Multiple fall of Pribram meteorites photographed. 12

Cosmic ray induced tracks have been studied in three Pribram stones Luhy, Velka and Hojsin. Track densities, based on more than 500 tracks, are in the range (1 to 8)x105 tracks/cm2 in the pyroxene grains. Track densities were also determined in olivine grains in samples from Velka which also fall in the same range. To determine the fission contributions to track densities, the intrinsic uranium concentrations of Pribram pyroxenes and olivines were measured. These results clearly establish the primarily cosmic ray origin of the tracks. The track densities correspond to shielding depths ranging between 7 to 14 cm for the various specimens studied. The pre-atmospheric radius of Pribram has been estimated on the basis of (i) cosmic ray track density distribution in the fragments, (ii) track density and spallogenic 22Ne/21Ne ratio in Velka and (iii) concentrations of radioisotopes 3H, 22Na and 26Al in Velka. The track data indicate that the pre-atmospheric radius, R0 must be about 25 cm whereas the rare gas and radioactivity data place an upper limit of R0=50 cm. (author)