Variations of 129I in the atmospheric fallout of Tokyo, Japan: 1963-2003.

Atmospheric fallout samples collected from Tokyo between 1963 and 2003 were analyzed using accelerator mass spectrometry (AMS) in order to determine (129)I/(127)I ratios and to examine the deposition rate of (129)I and its secular variation in Tokyo. The (129)I/(127)I ratios in the atmosphere during 1963-1977 ranged from 1 × 10(-8) to 2 × 10(-8). This is roughly 4 orders of magnitude higher than pre-atomic levels, possibly due to atmospheric nuclear weapons tests. The calculated monthly atmospheric deposition rates of (129)I differed from those produced by nuclear fallout of (90)Sr and (137)Cs, indicating that the variations in (129)I deposition are not influenced exclusively by either nuclear bomb testing or by the Chernobyl accident. After 1978, high (129)I depositions (up to 0.13 mBq/m(2)/month) were observed. The (129)I depositions started to increase markedly at the latter half of the 1970s. The secular variation of the estimated annual (129)I deposition in Tokyo showed a close relationship between the annual atmospheric discharge of (129)I from the Tokai Reprocessing plant. Therefore, the atmospheric fallout collected from Tokyo after the late 1970s is influenced primary by the (129)I discharge from the Tokai Reprocessing plant.