The propagation of ultraheavy cosmic-ray nuclei using energy-dependent cross sections

Propagation from the source(s) to Earth of the heaviest nuclei in the cosmic radiation-those with Z greater than or equal to 70-depends critically on the interaction probabilities in the interstellar medium. Recent studies with beams of energetic gold nuclei from accelerators have shown that these probabilities vary rapidly with energy in the energy range where the majority of these nuclei have been, or are likely to be, observed in the cosmic radiation. The effects of applying the resulting energy-dependent cross sections to propagation calculations have been studied by developing a weighted slab model that takes account of the rapid energy changes experienced by these nuclei and adjusts the cross sections in a manner based on recent accelerator measurements. The sensitivity of the predictions of this model on the various parameters is investigated. It is shown that introducing energy-dependent cross sections makes major differences in the predicted yields of secondary fragments, and consequently in the deductions concerning the elemental composition in the source. It is also shown that this model is able to fit the current observational cosmic-ray data with a reasonable set of assumptions regarding the nature of the source material and the propagation model. It is clear that with more and better measurements of the cross sections, and better observations on the cosmic-ray nuclei, these assumptions can be refined, leading to an improved understanding of the nature of the source of the cosmic radiation, and the processes involved in propagation.