Study of fossil tracks due to VH and VVH groups of cosmic ray nuclei in olivine crystals from Luna-16 and Luna-24

The olivine crystals from lunar regolith samples taken by the Soviet unmanned spacecrafts Luna-16 and Luna-24 were investigated. Eleven 0.5 –1.0 mm size olivine crystals were mounted in epoxy, polished and then etched in modified WO₄ solution. The Fe-group track densities up to 10⁸ tracks.cm⁻² (Fe-group) were measured under optical microscope. The tracks of length greater than 30 microns due to Z ≥ 36 cosmic ray nuclei are counted for VVH tracks density for all the crystals. The VVH / VH track densities ratio for these lunar olivine crystals varies from 1.25×10⁻⁴ to 2×10⁻³. It corresponds to the averaged depth of these crystals in lunar soil of 2–8 cm during galactic cosmic ray exposure. Lunar crystals are well suited for VVH track studies due to a very high track density. Two crystals were annealed at 430°C for 32 hrs. This procedure eliminates iron group tracks completely and leaves etchable tracks of nuclei with Z 50 even in the olivine crystals with Fe-group tracks up to 1–2×10⁸ tracks cm⁻². We were able to measure two tracks with the length 195 and 210μm which were produced by Th---U group of Galactic cosmic ray nuclei.