Ulysses HET Measurements of Electron-capture Secondary Isotopes: Testing the Role of Cosmic Ray Reacceleration

Abstract

In recent publications, we have reported consistent cosmic ray Galactic confinement times from Ulysses HET measurements of 10Be, 26Al and 36Cl. The abundances of other secondary radio-isotopes, specifically electron-capture isotopes such as 49V and 51Cr, are a crucial test of cosmic ray reacceleration. Reacceleration is implicit in interstellar shock or MHD acceleration models, but its significance for interstellar propagation is largely unknown. Electron-capture is suppressed during interstellar propagation. However, if cosmic rays experience significant reacceleration, nuclei will have spent time at lower energies where electron pick-up, and hence electron capture, is more likely than at higher energies. Thus, electron capture secondary isotopes would be less abundant (and their daughters, more abundant) than otherwise predicted. We show that the abundance ratio of 49V to 51V is a particularly sensitive test of this effect. For the first time in the literature we present evidence of reacceleration using Ulysses High Energy Telescope (HET) data.