MEASUREMENTS OF REGIONAL DISTRIBUTION OF RN-222 CONCENTRATION

Outdoor Rn-222 concentrations were measured with electrostatic integrating radon monitors (EIRM) at 40 points around Nagoya, in which 15 sets of 2-month-exposure data over 2.5 yr were obtained. Seasonal variation of Rn-222 concentration showed a clear pattern which had a spring-to-summer minimum and an autumn-to-winter maximum. Annual means Rn-222 concentration ranged 3.5 approximately 11.7 Bq.m-3 depending on locations. Higher concentrations were obtained in mountainous regions while lower concentrations were obtained near the sea. The relation between atmospheric Rn-222 concentration and uranium content of granitic rocks was also discussed. High level Rn-222 concentrations were obtained over areas of granitic rocks which were comparatively uranium-rich. As an almost linear relationship was recognized between the Rn-222 concentration and exposure rate due to external natural radiation, a close correlation was anticipated between the concentration and Ra-226 content of soil. From the obtained regional distribution of Rn-222 concentration, per caput lung dose in the area was estimated to be about 0.44 +/- 0.11 mSv.yr-1.