Plutonium fallout observed in Tsukuba, Japan

Plutonium in monthly deposition samples collected in Tsukuba (the Meteorological Research Institute), Japan from 1990 to end of 2001 is reported. The annual deposition of 239,240Pu during the period from 1990 to 2001 shows no systematic inter-annual variation. However, monthly 239,240Pu depositions show a typical seasonal variation with a maximum in spring season (March to April), which corresponds to seasonal cycle of soil dusts originating from the East Asian arid area. Plutonium isotopic ratios in the deposition samples suggest that significant amounts of the recent 239,240Pu deposition observed in Japan are attributed to the resuspension of plutonium-bearing surface soil particles; resuspended plutonium originates from the East Asian arid areas. Recent increase tendency of 239,240Pu contents in residue in deposition samples may reflect desertification in the East Asian continent. (author)