Impact of ENDF nuclear data libraries on thermal neutron capture gamma-ray spectra simulated for geochemical well logging

Nuclear well logging plays a fundamental role in the characterization of geological formations and the exploration of hydrocarbons. The reliability of computational simulations used to model well logging tools depends directly on the quality of the nuclear data employed in particle transport. In this study, the impact of ENDF nuclear data libraries on neutron-induced gamma-ray spectra simulated using the MCNP 6.3 code was evaluated. A geochemical logging tool was modeled in a formation representative of a Brazilian petroleum basin. The elemental compositions of the rocks were obtained by means of XRF analysis. Thermal neutron capture spectra of seventeen isotopes of geological interest were generated and compared with the reference data provided by the National Nuclear Data Center. The results revealed significant discrepancies among the evaluated libraries, including the absence of characteristic emission lines, peak shifts, spectral degradation, and differences in the intensities of the detected photons. The analysis of the thermal neutron capture cross sections indicated divergences in the nuclear data capable of explaining part of the differences observed in the simulated spectra. Additionally, the GADRAS-DRF software was used to incorporate the response function of a NaI(Tl) detector, producing spectra more compatible with real experimental conditions. Although the spectral adjustment reduced the observed degradation, some discrepancies associated with the nuclear data libraries remained evident. Among the evaluated libraries, none showed consistent agreement with the reference data for all the isotopes considered. Therefore, it is recommended that each isotope be simulated using the library that presents the best performance.