Thoron exhalation rate in stony materials: A simplified approach

A significant percentage of the thoron present in indoor environments is often attributable to building materials. It is therefore interesting to estimate the thoron exhalation rate from these materials. However, the detection of thoron is not a simple task: the very short half life of thoron and the lack of strong gamma emissions, prevents the use of the simplest and powerful radiometric techniques normally utilized in radon detection. Specific instruments have thus been developed in order to measure this radionuclide: alpha delayed coincidence counters (measuring the alphas emitted in a short sequence by 220Rn and 216Po) and alpha spectrometry are the most widely used instruments. However, a more simple approach, allowing the evaluation of the thoron (220Rn) exhalation rate by measuring only the radon one (222Rn), would be very welcomed. This goal can be achieved taking into account that a mathematical relationship between the thoron and radon exhalation rates JTh and JRn (Bq/(m2·s)) can be theoretically established. The thoron exhalation rate of any building material can thus be calculated by measuring only the corresponding radon exhalation rate, provided that the activity concentrations of the parents radionuclides (224Ra and 226Ra) are known. Operating in this way, it would be possible to obtain the thoron exhalation rates for the large majority of the building materials normally employed: in fact, the activity concentration data of many building materials are, in general, easily available. Following this approach a formula is proposed as a screening tools for stony materials.