Abstract
Time variation of Sr-90 fallout together with that of Cs-137 is given through the end of 1983. The results at 12 stations in Japan indicate that the recent fallout in Japan was mainly derived from the 26th Chinese nuclear detonation. Relatively short stratospheric residence time of radioactive debris was obtained for the 26th Chinese detonation. The activity ratio of Cs-137 to Sr-90 in the fallout ranged from 0.8 to 6.0 with the average at 2.03. It is noted that the activity ratio of Cs-137 to Sr-90 decreased just after the detonation and then it increased. The meridional distribution of Sr-90 fallout over the Japan Islands indicates that it increased from south to north, whereas the amount of precipitation decreased in the same direction. After the thermonuclear detonation, the amount of fallout increased in the following year and the appearance of maximum fallout was delayed by one to two months from normal pattern of the spring maximum.
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