Anomalous xenon in zone 13 Okelobondo

In situ laser extraction techniques were applied for the study of heavy nob le gases in a polished section of Zone 13 from the natural nuclear reactor in Okelobondo. Three main mineral phases were identified in this polished s ection using SEM-EDX. The Xe and Kr isotopic structures were determined by multiple measurements in each of these phases. Twenty-four isotopic analyse s of the gases extracted from two different U-rich phases revealed nearly n ormal fission spectra. All 9 analyses of a U-free phase, consisting mainly of alumophosphates, demonstrated an unusual isotopic composition (Xe-136/Xe -134/Xe-132/Xe-131/Xe-130/Xe-129/ Xe-128 = 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 Chemic al Fractionation of Fission Xe (CFF-Xe) in the Okelobondo alumophosphates. CFF-Xe is a decay product of intermediate fission fragments that have migra ted out of the U-rich host phases into adjacent U-free minerals. The CFF-Xe spectra in the alumophosphates are also accompanied by Xe-130 excesses, wh ich are attributed to neutron capture on fissiogenic I-129 that apparently migrated out of the nearby U-rich minerals. The Xe-130/Xe-129 ratio allows us to estimate the thermal equivalent neutron dose of 1.1 x 10(21) n/cm(2). The presence of an unknown fission component remarkably similar in composit ion to CFF-Xe can be inferred from the atmospheric and terrestrial data. Th is leads us to the hypothesis that the CFF process has operated on a global scale on the Earth. Copyright (C) 2000 Elsevier Science Ltd.