The Elemental Abundance Ratios of Interstellar Secondary and Primary Cosmic Rays

Abstract

The rigidity dependence of the ratios. of abundances of interstellar secondary and primary cosmic rays contains important information on their confinement and propagation in the Galaxy. Recent results suggest that high energy cosmic rays (> 20 GeV/nucleon) have propagated through much less interstellar matter than lower energy nuclei (< 1 GeV/nucleon). We report new observations of abundances in the charge range 2 ≤ Z ≤ 10, which were obtained with a dE/dx-Cerenkov detector launched into a polar orbit on OGO-6 as part of the Caltech Solar and Galactic Cosmic Ray Experiment. Integral rigidity spectra of all the elements observed have shapes similar to that of the helium spectrum in the rigidity range ~ 2 to ~ 14 GV, approaching a power law with exponent -1.6 above 8 GV. Calculations of interstellar propagation assuming a steady-state model and including the presence of interstellar helium and the effects of solar modulation predict a variation with rigidity of ratios such as Be/O and B/O, which is not observed. The data can be explained by assuming a rigidity-dependent confinement of cosmic rays within the Galaxy.

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