Gamma-ray spectrometer measurement of 238 U/ 235 U in uranium ore from a natural reactor at Oklo, Gabon

Abstract

About 20 years ago, Kuroda theorized that a high-grade uranium deposit emplaced about 2x10 9 years ago could achieve criticality and sustain a nuclear chain reaction, given a sufficient thickness of high-grade ore and an appropriate water content. Such a natural reactor was found in 1972 at the Oklo deposit, Gabon. The ore contains as much as 60 percent uranium, but the isotopic abundance of 235 U is as little as 0.4 percent in contrast to the normal abundance of 0.7110 percent 235 U. A sample from the Oklo deposit containing about 0.51 atom percent 235 U (by mass spectrometer) was analyzed by a gamma-ray spectrometer system, using a high-purity planar germanium detector. The 235 U was determined from its daughter's ( 234 Th) 63.3 keV photopeak; the 235 U was determined from its 143.8 and 163.4 keV photopeaks. The ratios of these photopeaks were compared with that from a standard having normal uranium isotopic content; the resulting calculations give a 235 U abundance of 0.54 atom percent in the Oklo sample. The gamma-ray spectrum also contains lines from five other isotopes in the uranium series, which indicate the Oklo sample to be at or near secular equilibrium, as the time elapsed since the nuclear reaction ended was sufficient to permit the daughters to achieve equilibrium.