Isoscalar Dipole Resonance Research Articles

Вихрова октупольна мода у кінетичній моделі колективних збуджень в ядрах

The nature of a new isoscalar octupole resonance found within a kinetic model based on the Vlasov equation for finite Fermi systems with moving surfaces is studied. It is shown that this octupole resonance is due to dynamic effects of the nuclear surface, like the low-energy isoscalar dipole resonance (vortex dipole mode) observed in heavy nuclei. It is found that the velocity field associated with the new octupole resonance has a vortex character in the surface region of the nuclear liquid and, moreover, the vortex motion of nucleons is fragmented into three areas near the nuclear surface. At the same time, the velocity field associated with the high-energy octupole resonance found within our kinetic model displays an octupole deformation form and includes a compression within the nuclear fluid, which is consistent with the corresponding quantum calculations in the random phase approximation.

Dynamic effects of nuclear surface in isoscalar dipole modes

Dynamic surface effects in the isoscalar dipole modes of heavy nuclei are studied within a semiclassical model based on the solution of the Vlasov kinetic equation for finite Fermi systems with a moving surface. The isoscalar dipole strength function and the velocity field associated with the isoscalar dipole modes obtained in previous papers using the semiclassical model are considered in the dynamic-surface approximation, which takes into account only the part of the variation of the phase-space distribution function caused by the dynamic surface. It is found that the dynamic surface effects have an essential influence on the features of the isoscalar dipole modes. The isoscalar dipole strength function has a two-resonance structure already in the dynamic-surface approximation, and the centroid energies of both the low-energy resonance and the high-energy resonance are close to corresponding centroid energies of exact strength function. Calculations of the velocity field in the dynamic-surface approximation show the vortex character of the low-energy isoscalar dipole resonance and the compression character of the high-energy one, which are similar to the corresponding exact velocity fields.

Nature of Isoscalar Dipole Resonances in Heavy Nuclei

The isoscalar dipole nuclear response reveals low- and high-energy resonances. The nature of isoscalar dipole resonances in heavy spherical nuclei is studied, by using a 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 as a function of 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 this collective excitation also involves a compression, as the energy increases. 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.

Description of decay mechanisms of the giant dipole resonances with a RPA + FKK approach

The decay mechanisms of giant resonances have been revisited to investigate the isoscalar dipole resonance in 208Pb nucleus.

Effects of coupling between particles and surface vibrations on isoscalar response of nuclei

The isoscalar quadrupole and octupole response of heavy spherical nuclei is studied in a semiclassical model that includes a coupling between the motion of nucleons and surface vibrations. By using a separable approximation for the residual interaction, an analytical expression has been obtained for the isoscalar response function of different multipolarity. A unified description of the low-and high-energy response is achieved within our semiclassical model. The inclusion of surface vibrations leads to the low-energy modes. The low-lying quadrupole mode is strongly affected by the surface tension but its frequency is still finite if, in the absence of Coulomb repulsion, the surface tension is set to zero. The frequency of the low-lying octupole mode, instead, goes to zero for vanishing surface tension, which would correspond to shape instability with respect to octupole deformation. Closed classical trajectories of triangular shape play an essential role in the damping of the low-lying octupole mode. Our octupole response displays also a novel resonance-like structure between the low-energy octupole resonance and the high-energy one (at around 13 MeV for a system with A=208 nucleons) that might have a toroidal character like the low-energy isoscalar dipole resonance.

Compressional-mode giant resonances in deformed nuclei

Background-free inelastic scattering spectra have been obtained for the Sm isotopes with 400 MeV α particles at forward angles (including 0°) to investigate the effect of deformation on the compressional-mode giant resonances. The strength distributions for the isoscalar giant resonances (L⩽3) have been extracted for the spherical nucleus 144Sm and the deformed nucleus 154Sm. We have observed that the effects of deformation are different for the low- and high-excitation-energy components of the isoscalar giant dipole resonance in 154Sm. Evidence for the theoretically predicted coupling between the isoscalar dipole resonance and the high-energy octupole resonance is reported.

Dipole torus mode in nuclear fluid-dynamics

The nuclear fluid-dynamics is applied to the study of a dipole electric response related to the transverse poloidal oscillations of a particle flow. This response is associated with excitation in the nucleus volume of the spherical Hill vortex-torus-like current structure harmonically oscillating in time; the nucleus shape remains spherical. The energy evaluated within the distorted Fermi-surface model is predicted to be E(1 torus −) = 65–85A −1 3 MeV , i.e. the torus excitation is expected to be identified experimentally as 2khω isoscalar dipole resonance. An analytic form is derived for the electric form factor and numerical calculations of the transition current density are presented. The contribution to the cross section of photoabsorption computed in the PWBA is estimated to be σ γ(1 torus −)= 10 −3−10 −4Z 2 A MeV·fm 2 . Some hints for further experiments are suggested.

The low-energy isoscalar dipole resonance in spherical nuclei

It is shown that localization of the low-energy isoscalar 1−strength observed in a recent Groningen experiment may be caused by incompressible flow vibrations with considerable contribution of a solenoidal component. Calculations performed here without free parameters predict the energy of the isoscalar low-energy dipole resonance of the order of 40A−1/3,Me V. This estimate agrees well with data obtained on90Zrand208Pb.

Exotic isoscalar dipole resonances in the Walecka model.

We have investigated possible exotic dipole resonances for finite nuclei in the Walecka model by performing a small amplitude time-dependent mean-field calculation with an ansatz based on a few collective degrees of freedom. We find that the frequencies of such resonances are several hundred MeV and that the resonances are dominated by the mesonic degrees of freedom.

Isoscalar dipole resonance: Form factor and energy weighted sum rule

A collective formalism first proposed by Deal to describe isoscalar dipole excitations in electron scattering is extended to include excitations of these modes by hadron scattering. Distorted-wave Born approximation calculations are performed using this collective isoscalar $L=1$ form factor to describe the excitation of the ${1}^{\ensuremath{-}}$, $T=0$ state at ${E}_{x}=7.12$ MeV ($1\ensuremath{\hbar}\ensuremath{\omega}$ character) in $^{16}\mathrm{O}$ and a newly proposed candidate for the isoscalar giant dipole resonance ($3\ensuremath{\hbar}\ensuremath{\omega}$ character) at ${E}_{x}=21.3$ MeV in $^{208}\mathrm{Pb}$ excited in inelastic $\ensuremath{\alpha}$ scattering. A good description of the cross sections of the ${1}^{\ensuremath{-}}$, 7.12 MeV state in $^{16}\mathrm{O}$ and the ${E}_{x}=21.3$ MeV bump in $^{208}\mathrm{Pb}$ is obtained in this model exhausting 4.2% and 130% of the energy-weighted sum rule, respectively.NUCLEAR STRUCTURE Isoscalar dipole resonance; derived form factor, energy weighted sum rule. Excitation in hadron scattering.