The composition of ultra-heavy cosmic rays

Preliminary results on the composition of ultraheavy nuclei in the cosmic rays are now available from experiments on HEAO-3 and Ariel-VI satellites. These results are compared with the composition predicted for reasonable propagation models using source abundances similar to those of solar system material. The effects of varying model parameters and of preferential acceleration are discussed. Discrepancies between predicted and observed abundances are larger than can be accounted for solely by first ionization potential effects and are interpreted in terms of a nucleosynthesis process.