Concentrations and their ratio of 222Rn decay products in rainwater measured by gamma-ray spectrometry using a low-background Ge detector

The concentrations and the concentration ratios of individual short-lived 222Rn decay products (214Pb and 214Bi) in rainwater were measured at Kumatori village (34.39°N, 135.35°E, approximately 70 m above sea level) in Osaka, Japan, by gamma-ray spectrometry using a low-background Ge detector. The dependence of the time variations of the concentrations and their ratios on rainfall rate was investigated. It was observed that the concentrations were negatively correlated with the rainfall rate in some rainfall events, and that there was no clear correlation in other rainfall events. The changes in the dependence of the concentration on the rainfall rate occurred after the passage of a cold front during a single rainfall event. The concentration ratios showed a weak negative correlation with the rainfall rate for most of the observed rainfall events. A scavenging model was designed in this study in order to explain the observation results. Based on the relationship between the concentrations of 214Pb and 214Bi in the rainwater and the rainfall rate for an individual rainfall event, the increase in the environmental gamma-ray dose rate from 214Pb and 214Bi deposited on the ground was calculated, and the calculated increase agreed well with that observed by the in situ measurement on flat ground.