The isotopic composition of galactic cosmic-ray elements from carbon to silicon: The combined release and radiation effects satellite investigation

The separation of the isotopes of carbon, nitrogen, oxygen, fluorine, neon, magnesium, and silicon has been achieved over the energy range similar to 100 to similar to 350 MeV nucleon(-1) with an instrument of new design carried on the Combined Release and Radiation Effects Satellite (CRRES). The Galactic source composition was derived from the measured isotopic abundance ratios for these elements, taking account of interstellar propagation and heliospheric modulation. It is concluded that, except for neon, the isotopic source abundances are substantially in agreement with those of the solar system. This conclusion appears to be valid both for single and double exponential path length distributions in the interstellar medium. The derived source isotopic composition for these cosmic-ray elements is compared with calculated abundance ratios obtained from earlier cosmic-ray investigations as well as radio or optical investigations of the local interstellar medium. Solar modulation reached an unprecedented level during the CRRES mission in 1990 and 1991, and the heliosphere was far from an equilibrium state. However, the relatively simple steady state model of heliospheric modulation we used yielded a remarkable consistency among the recently published cosmic-ray isotopic ratios by other investigators covering a wide range of energy and modulation levels, when compared with the CRRES measurements after corrections to a common level of modulation. The instrument and mission are described briefly.