Fossil Tracks in the Meteorite Angra dos Reis: a Predominantly Fission Origin

INVESTIGATIONS of recordable lengths (maximum etchable lengths) of fossil tracks in meteorites and in lunar samples have provided information on the charge composition of cosmic ray iron group and heavier nuclei 1–3 and on the spontaneous fission contributions since the solidification of these extra-terrestrial samples 4–6 . The concentration of uranium (and also transuranic elements) is usually very small (0.01–10 p.p.b. by weight 4,5 ) in the silicate crystals where the fossil tracks are studied, so most of the tracks are due to cosmic ray nuclei 6 . We have studied a few meteorites having relatively high uranium concentrations for fossil tracks as well as for neutron induced tracks in previously annealed samples. In particular we studied the meteorite Angra dos Reis (ADR), an augite achondrite, which is known to have a high uranium concentration (> 170 p.p.b. by weight) 7–9 and for which the concentrations 10–12 of rare gases, both fissiogenic and cosmogenic, are well known. ADR is rich in pyroxene (the only accessory mineral present is olivine 13 ) occurring as dark brown but fairly large thin transparent crystals having dimensions of 300–500 µm.