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

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 212Pb 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 220Rn source and certain aerosol concentration. A recirculation loop with various flow-rates below 1.6 m3 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 212Pb. Variable but stable activity concentrations of 212Pb 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(212Bi)/C(212Pb), ranging between 0.33 and 1, are achieved under different ventilation rates for the system.