Measurement of indoor thoron gas concentrations using a radon-thoron discriminative passive type monitor: Nationwide survey in Japan

Abstract

As part of a nationwide survey of thoron (220Rn) in Japan, the indoor220Rn gas concentrations in 940 dwellings were measured throughout one year, from 1993 to 1996, using a passive type222Rn-220Rn discriminative monitor. The monitor was placed in a bedroom or a living room in each house for four successive three-month periods. The mean annual indoor220Rn concentration was estimated from the four measurements in each house. The arithmetic mean, the median and the geometric mean for indoor220Rn concentrations in 899 dwellings were 20.1, 9.6 and 10.0 Bq m−3, respectively. The220Rn concentrations exhibited a log-normal distribution. It was found that the220Rn concentrations were dependent on the nature of the materials used for wall construction and also on the distance of measurement from the wall. Significant seasonal variations in the220Rn concentration were not observed. It would seem that the nature of the wall material contributed to the increased indoor220Rn concentrations.