COSMIC-RAY ISOTOPIC COMPOSITION OF C, N, O, NE, MG, SI NUCLEI IN THE ENERGY-RANGE 50-200 MEV PER NUCLEON MEASURED BY THE VOYAGER SPACECRAFT DURING THE SOLAR MINIMUM PERIOD

The isotopic composition of C, N, O, Ne, Mg, Si cosmic-ray nuclei has been measured in the energy range 50-200 MeV per nucleon using data collected by the High-Energy Telescope of the cosmic-ray subsystem experiment on the Voyager 1 and 2 spacecraft. These data were collected during the period of minimum solar activity in 1986-1988 at an average distance of 27 AU with an effective solar modulation that was much less than at the Earth. The isotope analysis, based on the energy loss - total energy method, has a mass resolution of 0.2 amu for carbon and 0.4 amu at silicon. We find a C-13/C-12 ratio slightly lower and a O-18/O-16 ratio slightly enhanced over their solar system value. We also observe the previously reported enhancement of the Ne-22/Ne-20 ratio relative to solar at the cosmic-ray source but only a weak, if any, enhancement of the Mg-25/Mg-24, Mg-26/Mg-24, and Si-30/Si-28 ratios.