Analysis of Rn-222 concentration in the upper atmosphere during rainfall using the wet scavenging model for its decay products

Takeyasu, Masanori ; Iida, Takao*; Takeishi, Minoru 

The atmospheric Rn-222 concentration in rain clouds was estimated using the wet scavenging model of Rn-222 decay products with the observed data of the environmental ray dose rate. In this method, the pattern of the increase of the dose rate calculated by the model was adjusted to the observed one, in order to estimate the atmospheric Rn-222 concentration which was one of parameter of the model. The Rn-222 concentration was estimated for the rainfall event on Dec. 27, 2003, when a large increase of the dose rate was observed in Tokai-mura. The concentration was estimated to be 24 Bq m, which was higher than the measured one at a height of a few kilometers around central Japan by aircraft. From a back trajectory analysis, the origin of Rn-222 atoms for the events on Dec. 27 was back-tracked to the northeastern part of China, and it was guessed that the large amount of Rn-222 was transported by the Eurasian continental air mass.