Recent deposition of 90Sr and 137Cs observed in Tsukuba

Recent observations of 90Sr and 137Cs deposition at the Meteorological Research Institute, Tsukuba, are presented. The annual deposition of 90Sr and 137Cs in the 1990s was around 0.15 and 0.3 Bq m−2, respectively. Although a spring peak was found in the deposition, it is difficult to explain this seasonal pattern only by the stratospheric fallout. The annual deposition trend of 137Cs was found to be well fitted by the stratospheric constant and an additional constant term. The variation in the ratios of the radioactivity and corresponding stable elements did not show the maxima in the deposition peak months in spring. It was concluded that most of 90Sr and 137Cs deposited recently are the resuspended soil origin. The activity ratio (137Cs/90Sr) suggested that plural sources are present for resuspended radioactivity in Japan.