Dose rates evaluation of some granitic rocks from the Paraná State, Brazil

Granitic rocks, widely used as building materials, are known to contain natural radionuclides and can be an important source of radiation for the population. Thirty-four samples of granite rocks from geological occurrences in Paraná state were measured with detector for evaluation of ²²⁶Ra, ²³²Th and ⁴⁰K activity concentrations. The effective annual external dose was evaluated from these radionuclides activities using a dosimetric room model with dimensions 4 m x 5 m x 2.8 m in which all walls are internally coated with 2 cm thickness granites and considering the annual exposure time of 7000 h, suggested by the European Commission on Radiological Protection for superficial coating materials. The internal exposure was evaluated from radon air concentration of the model room, simulated from an exhalation rate of ²²²Rn, determined with CR-39 solid state nuclear track detectors by the sealed can technique, considering a ventilation rate of 0.5 h^-1 and the same annual exposure time of 7000 h. The results for external gamma rays showed an increase in the annual effective dose ranging from 96 ± 4 μSv.a^-1 to 223 ± 7 μSv.a^-1 and, for radon inhalation, an increase in the ranging from 0.4 ± 0.04 μSv.a^-1 to 70 ± 4 μSv.a^-1. All results stayed below the recommended value by the European Commission on Radiological Protection, of 1 mSv.a^-1. (author)