Distribution of natural radiation in Japan in relation to geologic properties.

Abstract

A geological interpretation on the survey data of natural radiation in Japan was done. As terrestrial radiation is the major source of natural radiation, a systematic correlation was expected between the distribution of geology and the radiation. With a hope to find the correlation, Japan was divided into 376 pieces of 30 km meshes and a mean exposure rate of each mesh was obtained by averaging all values in it. The geographical distribution was then made clearer, where the differences between North-East Japan and Southwest Japan can be recognized. The radiation level of the latter is higher. This feature seems to be controlled mainly by granitic rocks and volcanism. The condition of weathering is implicit but important factor. Based on those meshes the mean exposure rate on each geologic belt, rock type and soil type were calculated. Every analysis has brought about a consistency in that the higher radiation level area corresponds to the old weathered region of grantic rocks. On the other hand, the lower radiation level area would correspond to cenozoic volcanic belts.