He-3 in galactic cosmic rays

Cosmic-ray He-3/He-4 observations, including a new measurement around 65 MeV per nucleon from ISEE-3, are compared with interstellar propagation and solar modulation calculations in an effort to understand the origin of cosmic-ray He nuclei. A survey of spacecraft and balloon observations of the He-3/He-4 ratio shows improved consistency among measurements in the 50-300 MeV per nucleon energy range when a previously neglected contribution from atmospheric secondary He-3 is taken into account. These low-energy observations imply a mean escape length of 6-8 g/sq cm in the standard 'leaky box' model for cosmic-ray propagation in the Galaxy, a value consistent with that derived from studies of heavier nuclei. No evidence is found for an excess of low-energy He-3 such as that reported at high energies.