Measurement of natural radioactivity in magmatic, metamorphic and sedimentary ornamental stones

The measurement of the natural radioactivity of 112 ornamental stones has been performed with gamma spectrometry. The rocks, representing the most diffused commercial varieties from Italy and foreign countries, are of magmatic, metamorphic and sedimentary origin. The natural rocks may contain common forming- and accessories minerals (i.e. feldspars or zircon) which have radioisotopes emitting alfa, beta particles and of gamma rays. In the previous works the gamma radioactivity measurements were performed on ground samples of ornamental stones, whereas in this work the rocks are analysed as 15x15 cm wide and 1 to 3 cm thick tiles, to simulate the real use conditions. First of all the samples are screened by means of a NaI(Tl) portable scintillation detector. Three representative samples were tested with HPGe gamma spectrometer using them as standards for the NaI(Tl) gamma spectrometer calibration. The measured gamma radioactivity (in Becquerel) refers both to the specific activity of each radioisotope and to the total activity of the samples. The sedimentary rocks show total activity values between 250 and 500 Bq, always lower than those of magmatic/metamorphic rocks. 40K is the most abundant radionuclide: its specific activity is < 900 Bq. Magmatic rocks have higher total activity than the previous ones; the maximum value is 2190 Bq in an alpine Porphyroid. They contain almost all the radioisotopes detected by NaI(Tl) spectrometer and 40K shows values up to 2000 Bq. The strongly variable values observed in all samples are always comparable with those of other building materials. Gamma spectrometry and scintillation detector data show a good correlation: so the use of scintillation technique could be encouraged in order to perform preliminary information on the ornamental stones radioactivity.