Thoron spike test for passive radon detectors

Passive radon(222Rn) detectors are commonly used for radon surveys. They provide the integrated exposure. Solid-state nuclear track detectors (SSNTD) are widely used as the detecting material.Usually, determination of radon concentrations is based on the number of tracks produced by alpha particles (track density). In order to obtain the relationship between the track density and the radon concentration (conversion factor: CF) calibration is necessary. The detectors are calibrated in a pure radon atmosphere with a standard radon chamber. In an actual environment, however, thoron (220Rn) exists as well as radon. In general, it is considered that the presence of thoron can be negligible because of its own small quantity compared to that of radon. Recent studies have shown that attention must also be paid to thoron because high thoron concentrations were occasionally observed in some areas. Although most of radon detectors are designed to minimize the entry of thoron, there are few reports on the thoron contribution to the detector response. After picking up some radon detectors, therefore, their responses are examined with our compact thoron chamber. The thoron chamber system consists of two parts: an exposure chamber and a thoron gas generator.
The exposure chamber is a 150 liter cylindrical vessel, made of stainless steel. Four metal tubes are attached to the lid of the chamber. They are used to supply/exhaust radon/thoron gas and to take air samples. The gas generator is a 10 liter stainless steel cylindrical vessel. The vessel is filled with thorium-rich ceramics. After connecting the exposure chamber and the generator, thoron gas circulates through the system with an external pump. The thoron concentration depends on the flow rate of the circulation. Radon and thoron concentrations are measured with scintillation cells after taking a sample promptly. For the prompt measurement, the scintillation cell is connected to the exposure chamber through a valve and a filter. Opening the valve (t=0), sampled air is drawn into the cell. 20 s later, the first measurement is performed for 100 s, which gives counts derived from both thoron and radon. 10 min later, the second measurement is performed for 5 min, which gives counts derived from radon only because most of the thoron has decayed (half life of thoron=55.6 s). With two counting data, radon and thoron concentrations are determined, respectively. Three passive radon detectors (A, B and C) were examined. Although the detector A and B have the same structure as the cup (cylinder), detecting materials are different (polycarbonate films and CR- 39). The detector A (polycarbonate films installed) was developed in Germany, which is called the Karlsruhe (KfK) passive radon dosimeter. Both detectors are covered with a glass-fiber filter. Such kind of filter will cause a large ventilation rate. The detectors are placed at the bottom of the cup. The detector C (CR-39) is commercially available, which is named Radtrak. They were exposed to thoron-rich air for 4 days. Average radon and thoron concentrations throughout the exposure period were 230 Bq/m3 and 2529 Bq/m3 with the scintillation cell, respectively. The detector A provided 2302 Bq/m3 as the measured radon concentration. The detector B (CR-39) provided 2913 Bq/m3. The detector C provided 1922 Bq/m3. The difference between the detector A and B seems to be derived from the detectable range of the material because the range of the CR-39 is wider than that of the polycarbonate film. Influence of thoron on the radon detector is discussed in detail.