A couple of questions about the setup and methodology

Submitted by Anonymous on Tue, 2011-03-29 19:07

Hey Brian and the rest of you, I just found this forum, and GREAT JOB! Unfortunately, something I've been wondering for a while hasn't been quite answered yet and I was hoping you could help:

1a. In your setup, you use particle filtering and a Ge detector, which is great for a university but lousy for a hobbyist like myself (due to the expense). Do you think either a gamma scintillator probe or LSC would work as well? Why or why not?

1b. Why did you choose not to look at betas at all?

2. Are the lead bricks you use in your cave natural isotopic lead or are they the isotopically-purified ones (relatively pure Pb-206) used for ultra-sensitive work? I'm guessing the answer is "natural" due to the large number of U and Th-series peaks present, but I just want this clarified.

3. On a semi-related note, on your graph for Setup "A", the peaks shown for I-131 and CS-137 seem to me to be well down in the noise. How can you tell they're there from the background?

Thanks again!

Mike

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A few answers

Submitted by bandstra on Tue, 2011-03-29 19:25.

Hi, thanks for your interest in our work! (1a) Unfortunately, observing the lines from these isotopes requires a detector with high energy resolution (~0.1-0.2% at 1460 keV (FWHM)), which is why we use germanium detectors. Scintillator detectors could in principle detect the lines from these isotopes, but they tend to have upwards of 5-10% energy resolution (FWHM), so lines would either be in the noise or confused with background lines. I am not too familiar with LSC, so I suppose the sensitivity would depend on what the natural background is from beta particles. (1b) We do not have a high-efficiency setup for detecting betas. (2) We use regular lead. We do not have a ultra-low-background setup; we just bring the background as low as is reasonable, which is sufficient for the measurements we are making. You can see our minimum detectable levels in our plots. (3) In the gamma-ray spectrum from Air Setup A, the regions where the I-131, Cs-137, and Te-132 lines would be are highlighted. They are were not detected by Setup A since it wasn't very sensitive (the data for A is at the bottom of the Air sampling page showing that the lines are indeed in the noise.