Neutron shell blocking of electron capture during gravitational collapse

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Abstract
Current ideas on the nuclear equation of state predict that early on in the collapse of the iron core of a massive star the nuclei present will become so neutron rich that allowed electron capture is blocked. This neutron shell blocking phenomenon and several unblocking mechanisms operative at high temperature and density, including forbidden electron capture, are discussed in terms of the simple shell model. Though the unblocking mechanisms are sensitive to details of the equation of state, typical conditions result in a considerable reduction of the electron capture rates on heavy nuclei, leading to dominance of electron capture on free protons and a large decrease in the overall neutronization rate. The results of one-zone collapse calculations are presented which suggest that the effect of neutron shell blocking is to produce a larger core lepton fraction at neutrino trapping which may lead to a larger inner-core mass and hence a stronger postbounce shock.

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