Protocolo para a medição de radiação gama em rochas ornamentais utilizando o espectrômetro gama NaI(TI)

Abstract

The assessment of radiological risk associated with the presence of naturally occurring radionuclides, such as 226Ra and 222Rn in ornamental rocks, is imperative to ensure regulatory compliance and safety in the use of these materials as building cladding. Given the high cost and operational complexity of high-resolution gamma spectrometry (HPGe), this work developed an alternative measurement protocol using the portable gamma spectrometer AT6101C [NaI(Tl)] under laboratory conditions, based on the experimental determination of weighted calibration factors (Fc) for the conversion of count rates into specific activity (Bq/kg). In parallel, 222Rn concentrations were measured using CR-39 solid-state detectors and estimated for a standard-room scenario, where, in the absence of air renewal (ACH = 0 h-1), values showed a wide range (6.87±0.53 to 1010±92 Bq/m3), exceeding the ICRP reference level only under conditions of total air stagnation. However, for air renewal rates of 0.5 and 1 h-1, concentrations decreased to negligible levels, demonstrating that minimal ventilation is the determining factor in mitigating radiological risk. The simplified physicalmathematical model allowed the inference of 226Ra activities for five selected samples, revealing a tendency toward negative systematic deviation in matrices with high specific surface area (SSA), where radon emanation induces the breakdown of secular equilibrium between 226Ra and 222Rn. HPGe validation confirmed that all samples remain below the 1000 Bq/kg limit established by CNEN, while the estimated annual effective dose values (derived from the Ambient Dose Equivalent) ranged from 0.842 to 0.955 mSv/year, significantly below the annual dose limit for members of the public (1 mSv/year). Granulometric and mineralogical analyses (SEM-EDS and XRF) demonstrated that compositional heterogeneity and the presence of iron and aluminum oxide-rich phases directly influence 222Rn exhalation and the variability of apparent activities. Statistically, the use of a single Fc per group proved viable only for carbonate rocks (metamorphic and sedimentary), which showed full internal compatibility (p > 0.05). It is concluded that the portable NaI(Tl) spectrometer, when properly calibrated by lithological group, is an effective radiometric screening tool, providing analytical agility and significant reduction in operational costs compared to high-resolution spectrometry.