Neutron capture effects on Sm and Gd isotopes in uraninites

Abstract Isotopic analyses of Sm and Gd in 10 uranium minerals with different formation ages were performed by thermal ionization mass spectrometry to investigate neutron capture effects due to spontaneous fission-released neutrons. The results from some uraninites show significant isotopic shifts on 150Sm/149Sm and 158Gd/157Gd due to neutron capture. Uraninites with high U contents, low Sm/U and Gd/U ratios, and geologically old ages (older than 1.5 Ga) have isotopic characteristic pointing to the occurrence of neutron capture reactions. Estimation of neutron fluences and differences of neutron energy spectrum in individual U ores are discussed from the isotopic data. The Shinkolobwe (Zaire) uraninite has especially clear isotopic deviations of Sm (e149Sm = −2.5, e150Sm = +3.7) and Gd (e157Gd = −2.3, e158Gd = +2.0), and its isotopic shifts correspond to a neutron fluence of 2.0 × 1015 n/cm2. The combination of isotopic data from Sm and Gd implies that fission-released neutrons of the Shinkolobwe sample are not well-thermalized relative to those of other uraninites. The Oklo sample taken from out of the natural fission reactor shows a fluence of 2.6 × 1016 n/cm2, which is 1.38 times higher than expected from other uraninites with the same formation age. Furthermore, it contains not only neutron capture products but also 2.9% of fissiogenic Sm relative to total Sm. This suggests that fission chain reactions began in this Oklo sample, but that criticality could never be sustained. There is a correlation between total rare earth element content and neutron fluxes in uraninites. As one of the geochemical characteristics, total rare earth element content in the Oklo sample is much lower than those in other uraninites. This suggests that rare earth elements in uraninites work as neutron absorbers, thereby preventing the fission process from reaching the critical point of a self-sustaining chain reaction.