The Passive Radon-Thoron Discriminative Dosimeter for Practical Use

A passive radon-thoron discriminative dosimeter for practical use has been developed. The body of the practical R-T dosimeter is made of two hemispheric diffusion chambers of carbonized plastic whose diameters are 110mm and 70mm, respectively. These diameters are determined to improve the detection efficiency of radon as well as thoron and also the discrimination ratio of radon to thoron. Inner surface of the detector housing is smooth and free from electrified charge to assure the uniform deposition of radon and thoron progeny, because the detector housing is molded out of carbonized plastic as an anti-static material. In addition, structure of an air inlet has improved to contact more tightly with a glass fiber filter to prevent dust from entering the detector housing. The air inlet of the detector housing is also covered with a half-cutted hemispherical windbreak to protect the glass fiber filter from weathering and to stabilize the influence of convectional air flow on the radon and thoron entry rate into two hemispherical diffusion chambers of the dosimeter. The results of calibration exercises showed that the lower detection limit of radon and thoron concentrations were estimated to be 5.1Bqm-3 and 7.9Bqm-3 respectively in 2 months exposure. And an interim measurement in the concrete cellar proved that the practical R-T dosimeter has enough specifications to be used in the large-scale radon-thoron discriminative survey.