P-1b-36 1 Sensitivity to Thoron on Passive Radon Detectors S. Tokonami 1 , H. Yonehara 1 , T. Sanada 2 , M. Yang 3 , M. Furukawa 1 and Y. Yamada 1 1 National Institute of Radiological Sciences, Chiba 263-8555, Japan 2 Japan Chemical Analysis Center, Chiba 263-0002, Japan 3 Research Institute of Uranium Mining, Hunan, China INTRODUCTION Although a lot of studies on radon have been done for a long time, there are few reports on thoron. It was considered that the presence of thoron could be negligible because of its own quantity compared to that of radon. However, recent studies have shown high thoron levels in our living environment (1-4). In particular, high thoron concentrations were occasionally observed in some areas, Japan (1,2). This fact made it clear that some of passive radon detectors were sensitive to the presence of thoron. They are commonly used for indoor radon surveys. It is possible that such detectors will give false values unless they are placed properly. Therefore, it is important to understand the detector response to thoron before practical use. A compact thoron chamber system was set up for the purpose. In this study, the followings are shown: configuration of the thoron chamber and sensitivity to thoron of several passive radon detectors. MATERIALS AND METHODS The thoron chamber system consists of two parts: an exposure chamber and a thoron gas generator. They are connected through an inline filter holder, a flow meter and an external pump. Figure 1 illustrates a sectional view of the thoron chamber system when thoron exposure tests are carried out for passive radon detectors. The exposure chamber is a 150 liters cylindrical vessel (565 mm diameter and 600 mm height), made of stainless steel. Four metal tubes are attached to the lid of the chamber. They are used to supply/exhaust thoron/radon gas and to take air samples. A fan is mounted on the lid to make the internal concentration uniform. The generator is a 10 liters stainless steel cylindrical vessel (240 mm diameter and 240 mm height). The vessel is filled with thorium-rich ceramics as the solid source. The ceramics were crushed into small pieces so as to promote thoron exhalation. After connecting the exposure chamber and generator, thoron gas circulates through the system with the external pump. The thoron concentration in the exposure chamber depends on the flow rate of the circulation. The flow rate was set at around 20 L/min in this system. Thoron and radon concentrations are measured with scintillation cells after taking an air sample through the metal tube. The measurement technique is based on two measurements using the difference of the half life between radon and thoron. Details on the technique are described in another paper (5).