Host: The Japan Radiation Research Society
We have shown the environmental factors affecting the leaching rate of 222Rn from the natural radioactive minerals to water that exist at Misasa in Japan and at Badgastein in Austria, which are world-famous for the radon therapy, as part of the study to elucidate the mechanism of the radon therapy. However, no data for the radon emanation fraction from these minerals have been reported. In this study, we measured the 222Rn emanation fraction from those samples under some environmental conditions (temperature and grain size) in order to elucidate the factors affecting 222Rn emanation. The radioactive sludge (0.9±0.0 Bq/g) was obtained from the hot bathroom with a high concentration of radon at Misasa Medical Center of Okayama University Medical School and the radioactive rock (6.4±0.0 Bq/g) was collected at Badgastein in Austria. Moreover, the soil (10.8±0.3 Bq/g) sampled at Ningyo-toge, Okayama Prefecture was prepared. The 222Rn emanation fraction from the samples was measured by the γ-ray spectrometry as follows. The dried samples of grain size of less than 63, 63-250, 250-500, 500-1000 or 1000-2000 μm in sealed glass bottles stood for about 30 days and γ rays from the samples were measured by a high-purity germanium detector. After the first measurement, the bottles were placed under the condition of temperature of 5, 25, 40, 60 or 80°C for 2 days and then the stopples of them were opened to escape 222Rn under the same temperature. Subsequently, γ rays from the samples were measured again. The 222Rn emanation fraction was estimated by calculating γ-ray count ratio of 214Pb or 214Bi between two count measurements. As a result, the 214Rn emanation fraction from the samples depended on the environmental temperature. In higher temperature, more amount of 222Rn exists in gas phase because higher temperature decreases adsorption of 222Rn on a solid surface. This may suggest that adsorption and desorption reactions of 222Rn dominate the emanation fraction in this experiment. We are currently discussing the 222Rn emanation fraction of those samples of the different grain sizes.
