Robert M. Moxham 1976 <p>About 20 years ago, Kuroda theorized that a high-grade uranium deposit emplaced about 2x10⁹ 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 ²³⁵U is as little as 0.4 percent in contrast to the normal abundance of 0.7110 percent ²³⁵U. A sample from the Oklo deposit containing about 0.51 atom percent ²³⁵U (by mass spectrometer) was analyzed by a gamma-ray spectrometer system, using a high-purity planar germanium detector. The ²³⁵U was determined from its daughter's (²³⁴Th) 63.3 keV photopeak; the ²³⁵U 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 ²³⁵U 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.</p> application/pdf en U.S. Geological Survey Gamma-ray spectrometer measurement of ²³⁸U/²³⁵U in uranium ore from a natural reactor at Oklo, Gabon article