Development of Radon and Thoron Exposure Systems at Hirosaki University

Abstract

Radon (²²²Rn) and thoron (²²⁰Rn) exposure systems were designed and developed for calibrating radioactive gas monitors, based on the quality assurance (QA) and quality control (QC) standards of the monitors at Hirosaki University, Japan. To monitor the concentration levels of radioactive gases, gas monitors were installed inside an exposure chamber together with a temperaturehumidity meter for recording inside and outside conditions during performance tests. In this study, radioactive gases generated from natural rock were used as the ²²²Rn source and commercial lantern mantles enriched with thorium (²³²Th) were used as the ²²⁰Rn source. The concentration levels were kept within a range between 0.2 and 10 kBq m^-3 for the ²²²Rn exposure system and between 3.5 and 29 kBq m^-3 for the ²²⁰Rn exposure system. The results obtained demonstrated that the concentration levels depended on such factors as the type of sources, flow rates of the radioactive gases generated into the chamber, gases emanating from the sources, and humidity in the gas generation system, and they are described in the paper. However, performance tests of these exposure systems were carried out and their results showed that ²²²Rn and ²²⁰Rn concentrations in the exposure chamber could be easily controlled through manually setting the flow rate and relative humidity and the distribution of gases was fairly homogenous.