Integrating Radon Progeny Monitor

INTRODUCTION The Japan Nuclear Cycle Development Institute (JNC) carried out research and development projects related to uranium exploration in Japan from 1967 to 1987. Several mining facilities have been retained after the end of projects. Hence, the study on the behavior and the environmental impact of radon and its progeny from the mining facilities is being continued at the Ningyo-toge Environmental Engineering Center of the JNC (JNC Ningyo-toge). The limitations against radon are determined with equilibrium equivalent radon concentration (EECRn) in current Japanese laws, including the Japanese Mine Safety Law. The Law requires a lower exposure dose than 1mSv/year out of the site, in excess of the natural background level. 1mSv/year is equivalent to 9 Bq/m of EECRn. It is therefore necessary to determine the long-term average concentration of radon progeny. Nevertheless, the environmental impact of radon is generally estimated from radon concentration by integrating radon monitors. This means that the radon progeny concentration monitors, which are suitable for long-term average concentration of radon progeny at outdoor fields where it is impossible to use an AC power source, have not appeared. Actually, long-term environmental surveillances of radon concentration have been carried out more frequently than that of radon progeny concentration. Radon progeny monitors are categorized into active monitors and passive monitors. In general, the response of the active monitor is more stable than the passive monitor. The equilibrium factor (F value) by grab sampling method on and around the mining sites is shown in Figure 1. The variation of the F values is vary widely. Thus, active monitors are suitable for environmental monitoring. Several different techniques using the active method, including integrating monitors, have been used to determine radon progeny concentration. Since some of them require precision systems with AC power sources, they are big and expensive. Although others are simple and use batteries, almost all of them are developed for short-term monitoring; for example, personal dosimetry or area monitoring while working. An integrating radon progeny monitor developed by the JNC Ningyo-toge collects radon progeny on a filter using a pump with a battery and detects alpha particles with a CR-39 detector. However, it differs from others in that it uses interval operation for sampling and it has a function to estimate the effect of components attached by diffusion during interval. This monitor makes possible the long-term measurements of radon progeny concentration for over a month without an AC power source. In this paper, the character of the monitor and the results of experimental measurement at outdoor fields are reported.