The Isotopic Composition of Hydrogen and Helium

The isotopes 2H and 3He have been identified in low-energy cosmic rays during solar-quiet periods from 1973 January to 1974 October. These observations, made with the Caltech Electron/ Isotope Spectrometer on IMP-7, cover the energy intervals 5-29 MeV per nucleon for 2H and 7-50 MeV per nucleon for 3He. We find that the energy spectra oflH, 2H, and 3He all fall rapidly with decreasing energy, giving 2H/1H and 3He/1H ratios which are essentially independent of energy as expected from current theories of the solar modulation of galactic cosmic rays. The measured 4He spectrum, however, is essentially flat below -40 MeV per nucleon, suggesting that there may be contributions from a local, nonsolar source of 4He. Comparisons ofthe 1 H, 2 H, and 3He observations with calculated spectra at 1 AU imply a mean interstellar path length of 7 ± 2 gem 2 • However, present low-energy measurements of Hand He isotopes at 1 AU do not discriminate between possible cosmic-ray source spectra. Subject heading: cosmic rays: abundances