Testing the Role of Cosmic Ray Reacceleration in the Galaxy

Cosmic rays constitute a super-thermal gas of charged particles magnetically confined within the Galaxy. While propagating though the interstellar medium (ISM), cosmic ray nuclei undergo nuclear spallation reactions, producing both stable (i.e., Be and B) and unstable secondary nuclei. Consistent cosmic ray confinement times of ~ 20 Myr have been reported from measurements of the radioactive secondary isotopes (10) Be, (26) Al, (36) Cl and (54) Mn using data from the High Energy Telescope (HET) on the Ulysses spacecraft. It is generally accepted that Galactic cosmic rays of energy less than ~ 10(14) eV are accelerated by supernova shocks in the ISM. Reacceleration of existing cosmic rays in the ISM is implicit in interstellar shock acceleration models, but whether reacceleration plays a significant role in cosmic ray production and interstellar propagation is largely unknown. The abundances of secondary electron-capture isotopes provide a crucial test of cosmic ray reacceleration. Electron-capture is suppressed during interstellar propagation because cosmic ray nuclei are essentially stripped of their electrons. If, however, 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. The abundance ratio of (49) V to (51) V is a particularly sensitive test of this effect. The latest Ulysses HET data is used to address this problem. This research was supported in part by NASA/JPL Contract 955432 and NASA Grant NAG5-5179.