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Published December 20, 2001 | Published
Journal Article Open

Measurement of the Secondary Radionuclides ^(10)Be, ^(26)Al, ^(36)Cl, ^(54)Mn, and ^(14)C and Implications for the Galactic Cosmic-Ray Age


We report on abundance measurements of ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn in the Galactic cosmic rays (GCRs) using the Cosmic-Ray Isotope Spectrometer (CRIS) instrument aboard the Advanced Composition Explorer spacecraft at energies from ~70 to ~400 MeV nucleon^(-1). We also report an upper limit on the abundance of GCR ^(14)C. The high statistical significance of these measurements allows the energy dependence of their relative abundances to be studied. A steady-state, leaky-box propagation model, incorporating observations of the local interstellar medium (ISM) composition and density and recent partial fragmentation cross section measurements, is used to interpret these abundances. Using this model, the individual galactic confinement times derived using data for each species are consistent with a unique confinement time value of τ_(esc) = 15.0 ± 1.6 Myr. The CRIS abundance measurements are consistent with propagation through an average ISM hydrogen number density n_H = 0.34 ± 0.04 H atoms cm^(-3). The surviving fractions, f, for each radioactive species have been calculated. From predictions of the diffusion models of Ptuskin & Soutoul, the values of f indicate an interstellar cosmic-ray diffusion coefficient of D = (3.5 ± 2.0) × 10^(28) cm^2 s^(-1).

Additional Information

© 2001 American Astronomical Society. Received 2000 October 6; accepted 2001 August 2. We gratefully acknowledge the individuals responsible for the development of the CRIS instrument (listed in Stone et al. 1998a). We appreciate the helpful discussion of Petr Vogel concerning theoretical calculations of the half-life of ^(54)Mn. We also thank the University of Chicago for providing Climax neutron monitor data (supported by National Science Foundation grant ATM 96-13963). This research was supported by the National Aeronautics and Space Administration at the California Institute of Technology (under grant NAG5-6912), the Jet Propulsion Laboratory, the Goddard Space Flight Center, Washington University, and the McDonnell Center for the Space Sciences at Washington University.

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