梅雨期における温帯低気圧の東進に伴う空間放射線量率の上昇機構

抄録

This paper proposes a meteorological mechanism for large increases in dose rate based on the findings from a high dose rate event that occurred during rainy season in northern Kyushu, Japan. The gamma-ray energy spectrum obtained by NaI(Tl) scintillation spectrometry confirmed the contribution of short-lived ²²²Rn decay products, such as ²¹⁴Pb and ²¹⁴Bi. The locations of monitoring posts, at which the large increases in dose rate were observed, corresponded to rainfall areas. Infrared satellite images and surface weather charts indicate that the intense precipitation was caused by cumulonimbus clouds along a stationary front, which accompanied a low pressure system. The low pressure system had formed in central China approximately 18 to 36 hours before the high dose rate event. Analyses of the backward trajectory and a long-range atmospheric transport model suggest that the low pressure system raised an air mass containing ²²²Rn to 1 to 2 km altitude above ground level. Then, the air mass that contained the ²²²Rn was transported laterally to Japan, as the low pressure system advanced to the east along the stationary front. Thus, the high dose rate event was attributed to the eastward advance of the low pressure system that carried the ²²²Rn and wet deposition of ²²²Rn decay products.