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.

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Because X-ray exposure doses to patients during X-ray diagnoses have been increasing with recent advances in medical technology, it is important that optimum control of the radiation dose be maintained during diagnoses. For an evaluation of an exposure dose, the effective energy of the X-ray must be determined, but this is difficult to accomplish during the diagnosis. Here we propose a new method to estimate the effective energy of an X-ray. The magnitude of energy released from an X-ray generator (2 peaks, 12 peaks, inverter, and constant potential) depends on various parameters, including tube voltage, tube current, tube voltage waveform, and total filtration of the X-ray tube. Therefore the measurement of an X-ray's effective energy was conducted by the half-value layer measurement method, which changes the values of these parameters. The data obtained by this method were analyzed to clarify the relationships between X-ray effective energy and the respective parameters. It was thus demonstrated that these relationships could be expressed by a simple linear approximation formula. For the calculation of X-ray effective energy by use of this approximation formula, errors were found to be within a range of -2.11% to 10.4%. Therefore, this method is considered usable for an accurate estimation of an X-ray's effective energy without the need for its direct determination during diagnosis.

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