Electron capture in core-collapse supernovae investigated through configuration mixing in neutron-rich nuclei

Electron capture on neutron-rich medium-mass nuclei is a key process where the electrons that impede the collapse of the core of massive stars are captured, thereby producing very neutron-rich nuclei. As the core collapses, the supernova is then initiated. For the electron capture to proceed, however, the allowed Gamow-Teller (GT) transition must be unblocked either by thermal excitations or by mixing of proton configurations from a higher-lying shell into the ground-state configuration of the nucleus. The present paper presents an experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University, in which we study the configuration mixing in the neutron-rich⁷⁶Zn isotope. The experiment utilised single-proton and single-neutron knockout with detection of reaction-residue γ rays and measurement of the parallel momentum of the residue. Through this we investigate the proton components of the ⁷⁶Zn ground state, with a particular interest in π-g_9/2, which may unblock the GT electron capture even at low temperatures and thereby open a new pathway for the initiation of the collapse of the pre-supernova stellar core.