Effects of density-dependent spin-orbit interactions in Skyrme-Hartree-Fock-Bogoliubov calculations of the charge radii and densities of Pb isotopes

TL;DR

This study introduces a density-dependent spin-orbit term in Skyrme interactions, improving the modeling of charge radii and surface densities of Pb isotopes, especially around N=126, and explores its effects on other nuclei like Ca-48.

Contribution

The paper develops and applies a new Skyrme interaction with a density-dependent spin-orbit term, enhancing the accuracy of nuclear charge radii and density predictions compared to previous models.

Findings

Improved modeling of the kink in charge radii at N=126 in Pb isotopes.

Better reproduction of surface proton densities in Pb-208.

Minor impact of isoscalar-to-isovector ratio changes on single-particle energies.

Abstract

I have investigated the $N$-dependence of the charge radii $r_c$ across $N=126$ in the Pb isotopes and the surface densities of $^{208}$Pb using new Skyrme interactions that contain a density-dependent spin-orbit term, which I have named the Skyrme-ddso interactions. I have compared the results obtained using Skyrme-Hartree-Fock-Bogoliubov calculations that employ the Skyrme-ddso interactions with the original Skyrme results and have discussed the effects of including the density-dependent spin-orbit term. The results for the kink behavior of $r_c$ at $N=126$ in the Pb isotopes were improved by the inclusion of the density-dependent spin--orbit term. Moreover, the new Skyrme calculations yield better results for the inner part of the surface proton density of $^{208}$Pb at $r\sim 5$ fm. The change in the $\ell s$ potentials from the original Skyrme calculation contributes to the kink behavior through its effects on the $\ell s$ splitting of the neutron $2g_{7/2,9/2}$ orbitals. It also affects the inner part of the surface proton density through its effect on the proton $1h_{11/2}$ orbitals. I have further demonstrated that the change in the isoscalar-to-isovector ratio of the spin--orbit term makes only a minor contribution to the single-particle energies and to the kink behavior in the Pb isotopes. In addition, I have investigated $^{48}$Ca using Skyrme-Hartree-Fock calculations with the new Skyrme interactions and have determined the effects of the density-dependent spin--orbit term on its radii and densities.