THE NUCLEAR TRACK RECORD OF CORE SAMPLES FROM THE H5 CHONDRITE JILIN

Based on 43 documented drill core samples of the H5 chondrite Jilin, t he depth profile of track densities was investigated. The study of oli vine and pyroxene crystals extracted from the two drill cores A and B of the main fragment no. 1 yields a track background for the Jilin int erior of 150 +/- 90 cm(-2). To estimate the contribution of different track origins, the U microdistribution of sample sections and single g rain sections was mapped by neutron-induced U-235-fission Using high-p urity quartz glass detectors. The average U content of 0.59 +/- 0.22 p pb for pyroxenes and 0.12 +/- 0.06 ppb for olivines is consistent with the observed fission track background in these minerals. There are no indications of spontaneous fission of Pu-244, secondary mobilisation of U in the bulk sample, or substantial track annealing. As deduced fr om the calculated fission track density, the track contribution from g alactic cosmic rays is negligible in the core samples investigated.