Natural radioactivity and radon exhalation rate in Brazilian igneous rocks

This paper reports the natural radioactivity of Brazilian igneous rocks that are used as dimension stones, following the trend of other studies on the evaluation of the risks to the human health caused by the rocks radioactivity as a consequence of their use as cover indoors. Gamma-ray spectrometry has been utilized to determine the 40K, 226Ra and 232Th activity concentrations in 14 rock types collected at different quarries. The following activity concentration range was found: 12.18–251.90 Bq/kg for 226Ra, 9.55–347.47 Bq/kg for 232Th and 407.5–1615.0 Bq/kg for 40K. Such data were used to estimate Raeq, Hex and Iγ, which were compared with the threshold limit values recommended in literature. They have been exceeded for Raeq and Hex in five samples, where the highest indices corresponded to a rock that suffered a process of ductile–brittle deformation that caused it a microbrecciated shape. The exhalation rate of Rn and daughters has also been determined in slabs consisting of rock pieces ∼10 cm-long, 5 cm-wide and 3 cm-thick. It ranged from 0.24 to 3.93 Bq/m2/h and exhibited significant correlation with eU (=226Ra), as expected. The results indicated that most of the studied rocks did not present risk to human health and may be used indoors, even with low ventilation. On the other hand, igneous rocks that yielded indices above the threshold limit values recommended in literature may be used outdoors without any restriction or indoors with ample ventilation.