Experimental Constraint on Stellar Electron-Capture Rates from the ${}^{88}\text{Sr}(t,{}^3\text{He}+{\hat{I}}^3){}^{88}\text{Rb}$ reaction at 115 MeV/u

Abstract

The Gamow-Teller strength distribution from ${}^{88}$Sr was extracted from a $(t,{}^3\text{He}+{\hat{I}}^3)$ experiment at 115 MeV/$u$ to constrain estimates for the electron-capture rates on nuclei around $N=50$, between and including ${}^{78}$Ni and ${}^{88}$Sr, which are important for the late evolution of core-collapse supernovae. The observed strength below an excitation energy of 8 MeV was consistent with zero and below 10 MeV amounted to $0.1\pm 0.05$. Except for a very-weak transition that could come from the 2.231-MeV $1^{+}$ state, no ${\hat{I}}^3$ lines that could be associated with the decay of known $1^{+}$ states were identified. The derived electron-capture rate from the measured strength distribution is more than an order of magnitude smaller than rates based on the single-state approximation presently used in astrophysical simulations for most nuclei near $N=50$. Rates based on shell-model and quasiparticle random-phase approximation calculations that account for Pauli blocking and core-polarization effects provide better estimates than the single-state approximation, although a relatively strong transition to the first $1^{+}$ state in ${}^{88}$Rb is not observed in the data. Pauli unblocking effects due to high stellar temperatures could partially counter the low electron-capture rates. The new data serves as a zero-temperature benchmark for constraining models used to estimate such effects.