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 indoor ²²⁰Rn gas concentrations in 940 dwellings were measured throughout one year, from 1993 to 1996, using a passive type ²²²Rn-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 indoor ²²⁰Rn concentration was estimated from the four measurements in each house. The arithmetic mean, the median and the geometric mean for indoor ²²⁰Rn concentrations in 899 dwellings were 20.1, 9.6 and 10.0 Bq m^-3, respectively. The ²²⁰Rn concentrations exhibited a log-normal distribution. It was found that the ²²⁰Rn 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 the ²²⁰Rn concentration were not observed. It would seem that the nature of the wall material contributed to the increased indoor ²²⁰Rn concentrations.

Keywords: SSNTD; environment; indoor; nationwide survey; radioactivity; radon; thoron.