Cause of the charge radius isotope shift at theN=126 shell gap

P.M Goddard,P.D Stevenson,A Rios,S Lunardi,J.J Valiente-Dobon,C Bucci,P Di Nezza,R Menegazzo,A Nannini,P.G Bizzeti,M Chiari,A Dainese,C Signorini

doi:10.1051/epjconf/20146602042

Abstract

We discuss the mechanism causing the `kink' in the charge radius isotope shift at the N=126 shell closure. The occupation of the 1$i_{11/2}$ neutron orbital is the decisive factor for reproducing the experimentally observed kink. We investigate whether this orbital is occupied or not by different Skyrme effective interactions as neutrons are added above the shell closure. Our results demonstrate that several factors can cause an appreciable occupation of the 1$i_{11/2}$ neutron orbital, including the magnitude of the spin-orbit field, and the isoscalar effective mass of the Skyrme interaction. The symmetry energy of the effective interaction has little influence upon its ability to reproduce the kink.

Highlights

The kink in the charge radius shift of even-even nuclei at the N=126 shell closure has been challenging to explain theoretically for several decades
Skyrme interactions which result in a significant population of the 1i11/2 neutron orbital for isotopes past the N = 126 shell closure reproduce the kink
This demonstrates that even for Skyrme interactions with a smaller symmetry energy, all that is required to reproduce the kink is an appreciable occupation of the 1i11/2 neutron orbital

Summary

Introduction

The kink in the charge radius shift of even-even nuclei at the N=126 shell closure has been challenging to explain theoretically for several decades. Popular explanations advocate the influence of a neutron 2g9/2 level with a large orbital radius [1, 2], or the role of ground-state quadrupole correlations [3]. Skyrme interactions which result in a significant population of the 1i11/2 neutron orbital for isotopes past the N = 126 shell closure reproduce the kink. Skyrme interactions which reproduce the kink always have a significant BCS occupation of the 1i11/2 neutron orbital compared to the 2g9/2. We shall discuss some of these factors in the following subsections

Spin-orbit field

Nuclear matter properties at saturation density

Conclusions