Natural Radiation Level in Japan Islands

Abstract

Extensive field survey on natural radiation was conducted from 1967 to 1991 over Japan Islands. The average level of the islands was found to be 79.7 nGy/h. In general, Southwest Japan has higher level compared with the Northeast one. From the data obtained at 1304 sites of the survey, a contour map of natural radiation level in the islands was made by simple interpolations, and a geological interpretation on the distribution of the level was done by comparing the contour map with geological information. The islands were divided into four areas according to whether the level is higher or lower than the average level. The boundaries between these areas were found almost exactly coincide with major geo-tectonic lines. This feature suggests that natural radiation level is controlled mainly by distribution of granitic and volcanic rocks.