Cosmic ray isotope abundances from chromium to nickel

IRIS, a telescope consisting of a large-area, precision Cerenkov detector, scintillators, spark chambers, and a passive stack of Lexan polycarbonate track detectors, was exposed to the primary cosmic radiation for 6.6 m/sup 2/ sr hr in a balloon flight in 1976. This measurement has yielded the isotopic composition of the iron-group elements Cr, Mn, Fe, and Ni with an average mass resolution of 0.65 amu for the energies 320--500 MeV per amu at the detector. This experiment places the most severe constraints to date on deviation from solar system composition of the source of the cosmic radiation. We place upper limits of 10% on the ratios /sup 54/Fe//sup 56/Fe and /sup 58/Fe//sup 56/Fe, which are inconsistent with large departures from solar system source composition reported by other workers. The isotopic measurements of Fe and Ni preclude the possibility that these cosmic rays were produced in a single e-process zone. The mean mass of Mn is lesss than 55, indicating the presence of electron capture species produced by spallation.