The g-ray and Cosmic Ray Connection: Ulysses HET Secondary Radioisotope Measurements and Cosmic Ray Propagation
J. J. Connell
J. A. Simpson
Cosmic rays constitute a super-thermal gas of charged particles magnetically
confined within the Galaxy.
Galactic cosmic ray nuclei and electrons are a major source of the diffuse
Thus, understanding diffuse g-ray production requires understanding
cosmic ray propagation in the Galaxy.
While propagating though the interstellar medium, cosmic ray nuclei undergo
nuclear spallation reactions, producing both stable and unstable secondary
Measurements of secondary radioisotopes are crucial probes of cosmic ray
The abundances of some secondary radioisotopes (10Be, 26Al,
36Cl, etc.) show the average density of material cosmic rays traverse
and relate to the confinement times of cosmic rays in the Galaxy.
The fact that the average density ( ~ 0.25 atom/cm3) found from these
measurements is much lower than the density in the Galactic disk suggests
that cosmic rays spend most of their time in a Galactic halo.
Thus, these measurements are a critical constraint on Galactic halo models.
The abundances of electron capture isotopes and their daughter nuclei (for
example, 49V and 51V) test the role of cosmic ray reacceleration.
Electron capture is strongly suppressed during cosmic ray propagation because
the nuclei are effectively stripped of their electrons.
Electron capture is only possible via electron pick-up during propagation,
a highly energy dependent process more likely at low energy.
If cosmic rays experience significant reacceleration, the observed cosmic ray
electron capture isotopes will be less abundant than otherwise expected
and their daughters correspondingly more abundant.
The Ulysses High Energy Telescope (HET) is a cosmic ray isotope
spectrometer with sufficient mass resolution ( ~ 0.28 amu at Fe) and
collecting area to measure these rare isotopes.
The latest HET measurements of the radioactive secondary cosmic ray isotopes
will be presented, and the implications for cosmic ray propagation discussed.
File translated from TEX by TTH, version 2.32.
On 16 Jul 1999, 09:19.