  The abundances of beryllium and boron have been measured in halo stars of
metallicities as low as [Fe/H] =-3. The observations show that the ratios Be/Fe
and B/Fe are independent of metallicity and approximately equal to their solar
values over the entire range of observed metallicity. These observations are in
contradiction with the predictions of simple models of beryllium and boron
production by spallation in the interstellar medium of a well mixed galaxy. We
propose that beryllium and boron are produced by spallation in the ejecta of
type II supernovae. In our picture, protons and alpha particles are accelerated
early in the supernova event and irradiate the heavy elements in the ejecta
long before the ejecta mixes with the interstellar medium. We follow the
propagation of the accelerated particles with a Monte-Carlo code and find that
the energy per spallation reaction is about 5 GeV for a variety of initial
particle spectra and ejecta compositions. Reproducing the observed Be/Fe and
B/Fe ratios requires roughly 3 times 10^{47} ergs of accelerated protons and
alphas. This is much less than the 10^{51} ergs available in a supernova
explosion.
