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All
24 hours of decay is fine if the Ra-220
concentration is low, but the thoron progeny are longer lived that the radon
progeny.
John R. Johnson
Radsafe: The short-lived radon daughters collected on any air filter have an approximate half life of about 30 minutes. An hour would reduce activity by a factor of about 4 and that would meet the comment by Dr. Cohen about "most" [ie: about 75%] of radon daughter activity having decayed away. Most but not all. I recall in rounting REMP programs in counting environmental particulate air sampling filters for long-lived beta activity, the EPA recommended that filters be counted after 24 hours of daughter product decay, a time when essentially all the particulate radon daughters from the original sampling would have decayed away allowing only long-lived beta activity to be counted. Stewart Farber email: SAFarberMSPH@cs.com ===== |