The Abundance of Nuclei Heavier than Iron in the Cosmic Radiation in the Geological Past

We have studied 198 tracks of cosmic rays with Z > 30, produced during the last 5 X 10~ years at a depth of `-`1 cm in the Patwar meteorite. Estimates of atomic number, Z, were based on an improved relationship between etched track length and Z, derived from recent bombardments of meteorite crystals with Fe ions iii an accelerator. After correcting for the difference in track-formation efficiency of cosmic rays of different Z, we obtain the value [Z> 30J/[Fe] = 4.4 X 1O~ for the ratio of the abundance of cosmic rays with Z > 30 to the abundance of Fe cosmic rays, averaged over the last 5 X 1O~ years. This is consistent with the latest estimate of the ratio of the universal abundances of the same group of elements, given by Cameron (1967) as 5 X 1O-~. Some implications and suggestions for future measurements are discussed. I. INTRODUCTIO