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

Journal of Research of the U.S. Geological Survey
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Abstract

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.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Gamma-ray spectrometer measurement of 238U/235U in uranium ore from a natural reactor at Oklo, Gabon
Series title Journal of Research of the U.S. Geological Survey
Volume 4
Issue 5
Year Published 1976
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Description 4 p.
First page 589
Last page 592
Country Gabon
Online Only (Y/N) N
Additional Online Files (Y/N) N