Characterization of Nd, Te and U isotope ratios in uraninite using SIMS

Fayek, M., Palenik, C.S. and Ewing, R.C. (2005) Characterization of Nd, Te and U isotope ratios in UO2 using SIMS. Goldschmidt Conference (Moscow, ID).

Presented on: 5/1/2005

Isotopes provided the initial evidence for the 2 Ga naturally occurring fission reactors discovered at Oklo and Okelobondo in Gabon, Africa and have been subsequently applied to characterize reactor operation conditions and the migration of actinide and fission product elements. However, the application of SIMS has been largely limited due to analytical challenges. Uraninite (UO2), which forms the core of these natural reactor zones represents an extremely complex matrix, containing measurable concentrations of most elements between 70<A<160. Furthermore, the isotopic ratios for a given element have been measurably altered from the range in typical terrestrial rocks. Therefore, nearly all isotopic measurements of reactor zone material have been based on chemical separation and analysis using TIMS or ICP-MS. This has limited the spatial resolution as well as the number of elements and samples analyzed. Note that these same complicating analytical factors also provide the evidence for our understanding of reactor operation.

Here we present the use of SIMS to measure isotopic ratios of Nd, Te and U isotopes (Table). Elements and isotopes were selected based on instrumental considerations (e.g., interferences and relative abundances) and for their sensitivity to reactor operation conditions. To minimize compound ion
interferences, extreme energy filtering (100 – 200V) was used with O-
(for U and Nd) and Cs+ (for Te) primary beams. Instrumental mass fractionation was corrected by measuring reactor zone uraninite previously characterized by TIMS or ICP-MS. Analysis of samples distributed across the
Okelobondo reactor zone provided insights into the heterogeneity of isotopic ratios at various length scales (µm to m). In conjunction with computer simulations of reactor operation, these isotopic ratios were used to constrain the reactor operating conditions.

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