Collective transition densities in neutron-rich nuclei

Abstract

Quadrupole transition densities in neutron-rich nuclei in the vicinity of the neutron drip-line are calculated in the framework of the Random Phase Approximation. The continuum is treated by expansion in oscillator functions. We focus on the states which contribute to the usual Giant Quadrupole Resonance, and not on the lowlying strength which is also expected in such nuclei and whose collective character is still under debate. We find that, due to the large neutron skin in these nuclei, the isoscalar and isovector modes are in general strongly mixed. We further show that the transition densities corresponding to the GQR states can be reasonably well described by the collective model in terms of in phase and out of phase oscillations of neutron and proton densities which have different radii.During recent years, in view of the prospects of new dedicated radioactive beam facilities, considerable interest has been focussed to the study of the effect of neutron skin on the collective properties of neutron-rich nuclei [1]. Largely the interest has been directed towards the possible occurrence of sufficient multipole strength at low excitation energies in such nuclei. It is now excepted that this feature is primarily a consequence of the extremely weak binding of the neutrons close to the Fermi surface, whereas the collective or single-particle nature of these excitations is still unclear [2]. It is also of interest to study neutron-rich nuclei which are still far from the neutron drip-line, so that the last nucleons are not so weakly bound, and ask what the effect of the neutron skin is on the traditional collective oscillations of these systems. In view of the presence of the neutron skin one would expect an enhanced mixing of the isoscalar and isovector modes.To investigate this point, we have performed microscopic calculations for nuclei 28 O and 60 Ca, based on spherical Hartree-Fock model with Skyrme SGII interaction. These interaction predicts the last neutron to be bound by 3.25

Multiphonon excitations and pygmy resonances in tin isotopes

“…8, where the proton, neutron, isoscalar, and isovector transition densities for the two states, calculated with the SLY4 interaction, are reported. The behavior for the low-lying state is similar to what has been found by several authors [3][4][5]17]. In the surface region, the proton and neutron densities are not out of phase, and the isoscalar transition density dominates over the isovector one; in the external region, only the neutrons give a contribution to both isoscalar and isovector transition densities which have the same magnitude.…”

“…While the dipole ones are almost purely isovector, states of higher multipolarities show a rather strong isospin mixing, especially for the isotopes with a thicker neutron skin. This effect is not new and has been extensively discussed in the literature [4,17]. Such behavior is similar for the two used interactions.…”

“…A clear connection of the low energy B(E1) strength and the thickness of the neutron skin has been shown and its effect on the transition densities amply discussed in Refs. [4,5,17,18]. Below we will present, for 132 Sn, a comparison between the transition density associated with the low-lying dipole state and that of the giant dipole resonance (GDR).…”

Excitations of pygmy dipole resonances in exotic and stable nuclei via Coulomb and nuclear fields

“…Previous calculations [2] have shown that as soon as the neutron number increases, some strength appears at low energies in the dipole strength distribution, well below the dipole giant resonance. This strength, carrying few percent of the isovector EWSR, is present in many isotopes and has been often associated to the possible existence of a new collective mode of new nature: the pygmy dipole resonance (PDR).…”

“…This is mainly due to isoscalar part and to the fact that the isoscalar dipole transition density displays nodes [2,31]. The interference is less pronounced in the 48 Ca case because of the presence of the isospin dependent part of the nuclear form factor that 064602-4 132 Sn + (α, 40 Ca, 48 Ca) as function of the energies.…”