Evolution of the Light Elements and the Cosmic‐Ray Flux in the Galaxy

Simple models for cosmic-rayinduced chemical evolution of the Galaxy prior to 1992 predicted a quadratic dependence of the Be and B abundances on metallicity among halo stars, whereas observations have shown that a linear trend obtains. Among the many possible solutions suggested so far for this apparent discrepancy, we have reexamined (1) the spallation by heavy cosmic rays impinging on protons and alphas in the interstellar medium and (2) the specic manner of evolution of the cosmic-ray energy spectrum during the collapse of the halo. We nd that these solutions, which account equally well for the linear trend, give rise to di erent evolutionary behaviors of the lithium isotope 6Li as a function of metallicity. Precise measurements of 6Li in halo stars might enable one to overcome such a degeneracy in the solutions and provide new insight into the early energetic processes in the Galaxy.