Abstract
The accumulated amount of Sr-90 deposition and the infinite gamma ray dose due to the artificial radioactive substance originated from nuclear tests were calculated. The amount of Sr-90 was derived from the values of gross activity of rain water and fallout which have been observed daily since May 1954. The gross activity is reduced to the value at the time one year after fission and Sr-90 content is calculated based on Hunter-Ballou's Tables. The contribution of the older fission products falling slowly from the stratosphere was taken into consideration under a few assumptions in which the values of five years and two months as the residence time of dust particles in respectively the stratosphere and the troposphere were given. Agreement among the calculated values by the present authors, those by US Atomic Energy Commission and the observed monthly deposition of Sr-90 by the chemical method (by IZAWA,1957) was fairly good (Fig.1). The prediction of the probable level of Sr-90 deposit in future was also given (Figs.2 and 3). Calculation of the infinite gamma radiation dose at a given point one meter above the ground was made taking also the stratospheric fallout into computation as above. The result of calculation is given in Table 4 and Figs.4 and 5.
