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M. E. Wiedenbeck, N. E. Yanasak (JPL), A. C. Cummings, J. S. George, R. A. Leske, R. A. Mewaldt, E. C. Stone (Caltech), E. R. Christian, T. T. von Rosenvinge (NASA/GSFC), W. R. Binns, P. L. Hink, J. Klarmann, M. Lijowski (Washington U.)
The Cosmic Ray Isotope Spectrometer (CRIS) carried aboard the Advanced Composition Explorer (ACE) spacecraft has been making isotopically-resolved measurements of galactic cosmic-ray nuclei in the energy range ~50-500 MeV/nucleon since August 1997. As a result of the large geometrical acceptance of CRIS (~250 cm 2sr) it has been possible to obtain statistically-accurate determinations of the abundances of essentially all of the stable and long-lived isotopes of Fe, Co, and Ni, as well as first measurements of Cu and Zn isotopes.
These data are providing new views of the origin and acceleration of cosmic rays in supernovae or other galactic sites. We find that the electron-capture nuclide 59Ni has decayed into 59Co, showing that a time longer than the 76,000 year halflife of 59Ni must have elapsed between nucleosynthesis and cosmic-ray acceleration. The stable isotopes of Fe (A=54, 56, 57, 58) and Ni (A=58, 60, 61, 62, 64) sample a wide range of neutron excesses and their relative abundances should serve as sensitive discriminators among alternative sites of nucleosynthesis. The general pattern of these abundances is similar to that found in solar-system material, suggesting that cosmic rays contain contributions from multiple stellar environments. We will present a detailed comparison between the compositions of cosmic-ray and solar-system matter and discuss the implications for models of the origin of iron-group cosmic rays.
This research was supported by NASA at Caltech (under grant NAG5-6912), JPL, GSFC, and Washington U.
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