The geographical distribution and features of 7Be deposition in Japan

The average value of annual 7Be deposition in Japan from 1989 to 1995 was 1,500 Bq/m2, the overall range being 290-6, 500 Bq/m2. The 7Be concentration in rain obtained by dividing the annual amount of 7Be deposition by annual precipitation was 0.2-2.8 Bq/l, giving an average of 1.0±0.4 Bq/l. A large amount of 7Be deposition was found along the Japan Sea, such as in the prefectures of Ishikawa, Toyama, and Fukui, while lesser amounts were found along the Pacific Ocean, and the lowest amounts were found inland and/or in the Setonaikai area. In the areas along the Japan Sea, both precipitation and 7Be concentration in the rain peak during the winter months, which causes large amounts of 7Be to be deposited. Along the Pacific Ocean-side of eastern Japan, an increase in 7Be deposition was observed during spring and autumn, while 7Be deposition in western Japan was in the spring. In the basins of central Japan, as well as in the Setouchi district, only slight 7Be deposition was seen throughout the year. (author)