Isotopic abundances of Fe and Ni in Galactic cosmic-ray sources

With a new instrument, designed to determine the isotopic composition of Galactic cosmic-ray nuclei, on board the Ulysses spacecraft, we report relative abundance measurements for Fe-54, Fe-55, Fe-56, Fe-57, Fe-58 and Ni-58, Ni-59, Ni-60, Ni-62 in the overall energy range between similar to 200 and similar to 420 MeV nucleon(-1). These measurements combine excellent mass resolution (sigma similar to 0.28 amu at Fe-56) with good statistical significance (more than 6000 Fe events). The high mass resolution is achieved by utilizing solid-state, position-sensing detectors to measure-with less than 1 degrees angular error-the trajectory of each nucleus entering the charged-particle telescope. The cosmic-ray source abundances are derived from the measurements using models for propagation from distributed sources in the Galaxy (''leaky-box'' model) taking into account solar modulation during penetration of the heliosphere. Overall, except for Fe-54/Fe-56 and Fe-57/Fe-56, we show that the principal Fe and Ni isotopic source ratios have values close to the solar system ratios derived from meteorites. In particular, we note that Fe-58 and Ni-62 display no evidence of neutron enrichment. We discuss the role of the predominantly electron-capture nuclide Mn-54, which decays mainly by beta(-) in the cosmic rays, and contributes to the abundance of Fe-54 measured in the solar system.