Secondary Electron-Capture Decay Isotopes and Implications for the Propagation of Galactic Cosmic Rays
S.M. Niebur1, W.R. Binns1, E.R. Christian2, A.C. Cummings3,
J.S. George3, P.L. Hink1, J. Klarmann1, R.A. Leske3,
M. Lijowski1, R.A. Mewaldt3, E.C. Stone3, T.T. von Rosenvinge3,
M.E. Wiedenbeck4, N.E. Yanasak4
1Washington University, St. Louis, Missouri, USA
2Goddard Space Flight Center, Greenbelt, Maryland, USA
3California Institute of Technology, Pasadena, California, USA
4Jet Propulsion Laboratory, Pasadena, California, USA
We report the first observations of galactic cosmic rays that can be used
to directly study energy-dependent electron capture of secondary isotopes.
Secondary nuclei that decay only by electron capture can be used to study
the energies at which galactic cosmic rays propagate through the interstellar
medium. Electron-capture decay occurs predominantly at low energies where
there is a significant probability of electron attachment. Consequently,
propagation models which incorporate energy-dependent electron attachment
cross sections predict that the abundances of these decay nuclei will be
depleted and the corresponding abundances of their daughter nuclei will be
elevated at low energies. Evidence of energy-dependent electron-capture decay
over and above that expected for a standard leaky-box propagation (excluding
reacceleration) could provide evidence of reacceleration of galactic cosmic
rays. The 51Cr --> 51V and 49V --> 49Ti decays are particularly suitable for
this study, since the amount of electron-capture decay expected for 51Cr and 49V
varies significantly over the interstellar energy interval measured by the
Cosmic Ray Isotope Spectrometer (CRIS) on ACE. These measurements and additional
modeling calculations will be used in the future to study the possible effects
of reacceleration on cosmic rays.