Abstract
The Cs‐137 and Cs‐134 monthly deposition at Tsukuba through August 1987 are reported and the origins of caesium isotopes in spring 1987 are discussed. The monthly Cs‐137 deposition increased to 131 Bq m−2 in May 1986 just after the reactor accident at Chernobyl, and thereafter it reached a minimum at the end of 1986. Cs‐134, which is difficult to identify in the fallout from atmospheric nuclear weapon tests, has been detected in all fallout samples since May 1986, and it shows a trend similar to the Cs‐137 deposition. The caesium isotopes fallout shows a maximum in spring 1987 due to the stratospheric fallout. In 1987, about 80 percent of the Cs‐137 fallout came from Chernobyl radioactivity transported into the stratosphere, and 20 percent came from previous atmospheric nuclear tests. It is estimated that several times 10−3 of released caesium isotopes from the Chernobyl reactor have been transported into the stratosphere.
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