隠岐島の地表γ線強度分布と表層地質分布の関係

抄録

To study the relation between the terrestrial γ-ray dose rate and subsurface geology, the γ-ray dose rate has been surveyed at 213 points in Okinoshima, an island situated in the Japan Sea, with a γ-ray survey meter. The results of this observation are the mean value of terrestrial γ-ray dose rate at 213 points = 57.1 nGyh^-1, the standard deviation =15.2 nGyh^-1, the maximum value =107.2 nGyh^-1, and the minimum value=23.1 nGyh^-1. On the soil of the basement rock in Okinoshima, the terrestrial γ-ray dose rate of Oki-gneiss is 72±15 nGyh^-1, which is about 1.6 times as big as the reported value for metamorphic rock and others are the same in the literatures. It is considered that the terrestrial γ-ray dose rate of Okigneiss has been more strongly affected by the granitic magma which has been interpenetrated in the process of this formation. In Okinoshima, it is suggested that the geological features of the basement rock remain in the surface soil, due to a good correspondence between the geographical distribution of terrestrial γ-ray dose rate and the subsurface geology. And then, in the relation between the terrestrial γ-ray dose rate and the atmospheric ²²²Rn concentration, the magnitude of ²²²Rn concentration is smaller than this of place on large land area. Because of a small land area in Okinoshima, this is caused that the land air is mixed with the sea air by the advection for a long time except the time from the sunset to around the midnight. It is suggested that the time and space change characteristic of the diffusion and mixture process is different in contrast to a place of large land area.