Terrestrial Gamma Radiation in Kochi Prefecture, Japan

Absorbed dose rates in air (dose rate) from terrestrial gamma radiations obtained using a scintillation survey meter ranged from 13.8 to 187 nGy/h depending on the geological features in Kochi Prefecture. Mean dose rates of four geological belts were found in the following order; Alluvial Deposits < Sambagawa Belt = Chichibu Belt < Shimanto Belt. The mean dose rate of the prefecture was about 60 nGy/h, which was 20% higher than that of Japan as a whole. The mean effective dose rate per person in this prefecture was about 0.37 mSv/y, which was the same as the Japanese mean. The ratio of the mean dose rates of 40K, nuclides of the thorium series and the uranium series in the soil determined using a germanium detector were 2 : 2 : 1. The relationship between 40K, 208Tl and 214Bi was positive, and therefore 40K, nuclides of the thorium series and the uranium series were considered to have the same behavior in the soil. 137Cs in the soil ranged from 1.4 to 150 Bq/kg with a mean of 24.5 ± 23.1 (S.D.) Bq/kg, suggesting that we continue to be under the influence of nuclear tests in the atmosphere.