Abstract
A comprehensive data set of plutonium deposition samples collected monthly (partly quarterly), from 1957 to 1997, by the Meteorological Research Institute (Tokyo until March 1980 and Tsukuba since April 1980), Japan is reported, together with information on the geochemical and meteorological factors controlling the plutonium fallout. The maximum fallout rate of 239,240 Pu during the sampling period was observed in June 1963, after the 1961–1962 period of US/former USSR atmospheric nuclear weapons testing. Annual 239,240 Pu deposition decreased in the period from 1963 to 1967 according to the stratospheric residence time of nuclear debris. From 1968 to 1984, the level of 239,240 Pu deposition was mainly controlled by stratospheric fallout from atmospheric nuclear weapons testing in China. After 1985, the 239,240 Pu fallout showed no decrease, which may be attributed to resuspension of deposited plutonium. In May 1986, the Chemobyl radioactivity was observed in rain and air samples in Japan. The Chemobyl-defived Pu isotopes, which are characterized by higher 238 Pu/ 239,240 Pu and 241 Pu/ 238 Pu activity ratios than those of the nuclear-test-derived 239,240 Pu , were detected in deposition samples in Japan. However, the level of Chemobyl 239,240 Pu in Japan was very low compared to that in Europe.
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