The Isotopic Composition of Cosmic-Ray Calcium

The isotopic composition of galactic cosmic-ray Ca has been measured over the energy range ˜ 150 to 400 MeV/nuc on the Advanced Composition Explorer (ACE) mission. Using the measured abundances of the dominantly-secondary Ca isotopes with A = 41-46 as constraints, in combination with measured cross sections for the production of Ca isotopes by fragmentation of ^(56)Fe on hydrogen, we show that the two doubly-magic isotopes ^(40)Ca and ^(48)Ca consist mainly of primary material. We find that their relative abundances in the cosmic-ray source are very similar to those found in solar-system material, in spite of the fact that different types of stars are thought to be responsible for producing these two isotopes. This observation is consistent with the view that cosmic rays are derived from a mixed sample of interstellar matter.