Measurements of Thoron Concentration by Passive Cup Method and Its Application to Dose Assessment.

Abstract

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