Answer to Question #3752 Submitted to "Ask the Experts"

During the spring months after significant rainfalls I have seen elevated readings in my effluent sampling media (KI charcoal). We work primarily with ¹²⁵I which is all I would expect to see in the samples. The samples are downstream of charcoal filter banks. I have done a short half-life study on the sample and it is decaying at a faster rate than the rate of ¹²⁵I (I use a 59.6 day half-life). I am measuring the samples on a well NaI crystal single channel analyzer. Have you seen any naturally occurring radon daughters that might attribute to this anomaly? This happens routinely every spring and I am supposing that the radon is forced out of the ground by the rainfall and some of the material is somehow bypassing the filtration and being collected on the media.

Analyze the suspect effluent sampling media on a low-background, high-resolution gamma-ray spectrometer to identify the radionuclide(s). If you do not have access to such a spectrometer, measure the half-life (lives) quantitatively and compare it (them) with likely suspects, including ²²²Rn. It would seem likely that ²²²Rn is the problem, except that you are sampling air that has already passed though a charcoal filter bank, which would presumable remove most of the radon.

Tom Gesell, PhD