Abstract
Based on the results from an analysis of the fallout collected in Fukuoka Prefecture in fiscal years 1965-1994, further analysis was carried out to determine the relation between gross β activity and some radionuclides, such as 40K, 90Sr, 137Cs and so on, which are regarded as key nuclides in assessing radiological impact on human health. Gross β activity in fiscal years 1964-1984 decreased in accordance with a regression curve given by F(x)=2.58×106e-0.126x, where r=0.655. The gross β activity during the period of fiscal years 1965-1981, when the influence of atmospheric nuclear explosion tests prevailed, was found to be in the range of 3.70-61, 700MBq/km2, among which 64% of the data was lower than 200MBq/km2. The mean contributions of 90Sr and 137Cs to gross β activity were estimated to be only 1.9% and 3.4%, respectively. No significant correlation was found between the radioactivities of these two nuclides and gross β activity.
After 1982, gross β activity decreased to a level ranging from ND to 666MBq/km2, in which 50% of the data was evaluated to be ND. In recent years, natural 40K is found to be the major contribution to gross β activity.
