Influence of the symmetry energy on the giant monopole resonance of neutron-rich nuclei

TL;DR

This study investigates how the density dependence of the symmetry energy affects the giant monopole resonance energies in neutron-rich nuclei using a relativistic mean field model, revealing significant structural changes at high neutron numbers.

Contribution

It introduces a modified relativistic mean field model to explore the impact of symmetry energy density dependence on GMR in neutron-rich nuclei.

Findings

Softer symmetry energy leads to higher GMR excitation energies in stable nuclei.

In highly neutron-rich nuclei, softer symmetry energy results in lower GMR excitation energies.

The resonance width increases sharply in neutron-rich isotopes, especially with softer symmetry energy.

Abstract

We analyze the influence of the density dependence of the symmetry energy on the average excitation energy of the isoscalar giant monopole resonance (GMR) in stable and exotic neutron-rich nuclei by applying the relativistic extended Thomas-Fermi method in scaling and constrained calculations. For the effective nuclear interaction, we employ the relativistic mean field model supplemented by an isoscalar-isovector meson coupling that allows one to modify the density dependence of the symmetry energy without compromising the success of the model for binding energies and charge radii. The semiclassical estimates of the average energy of the GMR are known to be in good agreement with the results obtained in full RPA calculations. The present analysis is performed along the Pb and Zr isotopic chains. In the scaling calculations, the excitation energy is larger when the symmetry energy is softer. The same happens in the constrained calculations for nuclei with small and moderate neutron excess. However, for nuclei of large isospin the constrained excitation energy becomes smaller in models having a soft symmetry energy. This effect is mainly due to the presence of loosely-bound outer neutrons in these isotopes. A sharp increase of the estimated width of the resonance is found in largely neutron-rich isotopes, even for heavy nuclei, which is enhanced when the symmetry energy of the model is soft. The results indicate that at large neutron numbers the structure of the low-energy region of the GMR strength distribution changes considerably with the density dependence of the nuclear symmetry energy, which may be worthy of further characterization in RPA calculations of the response function.