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Cosmic-ray lifetime in the galaxy: Experimental results and models

Abstract

The containment lifetime of the cosmic radiation is a crucial parameter in the investigation of the cosmic-ray origin and plays an important role in the dynamics of the Galaxy. The separation of the cosmic-ray Be isotopes achieved by two satellite experiments is considered in this paper, and from the measured isotopic ratio between the radioactive 10Be (half-life = 1.5 × 106 yr) and the stable 9Be, it is deduced that the cosmic rays propagate through matter with an average density of 0.24 ± 0.07 atoms cm-3, lower than the traditionally quoted average density in the galactic disk of 1 atom cm-3.

This paper reviews the implications of this result for the cosmic-ray age mainly in the context of two models of confinement and propagation: the homogeneous model, normally identified with confinement to the galactic gaseous disk, and a diffusion model in which the cosmic rays extend into a galactic halo. The propagation calculations use:

  1. (1)

    a newly deduced cosmic-ray pathlength distribution.

  2. (2)

    a self-consistent model of solar modulation.

  3. (3)

    an up-to-date set of fragmentation cross sections.

The satellite results and their implications are compared with the information on the cosmic-ray age derived from other cosmic-ray radioactive nuclei and the measured differential energy spectrum of high-energy electrons.

It is a major conclusion of this paper that in a homogeneous model the cosmic-ray age is 15(+7, -4) million years, i.e., about a factor 4 longer than early estimates based on the abundances of the light nuclei Li, Be, and B and a nominal interstellar density of 1 atom cm -3. The lifetime is even longer when the satellite results are applied to a diffusion halo model.

The deduced traversed matter density, together with other astrophysical considerations, suggest the population of a galactic halo by the cosmic rays.

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Simpson, J.A., Garcia-Munoz, M. Cosmic-ray lifetime in the galaxy: Experimental results and models. Space Sci Rev 46, 205–224 (1988). https://doi.org/10.1007/BF00212240

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Keywords

  • Homogeneous Model
  • Galactic Disk
  • Galactic Halo
  • Measured Isotopic Ratio
  • Gaseous Disk