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) a newly deduced cosmic-ray pathlength distribution. (2) a self-consistent model of solar modulation. (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.- Publication:
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Space Science Reviews
- Pub Date:
- September 1988
- DOI:
- 10.1007/BF00212240
- Bibcode:
- 1988SSRv...46..205S
- Keywords:
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- Astronomical Models;
- Beryllium;
- Cosmic Rays;
- Interstellar Matter;
- Isotope Separation;
- Milky Way Galaxy;
- Abundance;
- Beryllium 10;
- Diffusion Length;
- Energy Spectra;
- Gaseous Diffusion;
- Half Life;
- Nuclei (Nuclear Physics);
- Time Dependence;
- Space Radiation