Numerical analysis of two-filter method for continuous measurement of airborne Rn-222

The two-filter method enables the continuous measurement of airborne Rn-222 via simultaneously sampling and counting. However, the slow time response of the two-filter detector and the overlapping counting data derived from the intervallic data acquisition make the interpretation of measurement results complicated. A valid data analysis to exactly match the series of counting data with the corresponding radon concentrations is desired. In this work, the continuous sampling and counting processes of the two-filter method is depicted numerically and the calculation of radon concentration from the raw counting data is given with detailed formulas. The numerical model is used to quantitatively investigate the influences of several key factors, including the volume of the radon delay chamber (5-10 000 l), the flow rate of air sampling (2.5-1000 l min^-1) and the interval time of data acquisition (5-120 min), on the measurement sensitivity and accuracy, which provides an approach for the optimization of the design and measurement settings of a specific two-filter detector in relation to these aspects.