The Cosmic-Ray Radioactive Nuclide ^36CL and Its Propagation in the Galaxy

Details The Cosmic-Ray Radioactive Nuclide ^36CL and Its Propagation in the Galaxy The Cosmic-Ray Radioactive Nuclide ^36CL and Its Propagation in the Galaxy

Dec 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...509l..97c&link_type=abstract

The Astrophysical Journal, Volume 509, Issue 2, pp. L97-L100.

Astronomy and Astrophysics

Astronomy

18

Ism: Cosmic Rays, Ism: General, Nuclear Reactions, Nucleosynthesis, Abundances

Scientific paper

Radioactive ^36Cl has been resolved from ^35Cl and ^37Cl for the first time in the cosmic radiation with measurements obtained from the high-resolution High-Energy Telescope (HET) carried on the NASA-ESA Ulysses spacecraft. Within the framework of a homogeneous propagation model wherein all ^36Cl is of spallation origin, the average density of the interstellar gas rho, through which all cosmic-ray nuclei propagate, was determined. Heliospheric modulation was included in the analysis. The abundance ratio ^36Cl/Cl=5.2+/-1.8% at an average energy of 238 MeV u^-1 yields rho=0.28^+0.12_-0.10 atoms cm^-3. Although ^36Cl has a relatively short half-life (tau_1/2=3.01x10^5 yr) compared with the approximately 20 Myr confinement or escape time T_esc for cosmic rays in the Galaxy derived from ^10Be or ^26Al analysis, it is shown that the confinement time based on ^36Cl analysis is 18^+10_-6 Myr. Radioactive ^10Be, ^26Al, and ^36Cl are all produced by nuclear interactions during propagation from Galactic sources to the observer. However, ^10Be, ^26Al, and ^36Cl are each predominantly products of different primary cosmic-ray nuclei. ^54Mn is the fourth radioactive isotope measured by the Ulysses HET. Unfortunately, its decay half-life to ^54Fe is difficult to determine. Recent measurements yielded tau_1/2=6.3x10^5 yr, leading to rho=0.40^+0.23_-0.15 atoms cm^-3 with a confinement time T_esc=11 Myr.

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