RADON CONCENTRATION, ABSORBED DOSE-RATE IN AIR AND CONCENTRATION OF NATURAL RADIONUCLIDES IN SOIL IN THE OSAKA DISTRICT OF JAPAN

Radon concentrations in outdoor air at 18 sites in the Osaka district, in the central part of Japan's main island, were measured with electrostatic integrating radon monitors which were developed by Y. Ikebe et al of the Osaka survey centre as part of a nationwide survey of radon indoors and outdoors in Japan conducted by the National Institute of Radiological Science (S. and M. Abe and K. Fujitaka). The mean radon concentration in outdoor air during 2-month periods was measured over a period of a year and a half. In addition, the absorbed dose rate in air and the concentration of natural radionuclides in soil were measured at 40 sites in Osaka Prefecture which is located in the central part of the Osaka district using thermoluminescence dosemeters and with gamma ray spectrometry, respectively. Radon concentration in outdoor air showed a seasonal pattern, reaching its maximum during the winter and its minimum during the summer, but this variation was not significant at the coastal sites. It was concluded that this variation is correlated with a seasonal wind which blows from the continental interior to the ocean in winter and in the opposite direction in summer, as well as with geographical factors. Radon concentration in outdoor air in the Osaka district ranged from 0.6 to 17.9 Bq.m-3 and mean annual radon concentration in outdoor air at the 18 sites ranged from 2.7 to 6.9 Bq.m-3. It was discovered that radon concentration in outdoor air decreased with wind speed in both winter and summer. The absorbed dose rate in air ranged from 66 to 114 nGy.h-1, and the concentration of Ra-226 in soil ranged from 20 to 60 Bq.kg-1 respectively.