Abstract
Cosmic-ray ^3He/^4He observations, including a new measurement at ~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. We survey spacecraft and balloon observations of the ^3He/^4He ratio and find improved consistency among measurements in the ~50-300 MeV per nucleon energy range when a previously neglected contribution from atmospheric secondary ^3He is taken into account. These low-energy observations imply a mean escape length of 6-8 g cm ^(-2) in the standard leaky box model for cosmic-ray propagation in the galaxy, a value consistent with that derived from studies of heavier nuclei. Thus, we find no evidence for an excess of low-energy ^3He such as that reported at higher energies. Recent propagation models designed to explain the excess of cosmic-ray antiprotons observed at GeV energies also predict an excess of ^3He that is not observed at low energies.
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