Nature of Isoscalar Dipole Resonances in Heavy Nuclei

Abstract

Isoscalar dipole nuclear response reveals the low-and high-energy resonances. The nature of isoscalar dipole resonances in heavy spherical nuclei is studied by using the translation-invariant kinetic model of small oscillations of finite Fermi systems. Calculations of the velocity field at the centroid energy show a pure vortex character of the low-energy isoscalar dipole resonance in spherical nuclei and confirm the anisotropic compression character of the high-energy one. The evolution of the velocity field in dependence on the excitation energy of the nucleus within the resonance width is studied. It is found that the low-energy isoscalar dipole resonance retains a vortex character while with increasing energy this collective excitation also involves compression.The high-energy resonance keeps the compression character with a change in the excitation energy within the resonance width but the compression-expansion region of the velocity field related to this resonance shifts inside the nucleus.

Isoscalar dipole response of heavy nuclei in low-energy region within kinetic model

“…The translation-invariant kinetic model of small oscillations of finite Fermi systems based on the direct solution of the Vlasov equation for a Fermi system with a moving surface is used to study the collective isoscalar dipole excitations of heavy nuclei [17,19,20]. In this model, a nucleus is treated as a gas of interacting fermions confined to a spherical cavity with a moving surface.…”

“…Within our kinetic model, we can find the explicit expression for the fluctuation of the phase-space distribution function related to the collective isoscalar dipole excitations. By using this function, we can calculate the response function [19] as well as the local dynamical quantities, in particular, the velocity field [20].…”

“…Our translation-invariant model allows for a clear way to evaluating the intrinsic response function associated with the field (1). By looking at the response of the center of mass induced by the external field (1), we can get [20] 4 (13) .…”

“…The main resonance is interpreted as a nuclear toroidal mode. The semiclassical dipole strength function also has several resonance structures in the low-energy region [17,20]. It is of interest to study the nature of these resonance structures in order to compare them with quantum studies.…”

“…In this paper, the velocity fields associated with low-energy collective isoscalar dipole excitations are studied within the translation-invariant kinetic model of small oscillations of finite Fermi systems [21]. This article is a continuation of [20], in which attention was focused on studying the nature of the main nuclear isoscalar dipole resonances (toroidal and compression modes). In Section 2, we briefly recall the formalism of the kinetic model of collective dipole excitations in nuclei.…”