Cosmic-Ray [TSUP]26[/TSUP]A[CLC]l[/CLC] and Its Decay in the Galaxy

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Abstract
Radioactive 26Al (τ1/2 = 8.7 × 105 yr) has been resolved from stable 27Al in the cosmic radiation for the first time using the high mass resolution, charged particle telescope on the Ulysses spacecraft. The average density of the interstellar medium through which the cosmic radiation propagates was determined within the framework of a homogeneous propagation model, where all 26Al is of cosmic-ray spallation origin. Heliospheric modulation of the two isotopes was included. The derived ratio 26Al/27Al = 0.061±0.007 yields an average interstellar medium density ρ = 0.26+0.05−0.04 atoms cm−3 and a Galactic confinement time of 19±3 million years. These values agree with the results from radioactive 10Be analysis and indicate that the cosmic radiation may spend part of its confinement time in a Galactic magnetic halo. The implications of the observed 1.809 MeV γ-rays from 26Al decay to 26Mg throughout the Galaxy are discussed with respect to 26Al, 27Al, and 26Mg in the cosmic radiation and their injection into the interstellar medium from nucleosynthesis in supernovae, novae, and red giant stars.