Radon and thoron exhalation rate from building materials: experimental evaluations and theoretical models

The exhalation of radon and thoron from stony and other building materials can be a relevant radiation protection issue if these materials contain high level of their parents, the radionuclides belonging to the uranium and thorium series. However, the simple knowledge of the activity concentrations (Bq/kg) of the natural radionuclides inside these materials is not enough for a comprehensive evaluation of the radiological hazard arising from the use of those materials in the building constructions. Actually, the estimation of the fraction of the radon and thoron produced inside the material that is able to escape from the solid matrix is a crucial parameter that is greatly influenced by the characteristics of the material itself, i.e.: density, porosity, inner composition, etc. In this work, a new method for the determination of the thoron emissions from building materials, calculated from the more easily available radon exhalation data, is proposed. The method is based on the knowledge of the activity concentrations of the radon and thoron parents, namely 226Ra and 224Ra and on a new description of the diffusion mechanism inside stony and building material, involving the concept of fractal random walk.