抄録
Natural background radiation in Japan was analyzed based on surveyed data which were rearranged from macroscopic points of view. The background levels were classified into seventeen groups from which a contour map was made by simple interpolations. The general pattern obtained showed that high levels are prominent in Hida Mountains-Wakasa Bay area and Joetsu-northern Kanto provinces, and low levels are prominent in Hokkaido and northern Tohoku districts along with Kanto Plain. The whole area was further divided into two classes according to whether the level is higher than the country-wide mean level. A boundary between the two classes seen in Chubu District was found almost exactly coincides with Itoigawa-Shizuoka Tectonic Line. Kanto Mountainland was found to have higher levels than its neighborhood, which supports the current idea that old geology of granitic rocks gives high level radiation. A good correspondence could be seen between the radiation contours and every tectonic line proposed as the eastern border of Fossa Magna which is important in the tectonics of Japan Islands. It was then anticipated that informations of the natural radiation will give valuable side evidences to infer the geologic history of Japan Islands.
