Investigation of Pygmy Dipole Resonances in the Tin Region

TL;DR

This paper investigates the evolution and characteristics of Pygmy Dipole Resonances in tin isotopes using advanced nuclear models, revealing their relation to neutron skins and matching recent experimental data.

Contribution

It introduces a comprehensive theoretical approach combining HFB and multi-phonon QPM to study PDR evolution across tin isotopes, highlighting the skin dependence and resonance fragmentation.

Findings

PDR is linked to neutron skin thickness in heavy Sn isotopes.

A transition from neutron to proton skin affects PDR character.

Theoretical results agree with recent experimental measurements.

Abstract

The evolution of the low-energy electromagnetic dipole response with the neutron excess is investigated along the Sn isotopic chain within an approach incorporating Hartree-Fock-Bogoljubov (HFB) and multi-phonon Quasiparticle-Phonon-Model (QPM) theory. General aspects of the relationship of nuclear skins and dipole sum rules are discussed. Neutron and proton transition densities serve to identify the Pygmy Dipole Resonance (PDR) as a generic mode of excitation. The PDR is distinct from the GDR by its own characteristic pattern given by a mixture of isoscalar and isovector components. Results for the $^{100}$Sn-$^{132}$Sn isotopes and the several N=82 isotones are presented. In the heavy Sn-isotopes the PDR excitations are closely related to the thickness of the neutron skin. Approaching $^{100}$Sn a gradual change from a neutron to a proton skin is found and the character of the PDR is changed correspondingly. A delicate balance between Coulomb and strong interaction effects is found. The fragmentation of the PDR strength in $^{124}$Sn is investigated by multi-phonon calculations. Recent measurements of the dipole response in $^{130,132}$Sn are well reproduced.