Cosmic-ray exposure history of tektites

Abstract

To establish whether tektites are extraterrestrial in origin, five composites including a total of 79 tektites from the Far East and Australia were examined for cosmic-ray-induced Al26 by γ-γ coincidence spectrometry. No activity was found in any of these samples, corresponding to 2σ upper limits for the Al26 content of ≤1.6 to ≤8 dpm/kg, or about ≤3 to ≤13 per cent of Al26 content of meteorites. A grand composite of all the above samples showed a marginal positron activity corresponding to an Al26 content of 0.48±0.26 dpm/kg, but this activity can apparently be accounted for by reactions of cosmic-ray secondaries at the surface of the earth with silicon and aluminum in the tektites. No evidence was found for a preterrestrial exposure to cosmic rays. These results do not completely rule out an extraterrestrial origin but do place the following restrictions on it: if the tektites arrive as small, unshielded bodies, their ‘flight’ or cosmic-ray exposure time was less than 90,000 years; if they arrived in a large, well-shielded body, this ‘prototektite’ body must have had a radius of ≤54 meters.