Passive methods for environmental radon and thoron monitoring

The present paper describes passive radon (Rn-222) monitor, some problems with measurements of Rn-222 concentrations by passive cup method, a new type of passive Rn-222 and thoron (Rn-220) cup monitors, and a simple assessment method of dose due to exposure of Rn-220 progeny. A passive cup monitor has been constructed by using a 50 mm radius stainless steel hemisphere. In dwellings sith high Rn-220 concentrations, it is Particularly necessary to measure Rn-222 concentration with a monitor having a Rn exchange rate less than 0.1 h(-1). On the basis of the Rn-222 cup monitor, a ner type of passive Rn-222 and Rn-220 cup monitors ras developed. By placing a pair of cup monitors at a 20 cm distance from wall, the concentrations of both gases could be assessed from the a-track densities on the solid state nuclear track detectors (SSNTD). The Rn-222 and Rn-220 concentrations were surveyed around Nagoya in Japan. The Rn-220 concentrations were rather high in the dwellings with soil wall. 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 sane dwellings. Then, the annual mean effective dose equivalent due to Rn-220 progeny ras expected to be 0.67 mSv.y(-1) in the dwellings with soil wall.