peak area minus the background. Then, for each peak, compute the net counting rate.
10. Determine the peak efficiencies. You will need the manufacturing date for the source, half-life, relative intensity of photopeak, counting time and peak area. Plot the efficiency as a function of energy. How does this efficiency curve compare to that generated by the NaI detector?
Further Questions
Why must liquid nitrogen be used for an HPGe detector? What would happen to the noise in the system as the temperature increases? Why would this occur (base this explanation on how semiconductors operate). Would this be a problem for a silicon-based counting system? Why or why not?
What is the approximate resolution of the HPGe detector? How does this compare to the energy resolution of the NaI detector? How does it compare to the ideal resolution for HPGe counting systems?
What benefit does better energy resolution have on peak analysis or peak determination? Think in terms of trying to discriminate between two peaks with similar energies.
What is the approximate efficiency of the HPGe counting system? How does it compare to a NaI detector?
If an HPGe system is better at spectroscopy (in terms of resolution and peak determination), why would we still use NaI detectors?