Masses of neutron-rich 52-54SC and Ti-54,Ti-56 nuclides: The N=32 subshell closure in scandium

Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the neutron-rich Sc52-54 and Ti-54,Ti-56 nuclei. The new mass excess values ME (Sc-52) = - 40525(65) keV, ME(Sc-53) = -38910(80) keV, and ME(Sc-54) = -34485(360) 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.