We revisit the problem of the kink in the charge radius shift of neutron-rich even isotopes near the $N=126$ shell closure. We show that the ability of a Skyrme force to reproduce the isotope shift is determined by the occupation of the neutron $1i_{11/2}$ orbital beyond $N=126$ and the corresponding change it causes to deeply-bound protons orbitals with a principal quantum number of 1. Given the observed position of the single-particle energies, one must either ensure occupation is allowed through correlations, or not demand that the single-particle energies agree with experimental values at the mean-field level.

• Received 9 October 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.032503