THE ELEMENTAL AND ISOTOPIC COMPOSITION OF GALACTIC COSMIC-RAY NUCLEI FROM SCANDIUM THROUGH NICKEL

Measurements of the relative elemental and isotopic abundances of iron-group Galactic cosmic rays at energies of approximately 325 MeV per nucleon have been made using data collected from 1978 to 1981 by the high-energy cosmic ray detector aboard the ISEE 3 spacecraft. Assuming a standard leaky box model of cosmic-ray propagation, it is found that the source abundance ratio of Ni-60/Ni-58 is 1.07 +/- 0.39, which is a factor of 2.8 +/- 1.0 larger than the solar system value. Our measurements imply the presence of Co-59 at the source, which can be reconciled with the predictions of conventional nucleosynthesis models if there exists a time delay of greater than or similar to 10(5) yr between nucleosynthesis and acceleration. Most of the Mn-54 produced by spallation during cosmic-ray propagation in the Galaxy is found to have decayed to Fe-54, indicating a confinement time of greater than 2 Myr. The source ratio of Fe-54/Fe-56 corrected for the Mn-54 decay is 0.046 +/- 0.020, which is consistent with the solar system value of 0.063. The source ratio Mn-55/Fe-56 = 0.025 +/- 0.010 is a factor of 2.1 +/- 0.8 greater than solar. Also, the value of the Cr/Fe elemental ratio is found to be marginally enhanced. The isotope Cr-52 appears to be present in the cosmic-ray source at almost the 95% confidence level. The limits obtained on the abundances of the lighter iron-group elements at the source are Sc/Fe < 0.005, Ti/Fe < 0.007, and V/Fe < 0.005.