Determination of MDA for a two count method for stripping short-lived activity out of an air sample

When measuring airborne activity of isotopes with very low annual limit of intakes, interference from natural radioactivity presents an obstacle to obtaining timely results. One method for solving this problem is to perform two separate counts of the sample and mathematically strip-out the contribution from the short-lived component. While this method can process a large volume of samples with little or no preparation time, the sensitivity of the analysis is hard to determine. In an attempt to determine the sensitivity of the analysis and to quantify the parameters that affect it, an equation for the minimum detectable activity was derived. Actual air sample data were then evaluated against this equation to determine its usefulness.