A Simple Rainout Model for Radon Daughters

This paper presents a simplified time-independent model which calculates radioactivities of short-lived radon daughters in rainwater as a function of rainfall rate for unit concentration of radon in cloud. The model incorporates processes of impact collection for cloud droplets by raindrops, removal of raindrops from cloud and transport time of raindrops from the cloud base to the ground as forms of semi-empirical formulas. Radon concentrations in rain clouds are estimated from comparisons between model calculations and ground-level observations made over a few years.