STUDY OF FOSSIL TRACES 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 u nmanned 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 10 (8) tracks.cm(-2) (Fe-group) were measured under optical microscope. T he tracks of length greater than 30 microns due to Z greater than or e qual to 36 cosmic ray nuclei are counted for VVH tracks density for al l the crystals. The VVH / VH track densities ratio for these lunar oli vine crystals varies from 1.25x10(-4) to 2x10(-3). It corresponds to t he averaged depth of these crystals in lunar soil of 2-8 cm during gal actic cosmic ray exposure. Lunar crystals are well suited for VVH trac k studies due to a very high track density. Two crystals were annealed at 430 degrees C for 32 hrs. This procedure eliminates iron group tra cks completely and leaves etchable tracks of nuclei with Z greater tha n or equal to 50 even in the olivine crystals with Fe-group tracks up to 1-2x10(8) tracks cm(-2). We were able to measure two tracks with th e length 195 and 210 mu m which were produced by Th-U group of Galacti c cosmic ray nuclei.