Source Spectra and Composition of Cosmic Rays Implied by an Analysis of Interstellar and Interplanetary Travel

Abstract

The implications of a simple set of assumptions related to galactic cosmic rays are examined and compared with the existing data. These assumptions are that (a) the multiply charged cosmic-ray nuclei all have the same spectral shape at the source, (b) the relative abundances of ${\mathrm{He}}^3$ and light nuclei ($3\le Z\le 5$) are negligible at the source, and (c) the average amount of interstellar material traversed is independent of particle energy. The results show that within the present uncertainties of the experimental data and of the interaction cross sections, the data agree with the predictions without additional assumptions if the differential source spectra are relatively flat at low energies and the average interstellar path is 2.8±0.4 g/${\mathrm{cm}}^2$. In particular, the nearly constant relative abundances of the helium, medium ($6\le Z\le 9$), and ($Z\ge 10$) nuclei for different values of energy/nucleon can be explained, and the observed variation of the flux ratio of light to medium nuclei with energy/nucleon can be brought into fair agreement with predictions. The calculations also imply that, if the analysis does represent the true situation, then there is little or no solar modulation of the cosmic rays near solar minimum, and protons and helium nuclei have different source spectra.