Proposal and optimization of method for direct determination of the thoron progeny concentrations and thoron equilibrium

Most studies focused on thoron have shown that indoor thoron originates mainly from building materials and that thoron concentrations exponentially decrease with a distance from a wall surface. On the contrary, due to longer half-lives of thoron progenies, Pb-212 and Bi-212, which are mainly responsible for the bronchial dose, it is expected that their spatial distribution is homogeneous. Therefore, direct measurements of thoron progeny concentrations or direct thoron measurement and determination of thoron equilibrium factor are necessary for accurate dose assessment. In this paper, we have optimized and validated a reliable and inexpensive method for direct measurement of thoron progenies concentrations based on an air filtration followed by gamma spectrometry measurements. With simultaneous measurements of indoor thoron by the RAD7 active device, the equilibrium factor for thoron in 15 different investigated dwellings was estimated. Obtained results for the equilibrium equivalent thoron concentrations in the range from 0.277 Bq/m(3) to 11.34 Bq/m(3) and thoron equilibrium factor in the range from 0.0047 to 0.098 with arithmetic mean value of 0.034 are in good agreement with already published results.