Seasonal and spatial variations of enhanced gamma ray dose rates derived from 222Rn progeny during precipitation in Japan

Enhanced gamma (Δγ) ray dose rates due to 222Rn progeny wet deposited on the ground surface measured at six monitoring sites in Japan were statistically analyzed to investigate their temporal and spatial variations. Variations of Δγ ray dose rates indicated significant regional differences between the sites on the Sea of Japan coast and those located inland and on the Pacific coast in Japan. At the sites located on the Sea of Japan coast, significant seasonal variation was evident, with higher dose rates in autumn and winter, and lower dose rates in summer. In contrast, however, no significant seasonal variation was observed at the inland and Pacific coast sites in Japan. The variation of Δγ ray dose rates was characterized by three major factors: climatologically changed air masses arriving in Japan, variation of 222Rn and its progeny concentrations in the air column, and the regional characteristics of precipitation. It is evident that the contribution from local 222Rn to Δγ ray dose rates was smaller than those from continental 222Rn. Meso-scale atmospheric disturbance, however, was suggested as an important process for extremely high Δγ ray dose rates, possibly by the accumulation of additional local 222Rn with stronger convergence. It is therefore possible that the variation of Δγ ray dose rates reflects the chemical climatology of 222Rn and its progeny.