Estimation of Major Origin for Gross α and Gross β Radioactivity in Air Dust

Abstract

As part of the environmental radiation monitoring around the nuclear fuel cycle facilities constructed in Rokkasho-mura, we are carrying out continuous measurements of gross α and gross β radioactivities in air dust samples. Considering there measuring conditions, it is suggested that 210Po and 210Bi mainly contribute to gross α and gross β, respectively. The higher concentration of gross β radioactivity was observed during fall and winter. But then gross α radioactivity lowered during winter, and fluctuated on spring and fall. It is suggested that surface soils are one of major origins of gross α radioactivity, because 210Po/210Pb ratio in the soil is higher than the ratio in the air and the ground is covered with snow in winter. Large peaks of gross a radioactivity and α/β radioactivity ratio were observed on May 2003. Satellite imageries showed forest fires around Lake Baikal in this period. We confirmed that the air mass passed the vicinity of Lake Baikal and flowed in Rokkasho-mura by backward trajectories. There was a positive correlation between gross α radioactivity and the frequency of air mass traveling from the vicinity of Lake Baikal to Rokkasho-mura. The result suggested that the large peaks of the concentration of gross α radioactivity and α/β radioactivity ratio were caused by the forest fire in Russia.