The origin of primary cosmic rays: Constraints from ACE elemental and isotopic composition observations

Cosmic-ray isotope observations from NASA's Advanced Composition Explorer (ACE) mission have been used to investigate the composition of cosmic-ray source material. Source abundances relative to ⁵⁶Fe are reported for eleven isotopes of Ca, Fe, Co, and Ni, including the very rare isotopes ⁴⁸Ca and ⁶⁴Ni. Although the source abundances range over a factor ∼ 10⁴, most of the ratios to ⁵⁶Fe are consistent with solar-system values to within ∼ 20%. However, there are some notable differences, the most significant being an excess of ∼ (70 ± 30)% relative to the solar system for the cosmic-ray source ratio ⁵⁸Fe/⁵⁶Fe. The possible association of such an excess with a contribution to the cosmic-ray source from Wolf-Rayet star ejecta is discussed.