The Isotopic Composition of Hydrogen and Helium in Low Energy Cosmic Rays

The Caltech Electron/Isotope Spectrometer on IMP-7 has been used to identify the isotopes ^2H and ^3He in low energy cosmic rays during solar quiet periods from October 1972 to October 1974. These observations cover the energy intervals 5 - 29 MeV/nuc for ^2H and 7 - 50 MeV/nuc for ^3He. The energy spectra of ^1H, ^2H and ^3He all fall rapidly with decreasing energy, giving ^2H/ ^1H and ^3He/ ^1H ratios essentially independent of energy as expected from adiabatic acceleration. The measured ^4He spectrum, however, was essentially flat over this energy interval, and therefore the ^2H/^4He ratio observed at 1 AU is not simply related to the interstellar abundances of these nuclei. However, comparisons of the ^2H/^1H and ^3He/^1H ratios with calculated spectra are possible