PRODUCTION AND EVOLUTION OF LIBEB ISOTOPES IN THE GALAXY

We reassess the problem of the production of the light elements lithium, beryllium, and boron, by energetic collisions between Galactic cosmic rays (GCR) and interstellar gas nuclei, in the framework of a consistent model for the chemical evolution of our Galaxy. We propose a new scenario for the propagation of GCR in the early Galaxy suggesting that, during its halo phase, GCR were more efficiently confined than today and had flatter spectra at low energy. Application of this scenario to our chemical evolution model leads to a satisfactory agreement between theory and the recent observations of Be and B in low-metallicity stars; an associated Li overproduction is avoided because of the peculiar GCR spectra implied by our scenario. We conclude that the observed Be and B abundances at low metallicities can still be explained in terms of GCR production, with no need for a less conventional mechanism, like, e.g., inhomogeneous big bang nucleosynthesis.