Activity concentration of natural radionuclides and radon and thoron exhalation rates in rocks used as decorative wall coverings in Japan

摘要

In Japan, many dwellings have decorative wall coverings made from granite, andesite, tuff, gabbro, and marble. However, information regarding activity concentrations and radon (222Rn) and thoron (220Rn) exhalation rates for such rocks is very scarce. Therefore, samples of the granite, andesite, tuff, and marble that are used as wall coverings in Japan were collected from mining companies, and their activity concentrations and 222Rn and 220Rn exhalation rates were measured. Dose estimations for inhabitants living in houses built with these materials were also carried out. The activity concentration of natural radionuclides in all the materials was lower than the critical values described by the International Atomic Energy Agency (IAEA) (10,000 Bq kg-1 for 40K and 1,000 Bq kg-1 for all other radionuclides of natural origin). The maximum values of 222Rn and 220Rn mass exhalation rates for the granite samples were 0.12 and 430 mBq kg-1 s-1, and those for the area exhalation rates were 1.8 and 6300 mBq m-2 s-1, respectively; these values are higher than those for other samples. The maximum value of effective doses to inhabitants was 0.68 mSv y -1, which is lower than the intervention exemption level (1 mSv y-1) given in the International Commission on Radiological Protection (ICRP) Publication 82.