Evolution of the pygmy dipole resonance in nuclei with neutron excess

TL;DR

This paper investigates how the pygmy dipole resonance evolves in nuclei with neutron excess using a Random Phase Approximation approach, revealing distinct low-energy excitations different from giant dipole resonances.

Contribution

It introduces a phenomenological RPA method to study the evolution of low-energy dipole strength in various isotopes, highlighting the nature of pygmy dipole resonances.

Findings

Neutron excess induces low-energy E1 strength.

Low-energy E1 excitations differ from giant dipole resonances.

Proton and neutron transition densities reveal isoscalar or isovector nature.

Abstract

The electric dipole excitation of various nuclei is calculated with a Random Phase Approximation phenomenological approach. The evolution of the strength distribution in various groups of isotopes, oxygen, calcium, zirconium and tin, is studied. The neutron excess produces $E1$ strength in the low energy region. Indexes to measure the collectivity of the excitation are defined. We studied the behavior of proton and neutron transition densities to determine the isoscalar or isovector nature of the excitation. We observed that in medium-heavy nuclei the low-energy $E1$ excitation has characteristics rather different that those exhibited by the giant dipole resonance. This new type of excitation can be identified as pygmy dipole resonance.