Optimizing the total-alpha three-count technique for measuring concentrations of radon progeny in residences

A technique for measuring 222Rn-progeny concentrations in air, involving counting alpha decays for three intervals on a filter through which air has been drawn, is optimized for measuring low concentrations typically found indoors. Sampling and counting intervals are selected to minimize a linear combination of the minimum measurable concentrations (MMCs)--concentrations at which the relative standard deviation in the measurement due to counting statistics is 20%. The effects on the MMCs of varying total measurement time, sampling and delay times, and of radon-progeny activity ratios are considered. The effect on measurement precision of variations in the collection rate of radon progeny on the filter is also evaluated. Previous work on this technique has often focused on measurements in uranium mines where concentrations of radon progeny are typically much higher than in residences. With sequential sampling and counting intervals, extending the total measurement time from 35 to 60 min reduces the MMCs by factors of 3, 7 and 4 for 218Po, 214Pb and 214Bi, respectively, thereby permitting precise measurement of indoor concentrations down to the order of one pCi/1. By employing an instrument that counts while sampling, the MMCs can be further reduced by factors of 5.5 for 218Po, 2 for 214Pb and 2.5 for 214Bi, again for a 60-min measurement period.