Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the neutron-rich ${}^{52–54}\mathrm{Sc}$ and ${}^{54,56}\mathrm{Ti}$ nuclei. The new mass excess values $ME\left({}^{52}\mathrm{Sc}\right)=-40525\left(65\right)$ keV, $ME\left({}^{53}\mathrm{Sc}\right)=-38910\left(80\right)$ keV, and $ME\left({}^{54}\mathrm{Sc}\right)=-34485\left(360\right)$ keV, deviate from the Atomic Mass Evaluation 2012 by $2.3\sigma , 2.8\sigma$, and $1.7\sigma$, respectively. These large deviations significantly change the systematics of the two-neutron separation energies of scandium isotopes. The empirical shell gap extracted from our new experimental results shows a significant subshell closure at $N=32$ in scandium, with a similar magnitude as in calcium. Moreover, we present ab initio calculations using the valence-space in-medium similarity renormalization group based on two- and three-nucleon interactions from chiral effective field theory. The theoretical results confirm the existence of a substantial $N=32$ shell gap in Sc and Ca with a decreasing trend towards lighter isotones, thus providing a consistent picture of the evolution of the $N=32$ magic number from the $pf$ into the $sd$ shell.

• Received 2 December 2018

DOI:https://doi.org/10.1103/PhysRevC.99.064303