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

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