The P, S, Ar, and Ca isotopic composition of the galactic cosmic ray source

Galactic cosmic ray (GCR) measurements of the phosphorus, sulfur, argon, and calcium isotopes made by the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA’s Advanced Composition Explorer are reported over the energy range from ∼100 to ∼400 MeV/nucleon. The propagation of cosmic rays through the Galaxy and heliosphere is modeled to determine isotopic source abundance ratios P/S , S/S , Ar/Ar , and Ar/Ca . By deriving the GCR source abundance of argon (a noble gas) and calcium (a refractory), it is determined that material in grains is accelerated to GCR energies a factor of ∼6.4 more efficiently than gas-phase material in this charge range. With this information, the interstellar dust fraction of phosphorus and sulfur at the cosmic ray source is shown to be consistent with astronomical measurements of hot galactic environments.