Host: The Japan Radiation Research Society
The measurement of 222Rn emanation fraction from a natural radioactive mineral has been performed by counting α ray from 222Rn of gas phase in the sealed chamber where mineral is placed. Lower 222Rn emanation fraction of a sample can be accurately measured because emanated 222Rn itself is measured in the α-ray method. However, this method needs a dedicated detector for 222Rn which is correctly calibrated. On the other hand, the simple method of measuring 222Rn emanation fraction was developed, which is based on two count measurement of γ ray using an uncalibrated NaI (Tl) detector. However, 220Rn can significantly affect 222Rn emanation fraction measured by the γ-ray method if a sample contains high concentration of Th-series nucleus. In this study, the 222Rn emanation fraction was measured by γ-ray spectrometry to remove completely the influence of 220Rn. 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. The 222Rn emanation fraction from the samples was measured by the (1) γ-ray and (2) α-ray method as follows. (1) The dried samples in sealed glass bottles stood for about 30 days and γ rays from the samples were measured by a high-purity germanium detector. Subsequently, the stopples of the bottles were opened to escape 222Rn and 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. (2) The dried samples and a small ionization chamber were placed in a sealed camber. The 222Rn concentration in the chamber was measured for about 2 weeks and the 222Rn emanation fraction was estimated from the 222Rn growth curve. As a result, the 222Rn emanation fraction of the Misasa's sludge and Badgastein's rock measured by the α-ray method was 30 and 2%, respectively. We are currently discussing the 222Rn emanation fraction of those samples by the γ-ray method and the lowest 222Rn emanation fraction which we can measure by γ-ray method. Moreover, we discussed the detection limit of 222Rn emanation fraction in case of the γ-ray method.
