The γ-ray and cosmic ray connection: Ulysses HET secondary radioisotope measurements and cosmic ray propagation

Cosmic rays constitute a super-thermal gas of charged particles magnetically confined within the Galaxy. Cosmic ray nuclei and electrons are a principal source of the diffuse γ-ray background in the Galaxy. Thus, understanding diffuse γ-ray production is directly related to understanding cosmic ray propagation in the Galaxy. While propagating though the interstellar medium (ISM), cosmic ray nuclei undergo nuclear spallation reactions, producing both stable and unstable secondary nuclei. Measurements of secondary radioisotopes are crucial tests of cosmic ray propagation. The abundances of some radioactive secondary isotopes (10Be, 26Al, 36Cl, etc.) measure the average density of material cosmic rays traverse and relate to the confinement times of cosmic rays in the Galaxy. The abundances of electron capture isotopes and their daughter nuclei (for example, 49V and 51V) test the role of cosmic ray reacceleration. The Ulysses High Energy Telescope (HET) is a cosmic ray isotope spectrometer with sufficient mass resolution (∼0.28 u at Fe) and collecting area to measure these rare isotopes. The latest HET measurements of the radioactive secondary cosmic ray isotopes are given, and the implications for cosmic ray propagation and diffuse γ-ray production are discussed.