Subshell gaps and onsets of collectivity from proton and neutron pairing gap correlations

TL;DR

This paper investigates how proton and neutron pairing gaps relate to collective behaviors and subshell gaps in open-shell nuclei, revealing new insights into nuclear structure and the onset of collectivity.

Contribution

It demonstrates that combined proton and neutron pairing gaps can indicate subshell gaps and collectivity, providing a novel macroscopic approach to nuclear correlations.

Findings

Large and close-lying pairing gaps signal subshell gaps and collectivity.

Observed B(E2) trends agree with experimental data.

Peak collectivity in $^{110}$Sn challenges traditional shell-model expectations.

Abstract

Throughout the nuclear chart, particle-hole correlations give rise to giant resonances and, together with the proton-neutron interaction, deformation and rotational bands. In order to shed light on many-body correlations in open-shell nuclei, I explore macroscopic properties that could manifest from the collective behavior of protons and neutrons. Intuitively, the correlation of proton and neutron Cooper pairs can be inferred from the respective pairing gaps, that can precisely be extracted from the AME 2020 atomic mass evaluation through odd-even atomic mass differences. This work shows that the combination of large and close-lying proton and neutron pairing gaps is sensitive to onsets of collectivity and subshell gaps in superfluid nuclei, away from major shell closures. Trends of reduced transition probabilities or B(E2) values -- which describe the collective overlap between the wave functions of initial and final nuclear states -- are revealed in overall agreement with data. Specially interesting is the peak of collectivity in the tin isotopes at $^{110}$Sn, instead of at midshell, as expected by large-scale shell-model calculations; a situation that has astounded the nuclear physics community for quite some time.