Effects of Compton scattering on gamma-ray spectrometry applied to rocks from Irati Formation, Paraná sedimentary basin, Brazil

Gamma-ray spectrometry is a useful technique that allows measuring the natural radiation emitted by K (40K) and radionuclides belonging to the decay series of 238U (eU = 226Ra = 214Bi) and 232Th (eTh = 228Th = 208Tl) present in rock samples. Its evaluation is especially important for sedimentary rocks like those found at Irati Formation, Paraná Sedimentary Basin (PSB), Brazil, due to the possibility of finding stratigraphic marks associated with strong abrupt variations in the radioactivity levels during the surveys of sediment profiles. However, among the processes involved when gamma rays interact with matter, the Compton scattering is significant as it leaves its “signature” in gamma spectra recorded by different experimental arrangements, chiefly those utilizing scintillation detectors such as the NaI(Tl). This study describes the use of established stripping factor (RC) models and multiple linear regression (RLM) analysis to correct Compton interference in gamma-ray spectra to improve the calibration steps of a portable NaI(Tl) gamma spectrometer installed at LARIN-Ionizing Radiations Laboratory, UNESPETRO-IGCE-UNESP, Rio Claro (SP), Brazil. Afterwards, radiometric data in 111 rock samples from Irati Formation, São Paulo State, Brazil, were obtained, allowing determine gamma interference factors within the spectral windows of natural radionuclides [K(40K), eU(214Bi), and eTh(208Tl)]. The statistical frequency distribution of the radioelement concentration data, considering the RLM approach, defined 11 classes whose range corresponded to 0–21.66% for K, 4.29–131.74 ppm for eU, and 12.01–388.46 ppm for eTh. Violin plots indicated that the data are normally distributed with one mode value and different shapes. For K, the highest difference was found for values obtained by a previous survey conducted in 2021, compared to those utilizing the methods RC and RLM. For eU and eTh data, the highest difference was verified for data obtained by the method RC relative to other approaches. The variation in the residues did not differ significantly for K among the three methods, whereas lesser variation was found between the RC and RLM methods for eU and eTh. Therefore, these findings allowed evaluating the influence of the Compton scattering on the monitoring realized, contributing to improving the accuracy and comprehension of gamma spectrometric data that are extensively used in geophysical surveys.