The isotopic composition of hydrogen and helium in low-energy cosmic rays

Abstract

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 of ^1H, ^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 of the ^1H, ^2H, and ^3He observations with calculated spectra at 1 AU imply a mean interstellar path length of 7 ± 2 g cm^(-2). However, present low-energy measurements of Hand He isotopes at 1 AU do not discriminate between possible cosmic-ray source spectra.