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 IzkwA, 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. 1. Estimated amount of Sr-90 deposition on the earth's surface in Japan The observation of the gross radioactivity of daily rainfall and falling dust has been carried on since the middle of May, 1954, at the Meteorological Research Institute, Tokyo (MIYAKE, 1954, 1955). Rain water is collected on a polyethylene sheet (1 m2) and falling dust is received in porcelain trays (37 x 28 cm2) filled with a thin layer of water. Water is evaporated to dryness in a planchet made of stainless steel to measure the activity with an end-window type G-M counter. The total amount of radioactive deposits per unit surface is calculated taking each rain-fall amount into consideration. The gross activity is reduced to the value at the time one year after fission and the estimated Sr-90 content is obtained using Hunter-Ballou's Table. The result was not multiplied by the factor 1.6 which was suggested by EISENBUD and HARLEY (1956) . 1957Estimation of the Amount of Sr-90 Deposition223 The contribution of the older fission materials which is falling slowly from the stratosphere is also computed. The test series which were detected in Japan by the barometric pressure waves were regarded as thermonuclear explosions of ten or more megatons which should have brought about a large scale contamination in the stratosphere. The effects , of such detonations of larger magnitude (Castle test, 1954 ; Russian test, November 1955 ; Redwing test 1956 ; Russian test, autumn, 1956) were taken up into calculation. Assuming that the residence times of radioactive dust particles in the stratosphere as well as the troposphere are respectively five years* and two months,* the rough estimation of the amount of Sr-90 which is attributed to the stratospheric fallout will be given. The radioactivity Ad of falling dust from the stratosphere which is collected on the ground t days after fission is expressed as follows : (1)Ad= at-'''exP (-0T) , where T is the mean residence time of radioactive dust in the stratosphere (5 years) . The mean value of a can be obtained by measuring the activity of falling dust daily within the 20 days from 50 to 70 days after a thermonuclear test. The radioactivity A, in rain water which is contaminated by stratospheric fallout depends on the rainfall amount (MIYAKE and SUGIURA, 1950). (2)Ar= A' {1— exp (-13P) } , P is a rainfall amount and p is a constant relating to the rate of capture of dust particles by rain drops. A' is an amount concerning the concentration of radioactive substance in the vertical air column on the ground up to the top of the rain-bearing layer which is expressed in the same formula as Eq. 1. (3)A' =cr't-''2exp(—t1T) , in which the mean value of a' may be deduced by measuring the activity (Ar) in rain water falling within the 20 days from 50 to 70 days after a thermonuclear explosion by assuming a certain value of p. As stated by LIBBY (1956), 90% of radioactive dust in the air of the rain-bearing layer is washed out by a rain with a precipitation amount of more than 3 mm. Based on this assumption, the value of p can be calculated by using Eq. 2 (i3=1.98). The calculated values of Ad and Ar corresponding to each thermonuclear detonation described above are added to the activity of the daily tropospheric fallout and rainout. When a thermonuclear test series is conducted during some time period, the time of fission is represented by the intermediate test of the series. The cumulative ground deposition of Sr-90 in Tokyo prior to May, 1954 is not known, but it might be around the level of 0.4 mcikm2 or 1 mcimi2. The results of estimation of monthly and cumulative Sr-90 deposition on the earth's surface up to June 1957 are shown in Table 1 and Fig. 1. * Calculations were done using a few different values both for stratospheric and tropospheric residence times. Among them the agreement between the calculated and the analytical results was best in the case of above-mentioned values. 224 Y. Miyake, Y. Sugiura, K. Saruhashi and T. Kanazawa Vol. VIII No. 3 Table 1. Monthly and cumulative amount of Sr-90 deposition in Tokyo (mc/km2).
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Miyake et al. (1957) studied this question.