MEASUREMENTS OF THORON CONCENTRATION BY PASSIVE CUP METHOD AND ITS APPLICATION TO DOSE ASSESSMENT

A new type of passive integrating cup monitors was developed by using a 50 mm radius stainless steel hemisphere to measured indoor radon (Rn-222) and thoron (Rn-220) concentrations. By placing a pair of cup monitors with an air exchange opening of diameter 5 mm and four openings of diameter 20 mm at a 20 cm distance from wall during about three months, the concentrations of both gases could be assessed from the alpha track densities on the cellulose nitrate (CN) films. The Rn-222 and Rn-220 concentrations were surveyed with the cup monitors in the different types of dwellings around Nagoya in Japan over three years. The Rn-220 concentrations were rather high in the dwellings with soil wall, and the mean Rn-220 concentration was 160+/-12 Bq . m(-3). The Rn-220 exhalation rate from wall and the Rn-220 diffusion were evaluated from the distribution of Rn-220 concentrations in the dwellings. The results of the surveys have also clarified the relationship between the Rn-220 concentrations at a 20 cm distance from wall and the Rn-220 progeny concentrations measured in the same dwellings. Then, the annual mean effective dose equivalent due to Rn-220 progeny was expected to be 0.67 mSv . yr(-1) in the dwellings with soil wail.