Surprising Charge-Radius Kink in the Sc Isotopes at N=20

Charge radii of neutron deficient 40Sc and 41Sc nuclei were determined using collinear laser spectroscopy. With the new data, the chain of Sc charge radii extends below the neutron magic number N = 20 and shows a pronounced kink, generally taken as a signature of a shell closure, but one notably absent in the neighboring Ca, K, and Ar isotopic chains. Theoretical models that explain the trend at N = 20 for the Ca isotopes cannot reproduce this puzzling behavior.