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

Abstract

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 abu ndances. 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 .