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Published 2001 | public
Journal Article

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 ^(59)Ni, present in the ejecta of supernovae, will decay to ^(59)Co via electron-capture with a halflife of T_(1/2) = 7.6 × 10^4 yr. However, if the cosmic ray acceleration time scale is shorter than the decay halflife, ^(59)Ni will become fully-stripped of electrons and will be present in the cosmic rays. Abundances of cosmic ray ^(59)Ni and ^(59)Co measured with the Cosmic Ray Isotope Spectrometer (CRIS) are consistent with the decay of all source ^(59)Ni, implying an acceleration time delay > 10^5 yr. Abundances of the β-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, τ_(esc), for cosmic rays. Using the abundances of the β-decay species ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn measured by CRIS, we find a confinement time τ_(esc) ∼ 15 Myr.

Additional Information

© Published by Elsevier Science Ltd on behalf of COSPAR. Available online 3 August 2001. This work was supported by NASA (under NAG5-6912} at the California Institute of Technology, the Jet Propulsion Laboratory, Washington University, and Goddard Space Flight Center.

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