Analysis of bulk and surface contributions in the neutron skin of nuclei

TL;DR

This paper investigates whether neutron skins in nuclei are primarily bulk or surface phenomena by analyzing density distributions from various nuclear models, providing insights into nuclear structure and symmetry energy.

Contribution

It introduces a method to distinguish bulk and surface contributions in neutron skins using Fermi distribution fits and compares predictions from different nuclear interactions.

Findings

Equivalent sharp radius better indicates bulk vs. surface contributions.

Different nuclear models predict varying neutron skin thicknesses.

Bulk and surface contributions vary across isotopic chains.

Abstract

The neutron skin thickness of nuclei is a sensitive probe of the nuclear symmetry energy having multiple implications for nuclear and astrophysical studies. However, precision measurements of this observable are difficult. The analysis of the experimental data may imply some assumptions about the bulk or surface nature of the formation of the neutron skin. Here, we study the bulk or surface character of neutron skins of nuclei following from calculations with Gogny, Skyrme, and covariant nuclear mean-field interactions. These interactions are successful in describing nuclear charge radii and binding energies but predict different values for neutron skins. We perform the study by fitting two-parameter Fermi distributions to the calculated self-consistent neutron and proton densities. We note that the equivalent sharp radius is a more suitable reference quantity than the half-density radius parameter of the Fermi distributions to discern between the bulk and surface contributions in neutron skins. We present calculations for nuclei in the stability valley and for the isotopic chains of Sn and Pb.