Elasticity of nuclear medium as a principal macrodynamical promoter of electric dipole pygmy resonance

TL;DR

This paper explores how the elastic properties of the nuclear medium promote the emergence of the low-energy electric dipole pygmy resonance as a soft mode resulting from core-layer oscillations, using a macroscopic elastic model.

Contribution

It introduces a macroscopic elastic medium model to explain the origin of the electric dipole pygmy resonance as a soft mode of core-layer oscillations in nuclei.

Findings

Derived spectral equations for dipole vibration frequencies.

Showed the fundamental nature of the soft dipole mode.

Linked elastic properties to resonance excitation strength.

Abstract

Motivated by arguments of the nuclear core-layer model formulated in [S.I. Bastrukov, J.A. Maruhn, Z.Phys. A 335 (1990) 139], the macroscopic excitation mechanism of electric pygmy dipole resonance (PDR) is considered as owing its origin to perturbation-induced effective decomposition of nucleus into two spherical domains - undisturbed inner region treated as static core and dynamical layer undergoing elastic shear vibrations. The focus is placed on the imprinted in the core-layer model mechanism of emergence of the low-energy dipole electric resonant excitation as Goldstone soft mode of translation layer-against-core oscillations. To accentuate this attitude we regain the obtained in the above paper spectral equation for the frequency of spheroidal elastic vibrations trapped in the finite-depth layer, but by working from canonical equation of elastic dynamics of continuous medium. The obtained analytic equations for the frequency of dipole vibrational state in question and its excitation strength exhibit fundamental character of this soft dipole mode of nuclear resonant response.