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Answer to Question #3088 Submitted to "Ask the Experts"Category: Instrumentation and Measurements — Instrument Calibration (IC) The following question was answered by an expert in the appropriate field: Q
My question is related to the selection of energy windows for 109Cd when a Packard II Gamma counter is used. The gamma counter I am using gives an option up to three energy windows, A, B, and C. When a sample spiked with 109Cd is counted using the windows, A: 15-150 keV, B: 15-32 keV, and C: 32-150 keV, we get three peaks: the first at 22.2 keV, the second at about 61 keV, and the third at 88 keV. I am really confused about these peaks and about the correct settings for the energy windows provided that the counting results I get are different with the energy windows, and I do not know which energy intervals I have to use. Can you please help me in this regard?
A
Cadmium-109 has an electron capture (EC) decay mode, with an 18 kiloelectron volt (keV) K-shell Auger emitted 13% of the time, and various 22 to 25 keV K-shell x-rays 65% of the time. This also feeds 109mAg, which has similar energy x rays 35% of the time, a low-yield 88 keV gamma ray 3.7% of the time, and a couple high-yield conversion electrons at 62.5 and 84.2 keV, 42% and 44% of the time respectively. In a gamma counter you will detect these x rays. The 22 keV peak is expected from the 109Cd and 109mAg x rays, and the same with the 88 keV peak from the 109mAg gamma. Depending on the amount of activity in your sample, I would not have been surprised if you said there was a 44 keV peak, from coincidence x-ray interactions in the crystal. However, given a three 22 keV x-ray coincidence interaction is unlikely, I would have to conclude that you have a fairly high atomic number sample or sample holder. And, the monoenergetic conversion electrons at 62.5 and 84.2 keV are being converted to x rays and contributing to the observed peaks. Regardless, you want to minimize background and maximize your sample net counts in a given counting period, to get the best sample counting statistics. If you only have these three preset windows, I’d examine channel "A" 15-150 keV and "B" 15-32 keV for the best figure of merit; or the square root of the ratio, sample plus background counts over background counts. I would recommend reviewing Chapter 4 in the text Radiation Detection and Measurement by G. Knoll cited in this topical area of ATE for an in-depth discussion of counting statistics and figure of merit. David J. Allard, CHP
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