Response of direct thoron progeny sensors (DTPS) to various aerosol concentrations and ventilation rates

Direct thoron/radon progeny sensors (DTPS/DRPS) are absorber mounted LR115 type track detectors for measuring the time-averaged progeny concentrations. Through a large number of experiments, the sensitivity factor of these sensors in natural indoor environment was found to be nearly constant at a value of 0.94 Tr cm−2 d−1/EETC (Bq m−3) for DTPS and 0.09 Tr cm−2 d−1/EERC (Bq m−3) for DRPS. The constancy of the sensitivity factor in the natural environments is attributed primarily to the presence of large aerosol concentrations and relatively low ventilation rates in time-averaged measurements. However, detailed model calculations suggest that in extreme scenario i.e. at high ventilation rate and low aerosol concentrations, the sensitivity factor can be quite different. Such situations are likely to occur in occupational plant areas. Therefore systematic chamber experiments were carried out to using DTPS, to estimate the variability of the sensitivity factor in these extreme conditions. In the first set, the sensitivity factor of DTPS was measured in 6 different aerosol concentrations at zero ventilation rates. The sensitivity factor showed a steep decrease as the aerosol concentration increased to about 8554 particle cm−3, after which it remained almost constant with increase in aerosol concentration. The second set of experiments was conducted at ∼5000 particles cm−3 at three different ventilation rates. The sensitivity factor was found to increase with increase in ventilation rate. The results are further discussed.