Emergence of pygmy dipole resonances: Magic numbers and neutron skins

TL;DR

This study systematically investigates pygmy dipole resonances in light to medium nuclei, revealing their dependence on neutron skin thickness, Fermi level position, and orbital occupation, with a linear correlation between PDR strength and neutron skin.

Contribution

It provides a comprehensive, self-consistent analysis of PDR emergence and its relation to nuclear structure factors across multiple isotopic chains.

Findings

PDR peaks appear below 10 MeV in all studied isotopes.

PDR strength correlates with neutron skin thickness and orbital occupation.

Linear relationship between PDR strength and neutron skin thickness (~0.2/fm).

Abstract

The pygmy dipole resonances (PDR) for even-even nuclei in 8=<Z=<40 are studied performing a systematic calculation of the random-phase approximation with the Skyrme functional of SkM*. The calculation is fully self-consistent and does not assume any symmetry in the nuclear shape of the ground state. In every isotopic chain, the PDR emerges by showing a peak of the E1 strength at energies less than 10 MeV. The E1 strength of the PDR strongly depends on the position of the Fermi level and shows a clear correlation with the occupation of the orbits with the orbital angular momenta less than 3\hbar (l =< 2). We also found a strong correlation between the isotopic dependence of the neutron skin thickness and the pygmy dipole strength. The fraction of the energy weighted strength exhausted by the PDR and the neutron skin thickness show a linear correlation with the universal rate of about 0.2/fm.