Anomalous xenon in zone 13 Okelobondo

Autor: A. P. Meshik, C. M. Hohenberg, K. Kehm
Rok vydání: 2000
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Zdroj: Geochimica et Cosmochimica Acta. 64:1651-1661
ISSN: 0016-7037
DOI: 10.1016/s0016-7037(00)00334-3
Popis: 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 ({sup 136}Xe/{sup 134}Xe/{sup 132}Xe/{sup 131}Xe/{sup 130}Xe{sup 129}Xe/{sup 128}Xe = 1/1.25/1.73/0.89/0.0045/0.274/0) with concentrations ranging up to 10{sup {minus}2} cm{sup 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 {sup 130}Xe excesses, which are attributed to neutron capture on fissiogenic {sup 129}I that apparently migrated out of the nearby U-rich minerals.more » The {sup 130}Xe/{sup 129}Xe ratio allows one to estimate the thermal equivalent neutron dose of 1.1 x 10{sup 21} n/cm{sup 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 one to the hypothesis that the CFF process has operated on a global scale on the Earth.« less
Databáze: OpenAIRE
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