Extraction of Gamow-Teller strength distributions from56Ni and55Co via the(p,n)reaction in inverse kinematics

Abstract

Background: Gamow-Teller (GT) transition strength distributions in stable and unstable $pf$-shell isotopes are key inputs for estimating electron-capture rates important for stellar evolution. Charge-exchange experiments at intermediate beam energies have long been used to test theoretical predictions for GT strengths, but previous experiments were largely restricted to stable nuclei. Since a large fraction of the nuclei relevant for astrophysical applications (including key nuclei such as ${}^{56}$Ni) are unstable, new methods are needed to perform charge-exchange experiments in inverse kinematics with unstable isotopes.Purpose: The ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were measured in inverse kinematics in order to extract GT strengths for transitions to ${}^{56}$Cu and ${}^{55}$Ni, respectively. The extracted strength distributions were compared with shell-model predictions in the $pf$ shell using the KB3G and GXPF1J interactions. By invoking isospin symmetry, these strength distributions are relevant for electron captures on the ground states of ${}^{56}$Ni and ${}^{55}$Ni to final states in ${}^{56}$Co and ${}^{55}$Co, respectively.Method: Differential cross sections and excitation energy spectra for the ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were determined by measuring neutrons recoiling from a liquid hydrogen target into the Low Energy Neutron Detector Array. GT contributions to the spectra were extracted by using a multipole decomposition analysis and were converted to strengths by employing the proportionality between GT strength and differential cross section at zero linear momentum transfer.Results: GT strengths from ${}^{56}$Ni and ${}^{55}$Co were extracted up to excitation energies of 8 and 15 MeV, respectively. Shell-model calculations performed in the $pf$ shell with the GXPF1J interaction reproduced the experimental GT strength distributions better than calculations with the KB3G interaction.Conclusions: A new technique for measuring ($p,n$) charge-exchange reactions on unstable nuclei was successfully developed. It can be used to study the isovector response of unstable nuclei in any mass region and for excitation energies beyond the particle decay threshold. In the first experiment, ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were studied and GT transition strengths were extracted for the purpose of testing shell-model calculations used to estimate electron-capture rates in simulations of late stellar evolution. The calculation using the GXPF1J interaction was found to best reproduce the experimental strength distribution.