  As an explosion develops in the collapsed core of a massive star, neutrino
emission drives convection in a hot bubble of radiation, nucleons, and pairs
just outside a proto-neutron star. Shortly thereafter, neutrinos drive a
wind-like outflow from the neutron star. In both the convective bubble and the
early wind, weak interactions temporarily cause a proton excess (Ye > 0.5) to
develop in the ejected matter. This situation lasts for at least the first
second, and the approximately 0.05 - 0.1 solar masses that is ejected has an
unusual composition that may be important for nucleosynthesis. Using tracer
particles to follow the conditions in a two-dimensional model of a successful
supernova explosion calculated by Janka, Buras & Rampp (2003), we determine the
composition of this material. Most of it is helium and 56Ni. The rest is
relatively rare species produced by the decay of proton-rich isotopes unstable
to positron emission. In the absence of pronounced charged-current neutrino
capture, nuclear flow will be held up by long-lived waiting point nuclei in the
vicinity of 64Ge. The resulting abundance pattern can be modestly rich in a few
interesting rare isotopes like 45Sc, 49Ti, and 64Zn. The present calculations
imply yields that, when compared with the production of major species in the
rest of the supernova, are about those needed to account for the solar
abundance of 45Sc and 49Ti. Since the synthesis will be nearly the same in
stars of high and low metallicity, the primary production of these species may
have discernible signatures in the abundances of low metallicity stars. We also
discuss uncertainties in the nuclear physics and early supernova evolution to
which abundances of interesting nuclei are sensitive.
