Nature of isoscalar dipole resonances in heavy nuclei

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.