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Re: [Phys-l] Geiger counter



This is a follow-up message to one sent a few minutes ago, if you are reading this first, you may want to read the previous message to put this message into context.

When I measure radon levels in my lab and get results between 1 and 2 pCi/L, I use my actively-pumped activated-charcoal-canister method. It is well established that this method is collecting radon gas itself, and we count those canisters with a NaI(Tl) detection system for ten minutes after letting them equilibrate for three hours or more in order to establish the 214-Pb and 214-Bi equilibrium with the 226-Rn. Otherwise, if the charcoal also collected radon daughters, an early count could be too high. Or, if the charcoal did not collect radon daughters (only collected radon itself), an early count would be too low. We have detected both increased and decreased counts in our canisters over a two to four hour period until the count stabilizes and then decays with the 3.8-day 226-Rn half-life.

This means that when I measure the radon level in my lab, I am collecting radon gas itself, and I am quite confident that this level fluctuates between 1 and 2 pCi/L.

I also now realize that cotton pads that have had air sucked through them with a vacuum sweeper are collecting the radon daughters, and not radon gas itself. I now realize it is possible there aren't any radon daughters in my lab air even though there is a little bit of radon in my lab air. This is because my lab air is heavily filtered. If our fume hoods are not running, my lab air is 90% recycled, and upon recycling it passes through filters. Therefore my lab air passes through filters at the rate of one filtered exchange per hour. Those filters are most likely removing radon daughters, and not the radon gas.

When I sample the air with a method that collects radon gas, I see traces of radon (1 to 2 pCi/L).

When I sample the air with the method people are using to collect radon daughters, I don't measure anything because there aren't any radon daughters in my air.

Today I tried detecting radon daughters on the air-handler filters. It's not clear I can detect it. There are two problems. First, it will be low, because the radon in the building is low. Second these filters have an active area of something like 50 m^2, so the daughters, if present, are quite spread out.

The easiest thing for me to do next will be to go to an area where I know the radon level is 1 to 2 pCi/L, but the air is not filtered, and I can try the sweeper/pad method in that location.

Michael D. Edmiston, PhD.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
Bluffton, OH 45817
Office 419-358-3270
Cell 419-230-9657