Impact of Electron-Captures on N = 50 Nuclei on Core-Collapse Supernovae

Abstract

Sensitivity studies of the late stages of stellar core collapse with respect to electron-capture rates indicate the importance of a region of nuclei near the N = 50 shell closure, just above doubly magic $^{78}$Ni. In the present work, it has been demonstrated that uncertainties in key characteristics of the evolution, such as the lepton fraction, electron fraction, entropy, stellar density, and in-fall velocity are about 50% due to uncertainties in the electron-capture rates on nuclei in this region, although thousands of nuclei are included in the simulations. The present electron-capture rate estimates used for the nuclei in this region of interest are primarily based on a simple approximation, and it is shown that the estimated rates are likely overestimated by an order of magnitude or more. More accurate microscopic theoretical models are required to obtain Gamow-Teller strength distributions, upon which electron-capture rates are based. The development of these models and the benchmarking of such calculations rely on data from charge-exchange experiments at intermediate energies. An experimental campaign to study Gamow-Teller strength distributions in nuclei at and near N = 50, including $^{86}$Kr and $^{88}$Sr, with the (t, $^3$He) reaction at NSCL is underway and preliminary results indicate the electron-capture rates in the weak rate library are indeed overestimated.