Secondary electron-capture-decay isotopes and implications for the propagation of galactic cosmic rays

We report the first observation of an energy dependence in the titanium, vanadium, and chromium isotopic abundances in cosmic rays. The observations were made in the 100–500 MeV/nucleon energy interval using data from the Cosmic Ray Isotope Spectrometer on the ACE spacecraft. The ^(51)Cr and ^(49)V isotopes in cosmic rays are produced by fragmentation of heavier cosmic ray nuclides and decay only by electron capture. The observations indicate that electron-capture decay occurred primarily at the lower energies measured, and that there is a resulting energy dependence in the abundances of these isotopes and their decay products.