A new calculation method is presented for determining the air concentrations of any serially decaying nuclides. This new method is easily incorporated into a Basic or spreadsheet program that is used to record the decay data. Specifically, the measurements were designed to illustrate radon progeny from a filter sample that is gross-alpha counted. The method is based on regressing the decay data, recorded in 1-min intervals, on the time after the end of sampling with an equation that describes the alpha activity as a function of the decay exponentials, e-lambdat. The results from both generated data and experimental filter samples were compared to the results obtained by two other well-known methods. All the methods give comparable estimates for the progeny air concentrations; non-linear regression and weighting by variances produces the smallest error estimates with the method presented here.
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