Cosmic-ray time scales using radioactive clocks

Radionuclides in the galactic cosmic rays serve as chronometers for measuring the characteristic time of physical processes affecting cosmic ray energy spectra and composition. The radionuclide Ni-59, present in the ejecta of supernovae, will decay to Co-59 via electron-capture with a halflife of T-1/2 = 7.6 x 10(4) yr. However, if the cosmic ray acceleration time scale is shorter than the decay halflife, 59Ni will become fully-stripped of electrons and will be present in the cosmic rays. Abundances of cosmic ray Ni-59 and Co-59 measured with the Cosmic Ray Isotope Spectrometer (CRIS) are consistent with the decay of all source Ni-59, implying an acceleration time delay > 10(5) yr. Abundances of the beta -decay radioactive secondaries, produced by fragmentation of the cosmic rays during transport in the interstellar medium (ISM), depend on the time scales for spallation and escape from the Galaxy. Consequently, measurement of these abundances can be used to derive the galactic confinement time, tau (esc) for cosmic rays. Using the abundances of the beta -decay species Be-10, Al-26, Cl-36, and Mn-54 measured by CRIS, we find a confinement time tau (esc) similar to 15 Myr. Published by Elsevier Science Ltd on behalf of COSPAR.