RESUMO
This paper describes the use of gamma ray spectrometry in the study of rock samples from the Irati Formation, Paraná sedimentary basin, São Paulo State, Brazil. This technique allowed to measure the natural radiation emitted by 40K, as well the radionuclides belonging to the decay series of 238U (eU = 226Ra = 214Bi) and 232Th (eTh = 228Th = 208Tl) which occur in the analyzed samples. Four gamma ray spectrometers have been utilized for comparing the results obtained: a portable sodium iodide [NaI(Tl)] scintillation detector (Digidart), a handheld bismuth germanate oxide (BGO) detector and two bench NaI(Tl) crystals, differing in their geometry (Planar and Well types). This study involved the calibration of the spectrometers, except in the case of the BGO that is factory calibrated. Afterwards, gamma ray analysis was done for 122 rock samples colleted at Partecal Quarry located at Assistência District, Rio Claro city, which have been of interest for the oil and gas sector. For comparison purposes, the obtained datasets were subjected to different statistical tests, including the analysis of variance (ANOVA) that proved to be of great value for checking the differences of the mean concentration values of eU, eTh and K. The results pointed out several factors that affect the gamma ray analysis for the natural radioelements uranium, thorium, and potassium such as the samples size, shape and geometry, detector type, shielding and counting time. These factors are of difficult control in order to get reliable radiometric measurements by this technique.