A cosmic-ray age based on the abundance of 10Be

A measurement of the isotopic composition of galactic cosmic-ray beryllium (60--185 MeV amu-1) has been made with high resolution (sigma/sub M/ < 0.2 amu). The isotope fractions of 7Be, 9Be, and 10Be obtained are 0.546 +- 0.029, 0.390 +- 0.029, and 0.064 +- 0.015, respectively. The abundance of the radioactive isotope 10Be (T/sub 1/2/=1.6 x 106 yr) is used to deduce a cosmic-ray confinement time of 8.4 (+4.0, : 2.4) x 106 yr and an interstellar gas density of 0.33 (+0.13, : 0.11) atoms per cm3 in the confinement volume. These uncertainties are due only to the measurement errors (primarily statistical). Errors in the propagation model parameters, which result in comparable uncertainties, are discussed