Determination of radon specific exhalation rate from Italian ceramic tiles

Ceramic tiles commonly used in Italy have been studied for both natural radionuclide content and radon emanation. Their body composition is made up of mixtures of clay, sand and other natural materials that are shaped into slabs and fired at high temperature up to 1250°C. From a radiological point of view, zircon sands are the most interesting raw materials used in the composition of the tiles as opacifer in the preparation of glazes in the percentage up to 20% and of porcelain tiles in concentration of 1-10%. Therefore the highest natural activity concentrations are found in glazed and porcelain tiles. High-resolution gamma-ray spectroscopy technique has been used for determining the activity concentration of 226Ra, as well as the E-PERM electret ion chamber has been employed for radon exhalation measurements. The activity concentration ranged from 46 to 257 Bq kg-1 for 238U, from 33 to 197 Bq kg-1 for 226Ra and from 39 to 76 Bq kg-1 for 232Th. Out of 12 samples, 9 reported specific exhalation rates lower than the LLD of 0.0006 Bq h-1 for about 15-20 days exposure time, and 3 very low (0.0010-0.0016 Bq kg-1 h-1). It seems highly probable that the manufacturing process makes extremely difficult the release of radon from tiles; in fact the main characteristics of these products are an extremely compact body, high density, low porosity and very strong intercrystalline coherence obtained by pressing process and firing process. It is recognised that vitrification chemically binds the contaminants in a glass-like matrix that significantly reduces contaminant mobility. Therefore, it is highly probable that gas radon present in the tile is released during vitrification whereas the gas radon produced through the decay of radium after the vitrification process remains locked up in the glass-like matrix.