In 1956, H.L. Kusnetz proposed a quick method for radon progeny concentration measurement in mine atmosphere using a single gross-alpha count of a membrane-filtered air. The method is still widely used today and is based on a number of impractical assumptions. An instantaneous sampling time (less than ten seconds), is one of these assumptions that ignores the build-up and decay of the progeny on the filter paper during the sampling period, which is typically in the order of a few minutes. Of special concern is the Bi-214 decay during the sampling period, since Po-214's alphas are lost during the sampling time and cannot be accounted for during the counting time. In addition, the method assumes that Bi-214 activity during the counting period is constant. This inaccurate assumption can result in a smaller count rate, especially when counting times are long. Hence, underestimated working levels are expected when using Kusnetz factors without correcting for the sampling and counting times. In this technical report, exact sampling and counting time corrections to the method are provided along with the updated Kusnetz factors that correspond to common equilibrium conditions to correctly estimate the Working Level in air. Additionally, time corrections to the commonly used self-absorption correction formula and the lower level of detectability (LLD) equation used for any sample measurement are given.
