Abstract
The monthly fallout of 90Sr at 12 stations from 24°N to 45°N in Japan is given for the period from January 1973 to April, 1981. The cumulative amount of 90Sr fallout is highest in Akita and lowest in Osaka. The cumulative amount of 90Sr in Tokyo from the beginning of nuclear testing to April, 1981 is estimated to be about 78 mCi/km2. After the cessation of the atmospheric tests by US and USSR in 1963, the 90Sr fallout in Japan decreased steadily, but because of the new addition of radioactive debris from the Chinese nuclear tests, the trend of decrease is considerably disturbed in recent years. The isotopic ratio of 89Sr to 90Sr in radioactive debris from each of the Chinese tests varied with time in a similar manner. The ratio increased abruptly several months after of each test and decreased after the inclination of e-(λ89-λ90)t in the case of thermonuclear explosions. On the basis of change of 89Sr/90Sr ratio in the monthly fallout, the residence time of nuclear debris in the atmosphere is estimated to be 30 to 50 days in the troposphere and 1.0 to 1.2 years in the stratosphere. The general trend of 90Sr fallout in Japan essentially depends on the altitude and the time of year of the injection of radioactive debris into the atmosphere.
