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We study the early evolution of the electron fraction (or, alternatively, theneutron-to-proton ratio) in the region above the hot proto-neutron star formedafter a supernova explosion. We study the way in which the electron fraction inthis environment is set by a competition between lepton (electron, positron,neutrino, and antineutrino) capture processes on free neutrons and protons andnuclei. Our calculations take explicit account of the effect of nuclearcomposition changes, such as formation of alpha particles (the alpha effect)and the shifting of nuclear abundances in nuclear statistical equilibriumassociated with cooling in near-adiabatic outflow. We take detailed account ofthe process of weak interaction freeze-out in conjunction with these nuclearcomposition changes. Our detailed treatment shows that the alpha effect cancause significant increases in the electron fraction, while neutrino andantineutrino capture on heavy nuclei tends to have a buffering effect on thisquantity. We also examine the effect on weak rates and the electron fraction offluctuations in time in the neutrino and antineutrino energy spectra arisingfrom hydrodynamic waves. Our analysis is guided by the Mayle & Wilson supernovacode numerical results for the neutrino energy spectra and density and velocityprofiles.Comment: 38 pages, AAS LaTeX, 8 figure

The Influence of Nuclear Composition on the Electron Fraction in the Post–Core Bounce Supernova Environment