Natural radiation exposures for cave residents in China

摘要

In order to understand the biological effects on human beings due to natural radiation exposures as a final goal, the following radiation measurements were preliminarily carried out in cave dwellings and their surrounding areas in China (Shanxi and Shaanxi provinces): (1) indoor and outdoor gamma doses and (2) radon, thoron and their decay product concentrations. 202 cave dwellings were subject to investigation. The gamma dose rates were measured with a compact gamma dose meter with a semiconductor detector at all the sites. Mean gamma dose rates indoors and outdoors were 150 and 110 nSv h~(-1), respectively. They were also measured with a 1″ x 2″ NaI scintillation spectrometer at 32 dwellings. The indoor and outdoor gamma dose rates ranged from 121 to 182 nSvh~(-1) indoors and 99 to 142 nSvh~(-1) outdoors, respectively. After making some corrections on conversion, the arithmetic means (AM) were eventually estimated to be 145 and 107 nGy h~(-1), respectively. After only four dwellings were rejected due to invalid data, statistical analyses have shown indoor radon concentrations ranging from 18 to 224 Bq m~(-3) with an AM of 64 Bq m~(-3), indoor thoron concentrations ranging from 8 to 1176 Bq m~(-3) with an AM of 219 Bq m~(-3) and the indoor equilibrium equivalent thoron concentration (ETTC) ranging from 0.05 to 7.6 Bq m~(-3) with an AM of 1.8 Bq m~(-3), respectively. The present survey has revealed the two following points: (1) gamma dose rates and radioactivities in soil were homogeneous in these areas; (2) the presence of thoron cannot be negligible from the viewpoint of accurate dose assess-, ment.