Abstract
The absorbed dose rate in air because of terrestrial gamma radiation in Miyako-jima, an island that is part of Okinawa Prefecture in the subtropical region of Japan, was estimated at 637 points by in situ measurements with spectrometers equipped with 3″φ×3″NaI (Tl) and 1″φ×2″NaI (Tl) scintillation detectors. The mean, minimum, and maximum dose rates were calculated to be about 79nGy/h, 3nGy/h, and 165nGy/h, respectively. The correlation of the dose rate and geology showed that the high-rate areas (>100nGy/h) and the distribution of the Holocene red soils (Onokoshi Clay) overlap each other. On the other hand, the low dose rates (<30nGy/h) were mainly found in an outcrop of the Pleistocene Ryukyu Limestone, the main geologic element in the foundation of the red soils. Recent studies (e. g., Inoue et al., 1993) concluded that most of the red soils were not residuals from the base rocks, but of eolian dust “Kosa (Yellow Sand)” origin. These results strongly indicate that the dose rate in Miyako-jima has been enhanced as a result of eolian deposits transported mainly from the arid region of China since the last glacial epoch.
