Measurement of the Isotopic Composition of Cosmic‐Ray Helium, Lithium, Beryllium, and Boron Up to 1700 MeV Per Atomic Mass Unit

We present data from the second flight of the superconducting magnet instrument for light isotopes (SMILI), which took place on 1991 July 24. This instrument was optimized to determine the isotopic composition of He, Li, Be, and B in the Galactic cosmic rays, up to an energy of 2 GeV amu(-1). The abundances of He, Li, and B are found to be consistent with standard models of cosmic-ray propagation. Our measurement of the abundances of the beryllium isotopes suggests an enhancement of the fraction of the isotope Be-10 over that found at low energy. Of 26 beryllium events, nine are found to be Be-10. Monte Carlo calculations based on this observation imply the mean lifetime of cosmic rays to be less than 6 Myr at the 97.5% confidence level.