Journal of Radiation Research, Jun 1961
Radioactivity in monthly fallout, which had been collected by the “pot” method, were measured for Sr-90 since October 1956 and Cs-137 since August 1957. Total accumulated amount up to July 1960 were 15 me/km2 for Sr-90 and 31mc/km2 for Cs-137. Distinct spring maxima of fallout rate were observed on May 1959 and 1960. Ratio of the total activities of Cs-137 and Sr-90 since August 1957 were 2.8, which is higher than the assumed ratio from nuclear fission processes, and ratio Cs-137/Sr-90 vary considerably from month to month (1.8-8.2). The high observed ratio can be explained mainly from the differences in the half-lives of the ancestor radionuclides of Sr-90 and Cs-137, i.e., Kr-90 (33 sec) and Xe-137 (3.9 min), respectively, while monthly variation of the ratios can be explained from meteorological conditions in the rain-bearing layer of troposphere.
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Masami Izawa, Hiroyuki Tsubota, Takashi Nagai. Strontium 90 and Cesium 137 in Fallout Deposits and Implications of Their Ratio*, Journal of Radiation Research, 1961, 29-41, DOI: 10.1269/jrr.2.29