Characteristics of the Environmental Radon and Thoron in Minamidaito-jima, a Comparatively High Background Radiation Island in Japan

Abstract

General survey for the environmental radon and thoron has been performed in Minamidaito jima, Okinawa prefecture, southwestern part of Japan. In situ measurements of the radon and thoron exhalation rates were conducted at a total of 7 points using an accumulation technique with a ZnS(Ag) scintillation detector. The medians of radon and thoron exhalation rates were estimated to be 31 mBq m^-2 s^-1 and 3,970 mBq m^-2 s^-1, respectively. The atmospheric radon concentrations were measured at 2 points with an ionization chamber, and the maximum concentration was estimated to be 94 Bq m^-3. For the surface soil samples collected at the measurement points of the exhalation rates, arithmetic means for ²³⁸U and ²³²Th series concentration were estimated to be approximately 145 Bq kg^-1 dry and 101 Bq kg^-1 dry, respectively. These results indicated that the environmental radon and thoron levels in Minamidaito-jima are fairly higher than those of the average of Japan. And it is strongly suggested that the exhalation rates and the concentrations are enhanced mainly due to the surface soil with the relatively high radioactivity.