Isospin Character of the Pygmy Dipole Resonance in 124Sn

TL;DR

This study investigates the isospin structure of the pygmy dipole resonance in 124Sn using coincidence methods and theoretical models, revealing a split into isoscalar and isovector components with different excitation properties.

Contribution

It provides the first detailed experimental and theoretical analysis of the isospin character of the pygmy dipole resonance in 124Sn, highlighting the splitting into distinct components.

Findings

Identification of two components with different excitation mechanisms.

Agreement between experimental data and theoretical models.

Prediction of neutron-skin oscillations as dominant in the low-lying isoscalar component.

Abstract

The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn with the (a,a'g) coincidence method at E=136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in (a,a'g) as well as in (g,g') reactions and a group of states at higher energies which is only excited in (g,g') reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.