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Published June 1, 1976 | Published
Journal Article Open

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


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.

Additional Information

© 1976 American Astronomical Society. Received 1975 August 13. Mr. W. E. Althouse, Dr. G. J. Hurford, and Dr. J. E. Lupton made significant contributions to the design and development of the Electron/Isotope Spectrometer. In addition, Dr. Hurford was responsible for many of the data analysis programs. We appreciate discussions with Dr. T. L. Garrard. This work was supported in part by the National Aeronautics and Space Administration under contract NAS5-11066 and grant NGR 05-002-160.

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Published - 1976ApJ___206__616M.pdf


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