On identification of the galactic cosmic-ray nuclear tracks due to Th-U in olivine crystals from meteorites

Earlier, in 1980-1987, during the investigations of fossil tracks in 12 cm3 of olivine crystals from the meteorites Marjalahti and Eagle Station the abundant groups of tracks with lengths of 120-140-mu-m, 200-220-mu-m and also a few tracks with L 350-mu-m have been measured. These crystals were annealed at 430-degrees-C during 32 h before etching. The necessary calibrations of olivine crystals with 25 and 70 MeV/N U-238 nuclei were carried out at the Bevalac accelerator (LBL, Berkeley) and with 19 MeV/N Pb-208 nuclei at UNILAC (GSI, Darmstadt). The subsequent annealing at 430-degrees-C during 32 h results in shortening down of U-238 nuclei tracks to 230 +/- 25-mu-m (Marjalahti olivines) and 220 +/- 20-mu-m (Eagle Station olivines), which agrees with the data on the 210 +/- 20-mu-m fossil track group within the experimental error bar. The annealing of crystals at 450-degrees-C during 32 h provides the shortening down of the Pb-208 tracks to 60-70-mu-m, on the U-238 tracks to 120 +/- 20-mu-m. This can be compared with corresponding fossil track length spectra (the approximately 50-60-mu-m and approximately 115-mu-m groups).