  A model of supernova-driven chemical evolution of the Galactic halo, recently
proposed by Tsujimoto, Shigeyama, & Yoshii (1999, ApJL, 519, 64), is extended
in order to investigate the evolution of light elements such as Be and B (BeB),
which are produced mainly through spallative reactions with Galactic cosmic
rays. In this model each supernova sweeps up the surrounding interstellar gas
into a dense shell and directly enriches it with ejecta which consist of heavy
elements produced in each Type II supernova with different progenitor masses.
We propose a two-component source for GCRs such that both interstellar gas and
fresh SN ejecta engulfed in the shell are accelerated by the shock wave. Our
model results include: (1) a prediction of the intrinsic scatter in BeB and
[Fe/H] abundances within the model, (2) a successful prediction of the observed
linear trend between BeB and [Fe/H], (3) a proposal for using BeB as a cosmic
clock, as an alternative to [Fe/H], and (4) a method for possibly constraining
the BBN model from future observations of metal-poor stars.
