Quantitative analysis of radioactivity and radiological hazards in ornamental stones

The novelty of this study originates from the combination of a thorough mineralogical description with a complete evaluation of radiation hazards to clarify the connection between raw materials and the distribution of naturally occurring radionuclides within the Imperial Porphyry, and thus to gain further information regarding their impacts on the environment and nuclear safety. The regional geological framework are predominated by Dokhan Volcanics consisting of platy, porphyritic flows from andesitic to rhyolitic, intruded by a body of monzogranite, and cut by felsite and dolerite dikes. The activity concentrations of 238 U, 232 Th and 40 K, as well as the associated radiological hazard indices, were assessed for fifty Imperial Porphyry samples in the field with a portable GS-256 gamma-ray spectrometer, and in the lab with NaI(Tl) gamma ray spectroscopy. The activity concentrations (mean ± standard deviation, SD) were calculated for 238 U, 232 Th and 40 K to be 47 ± 22, 52 ± 16 and 1289 ± 313 Bq·kg −1, respectively, with the average abundance order reported for 40 K > 232 Th > 238 U. The findings of this study are only slightly higher than world averages as reported by UNSCEAR. The computed mean values of radiological hazard indices were 220 Bq·kg −1 for radium equivalent activity (Ra eq ), 106 nGy h −1 for D air , 0.13 mSv y −1 for outdoor annual effective dose (AED out ), 0.52 mSv y −1 for indoor annual effective dose (AED in ), and 0.45 × 10 −3 for excess lifetime cancer risk (ELCR); all have values attributed to the applicable internationally accepted safety limits, depending on mineral composition and radionuclide content. There were low correlation coefficients with 238 U and 232 Th, which also reflect their different geochemical behaviours (along with mineral controls). The geochemical ratios (thorium content (eTh)/uranium content (eU) = 4.29; thorium content (eTh)/potassium content (K) = 3.36) indicate that uranium was moderately mobile in oxidizing conditions, but thorium was locally concentrated due to magmatic differentiation and hydrothermal alteration. From these findings, it is apparent that Imperial Porphyry has a moderate extent of natural radioactivity due to its magmatic origin, and it appears to be safe to use as a decorative or construction material with no significant radiological concern.