  During the first 20 seconds of its life, the enormous neutrino luminosity of
a neutron star drives appreciable mass loss from its surface. Previous
investigations have shown that this neutrino-driven wind could be the site
where the r-process occurs. The nucleosynthesis is sensitive to four physical
parameters characterizing the wind: its mass loss rate, the entropy per baryon,
the electron fraction, and the dynamic time scale. Different authors, using
numerical models for supernovae, have arrived at qualitatively different values
for these key parameters. Here we derive their values analytically and test our
analytic results by numerical calculations using an implicit hydrodynamic code.
Employing our analytic and numerical methods, we also investigate how various
factors can affect our results. The derived entropy typically falls short, by a
factor of two to three, of the value required to produce a strong r-process.
Various factors that might give a higher entropy or a more rapid expansion in
the wind are discussed.
