  Supernova simulations to date have assumed that during core collapse electron
captures occur dominantly on free protons, while captures on heavy nuclei are
Pauli-blocked and are ignored. We have calculated rates for electron capture on
nuclei with mass numbers A=65-112 for the temperatures and densities
appropriate for core collapse. We find that these rates are large enough so
that, in contrast to previous assumptions, electron capture on nuclei dominates
over capture on free protons. This leads to significant changes in core
collapse simulations.
