A simple method for determination of RaA, RaB and RaC has been developed which has a sensitivity of the order of 1 pCi/l. for each nuclide. An air sample is obtained and the total alpha disintegrations in three selected time intervals after the end of air sampling determined. The concentration of each nuclide is then obtained from an equation involving the alpha disintegrations, the sampling flow rate, and known numercal coefficients. Simple equations giving the standard deviation of the nuclide concentrations have also been derived. The method was optimized for accuracy by investigating a large number of sampling times and counting intervals, and it is recommended that 5 min air sampling be used, followed by alpha counts from 2 to 5, 6 to 20 and 21 to 30 min after the end of sampling.
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