CR-39 detector-based Radon dosimetry system calibration in the self-decay mode

Abstract

This paper describes cooperation of the Slovak University of Technology in Bratislava (STU) and the Slovak Institute of Metrology (SMU) in the verification of the calibration factors for the solid-state nuclear track detector utilized for radon dosimetry and establishing traceability for the STU laboratory. The SMU operates the Air Radon Standard which consists of a radon chamber and calibrated radon atmosphere monitoring system. A specification of the radon chamber is that the radon concentration changes during the exposure time. The radon atmosphere is created by the insertion of a specified amount of radon gas into the chamber, which subsequently decays over time. The STU laboratory is equipped with the TASLImage™ system for radon dosimetry with corresponding TASTRAK CR-39 type detectors and diffusion chambers. These chambers allow radon to diffuse inside and prevent dust and radon progeny to enter the container. Each diffusion container is characterized by its own diffusion rate, which should be considered in the metrology calibration approach, especially when the radon chamber is operated in the decay mode. Therefore, several measurements were carried out to determine the optimal condition for conversion factor verification. The experiments were conducted for low radon concentrations, but still in the range that the radon chamber can reliably provide (100 Bq m−3 to 100 kBq m−3) and in combination with several times of exposure. Based on the achieved results the default calibration factor was verified, and an optimal calibration approach was proposed. To improve the capability of the radon measurement, a custom diffusion container design was proposed. The design incorporates compact dimensions with an aerosol filter at the entrance to the container. This allows to increase in an entry hole and thus increases the diffusion rate. Subsequently, the homogeneity of the radon atmosphere in the chamber was tested.