Natural radionuclides content and radiological hazard of commercial ornamental stones: An integrated radiometric and mineralogical-petrographic study

Twenty samples of natural building materials commonly employed as ornamental stones in the international market have been investigated for natural radioactivity. External (gamma), as defined and used by the European Commission, and internal (alpha) hazard indexes were calculated and radon specific exhalation rate and emanation fraction were measured. The radiological investigation was complemented by an integrated mineralogical-petrographic and rock characterization approach. The most common radioactive accessory minerals occurring in the investigated samples are apatite, zircon and allanite, with minor monazite, thorite, thorianite, REE and Zr-oxides. Significant correlations with total activity concentration have been observed for K2O, Th and Ce concentrations. The emanation fraction is also influenced by both total porosity and porosity distribution. Radon exhalation rate and emanation fraction are very variable ranging from 0.0011 to 0.64 Bq kg−1 h−1 and from 0.2 to 62%, respectively. Most of the materials have radiological hazard indexes that do not exceed the European Commission limit values when used as ornamental or paving or flooring stones. However, three volcanic (Tufo Giallo Riano, Tufo Grigio Riano and Peperino Viterbese) samples could cause significant exposure both from excess radon indoor concentration (>200 Bq m3) and from gamma radiation (>1 mSv y−1) when used as structural materials. This study further indicates that limit values for hazard indexes based on natural activity concentration and Rn emanation should take into account the lithological properties and use of the materials.