Answer to Question #3088 Submitted to "Ask the Experts"

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 of 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?

Cadmium-109 has an electron capture (EC) decay mode, with an 18 kiloelectron volt (keV) K-shell Auger emitted 13 percent of the time and various 22 to 25 keV K-shell x-rays 65 percent of the time. This also feeds ^109mAg, which has similar-energy x rays 35 percent of the time, a low-yield 88 keV gamma ray 3.7 percent of the time, and a couple high-yield conversion electrons at 62.5 and 84.2 keV, 42 percent and 44 percent 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 Glenn Knoll, cited in this topical area of ATE, for an in-depth discussion of counting statistics and figure of merit.

David J. Allard, CHP