Stable control of thoron progeny concentration in a thoron chamber for calibration of active sampling monitors

摘要

Long-term stability and homogeneity of activity concentration in a calibration facility are vital to quality assurance and quality control for measuring thoron and its progeny. The long half-life of Pb-212 leads to difficulty for attaining highly stable activity; thus, the development of a compensation system based on air recirculation between the ageing chamber and thoron chamber is presented to make up for thoron progeny losses incurred due to plate-out, decay and sampling in the thoron chamber. Stable concentration of thoron progeny in the ageing chamber is dependent on a constant solid Rn-220 source and certain aerosol concentration. A recirculation loop with various flow-rates below 1.6 m(3) h(-1) simulates air sampling of instruments being calibrated in the thoron chamber to eliminate concentration fluctuations. A theoretical model is derived for estimating the effect of main variables on the stable concentrations of Pb-212. Variable but stable activity-concentrations of Pb-212 are produced under different compensation and simulation flow-rates by this method with the coefficient of variation below 10% for the activity concentrations of thoron progeny. Concentration ratios of C(Bi-212/c(Pb-212) ranging between 033 and 1, are achieved under different ventilation rates for the system. (C) 2017 Published by Elsevier Ltd.