Anomalous xenon in zone 13 Okelobondo

Abstract

In situ laser extraction techniques were applied for the study of heavy noble gases in a polished section of Zone 13 from the natural nuclear reactor in Okelobondo. Three main mineral phases were identified in this polished section using SEM-EDX. The Xe and Kr isotopic structures were determined by multiple measurements in each of these phases. Twenty-four isotopic analyses of the gases extracted from two different U-rich phases revealed nearly normal fission spectra. All 9 analyses of a U-free phase, consisting mainly of alumophosphates, demonstrated an unusual isotopic composition ( 136Xe/ 134Xe/ 132Xe/ 131Xe/ 130Xe/ 129Xe/ 128Xe = 1/1.25/1.73/0.89/0.0045/0.274/0) with concentrations ranging up to 10 −2 cm 3 STP/g. This is the highest Xe concentration ever measured in a natural material. Kr was also anomalous, although to a lesser extent. These results confirm the presence of Chemical Fractionation of Fission Xe (CFF-Xe) in the Okelobondo alumophosphates. CFF-Xe is a decay product of intermediate fission fragments that have migrated out of the U-rich host phases into adjacent U-free minerals. The CFF-Xe spectra in the alumophosphates are also accompanied by 130Xe excesses, which are attributed to neutron capture on fissiogenic 129I that apparently migrated out of the nearby U-rich minerals. The 130Xe/ 129Xe ratio allows us to estimate the thermal equivalent neutron dose of 1.1 × 10 21 n/cm 2. The presence of an unknown fission component remarkably similar in composition to CFF-Xe can be inferred from the atmospheric and terrestrial data. This leads us to the hypothesis that the CFF process has operated on a global scale on the Earth.