Recent trends of plutonium fallout observed in Japan: comparison with natural lithogenic radionuclides, thorium isotopes

In order to elucidate sources of present atmospheric plutonium, plutonium in monthly deposition samples from 2000 to end of 2002 collected to Tsukuba (the Meteorological Research Institute), Japan is determined, together with monthly thorium deposition. Plutonium measurements in deposition samples as did thorium isotopes were carried out by alpha-spectrometry after the radiochemical separation and purification. The annual deposition of 239,240Pu during the period from 1985 to 2002 shows no systematic inter-annual variation. However, a maximum annual 239,240Pu deposition (7.8 mBq·m-2·y-1) was observed in 2002. On the other hand, 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 desert and arid areas. Thorium, which is a typical lithogenic radionuclide, reflects soil-derived particles in the atmospheric dust. The monthly Th deposition showed a typical seasonal trend with a maximum in spring and minimum in summer. The 230Th/232Th activity ratios in the deposition samples significantly differed from that in surface soils near Tsukuba, which means that a significant part of thorium in deposition samples is not derived from resuspension of local soil particles. The result reveals that most of the resent 239,240Pu deposition observed in Japan is attributable to resuspension of deposited plutonium; resuspended plutonium originates from the East Asian continent desert and arid areas. These findings suggest that a significant amount of soil dust observed in Tsukuba results in the long-range transport of continental dust from the East Asian arid areas. (authors)