Study of a deposition-based direct thoron progeny sensor (DTPS) technique for estimating equilibrium equivalent thoron concentration (EETC) in indoor environment

A study has been carried out to examine the suitability of a passive, deposition-based technique for estimating the time-integrated equilibrium equivalent thoron concentration (EETC) in indoor environment using a direct thoron progeny sensor (DTPS). DTPS is an absorber-mounted LR-115-type nuclear track detector tuned to respond only to the 8.78 MeV alpha particles emitted from the deposited activity of 212Po on the absorber surface. The study has been carried out in two steps, the first step comprising the estimation of track registration efficiency (ηη) and the second step comprising the estimation of effective deposition velocity of thoron progeny aerosols on the detector surface in natural and laboratory environments. The track registration efficiency is found to be 8.3% for 212Po and less than 0.02% for all other alpha emitters including 222Rn (radon) progeny, thereby enabling a precise discrimination of thoron progeny. The deposition velocity of thoron progeny aerosols on DTPS surface has been measured in conjunction with active air sampling techniques for different orientations of the deposition surface. The geometric mean deposition velocity was found to be 0.077mh-1 having a narrow spread (geometric standard deviation of 1.35). Upon combining these factors, the overall sensitivity factor is estimated to be 0.94Trcm-2d-1/EETC(Bqm-3), which indicates ∼45∼45 times higher sensitivity as compared with conventional bare detector techniques (which register alpha particles emitted directly from air). The results obtained have been compared with currently used aerosol deposition models and progeny equilibrium models by using the aerosol number size and progeny activity distributions measured with scanning mobility particle sizer (SMPS) and low-pressure cascade impactor, respectively. The results are in good agreement with each other. A sensitivity analysis of the response characteristics of DTPS with respect to room ventilation rate and aerosol concentrations has been carried out to assess the variability of the sensitivity factor in the context of indoor dosimetry. The results are further discussed.