A Re-Examination of the Cosmic Ray Helium Spectrum and the ^3He/^4He Ratio at High Energies

We have reexamined the implications of a recent measurement of cosmic-ray ^3He at ~6 GeV per nucleon which reported an unexpected high abundance of this rare isotope. The implied ^3He/^4He ratio from this high-energy measurement depends critically on the assumed rigidity spectrum of helium nuclei between ~10 and 20 GV. A careful analysis of published measurements of the helium spectrum, along with new data obtained using a magnetic spectrometer, show that the appropriate spectral index (γ) to be used to interpret this measurement has a value of less than 2.55, for a differential rigidity spectrum dJ/dR α R^(-γ). Since this index is lower than originally used, the ^3He/^4He ratio of less than 0.17 that we derive (evaluated at constant energy per nucleon), is significantly lower than the original quoted value of 0.24 ± 0.05. The cosmic-ray escape pathlength in g cm^(-2) of interstellar material deduced from this revised ratio and from the ^3He/^4He measurements at low energies is now found to be consistent with that deduced from studies of heavier nuclei. The body of ^3He/^4He measurements now available thus severely restricts the types of models that might be invoked to explain the high intensity of cosmic-ray antiprotons that has been reported.