Why is lead so kinky?

TL;DR

This paper investigates the cause of the kink in charge radius shifts of neutron-rich lead isotopes, emphasizing the role of neutron orbital occupation and single-particle energy positioning in Skyrme force models.

Contribution

It demonstrates that the isotope shift kink depends on neutron orbital occupation and the alignment of single-particle energies, highlighting the importance of correlations or mean-field adjustments.

Findings

Occupation of neutron 1i11/2 orbital influences isotope shift

Single-particle energy positioning affects model accuracy

Correlations or adjusted energies are necessary for accurate predictions

Abstract

We revisit the problem of the kink in the charge radius shift of neutron-rich even lead isotopes. We show that the ability of a Skyrme force to reproduce the isotope shift is determined by the occupation of the neutron 1i11/2 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.