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Abstract

Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the neutron-rich ^52–54Sc and ^54,56Ti nuclei. The new mass excess values ME(⁵²Sc)=−40525(65) keV, ME(⁵³Sc)=−38910(80) keV, and ME(⁵⁴Sc)=−34485(360) keV, deviate from the Atomic Mass Evaluation 2012 by 2.3σ, 2.8σ, and 1.7σ, 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.