Electromagnetic transition strength in ${}^{53}$Ca: the lifetime of the $5/2^{-}$ state

Abstract

High-resolution in-beam $\gamma$-ray spectroscopy of ${}^{53}$Ca was performed at the Radioactive Isotope Beam Factory (RIBF) at RIKEN. Excited states in ${}^{53}$Ca were populated via one-neutron and one-proton removal reactions from a ${}^{55}$Sc radioactive beam, and the emitted $\gamma$ rays were detected using the HiCARI hybrid germanium detector array. The lifetime of the first excited $5/2^{-}$ state in ${}^{53}$Ca was measured for the first time through an analysis of Doppler-roadened $\gamma$-ray line shapes arising from relativistic recoil velocities. A lifetime of $\tau =11.3_{-11.3}^{+5.3(stat)+3(sys)}$ ps was deduced, corresponding to a reduced transition probability of $B(E2;5/2^{-}\to 1/2^{-})>2.6e^{2}f{m}^4$ at the 1$\sigma$ level, with a best-fit value of $4.5e^{2}f{m}^4$. The measured transition strength is compared with state-of-the-art shell-model and ab initio calculations, providing a stringent benchmark for shell-model Hamiltonians and effective charges and modern ab initio descriptions of the neutron-rich calcium isotopes approaching the potential $N$ = 40 shell closure.