Pygmy dipole resonances in the relativistic random phase approximation

Abstract

The isovector dipole response in 208 Pb is described in the framework of a fully self-consistent relativistic random phase approximation. The NL3 parameter set for the effective mean-field Lagrangian with nonlinear meson self-interaction terms, used in the present calculations, reproduces ground state properties as well as the excitation energies of giant resonances in nuclei. In addition to the isovector dipole resonance in 208 Pb, the present analysis predicts the occurrence of low-lying E1 peaks in the energy region between 7 and 11 MeV. In particular, a collective state has been identified whose dynamics correspond to that of a dipole pygmy resonance: the vibration of the excess neutrons against the inert core composed of equal number of protons and neutrons. 1The multipole response of nuclei with large neutron excess has been the subject of many theoretical studies in recent years. Exotic nuclei with extreme neutron to proton ratio exhibit many interesting and unique structure phenomena. In addition to exotic ground state properties, the onset of lowenergy collective isovector modes has been predicted and experimental evidence for these modes in light nuclei has been reported [1]. Catara et al. have studied the low-lying components in strength distributions of weakly bound neutron-rich nuclei [2], and the effect of large neutron excess on the dipole response in the region of the giant dipole resonance in O and Ca isotopes [3]. They have shown that the neutron excess increases the fragmentation of the isovector giant dipole resonance (GDR) and that the radial separation of proton and neutron densities leads to non-vanishing isoscalar transition densities to the GDR states. The fragmentation of the isoscalar and isovector monopole strength in neutron rich Ca isotopes has been studied in Ref. [4], and in Ref.[5] the onset of pygmy dipole resonances in Ca isotopes has been analyzed.The pygmy dipole resonance, which is also the subject of the present study, results from the excess neutrons oscillating out of phase with a core composed of equal number of protons and neutrons. A number of theoretical models have been applied in studies of the dynamics of pygmy dipole resonances. These include: the three-fluid hydrodynamical model (the protons, the neutrons of the same orbitals as protons, and the excess neutrons) [6], the two-fluid (the core fluid and the neutron excess fluid) Steinwedel-Jensen hydrodynamical model [7], density functional theory [5], and the Hartree-Fock plus random phase approximation (RPA) with Skyrme forces [3,8]. More recently, large scale shell model calculations have been performed in studies of pygmy and dipole states in O isotopes [9], and dipole and spin-dipole strength distributions in 11 Li [10]. There is also experimental evidence for possible pygmy dipole states in 208 Pb. Studies of the low energy spectrum by elastic photon scattering [11], photoneutron [12], and electron scattering [13] have detected fragmented E1 strength in the energy region between 9 and 11 MeV. The fine...

Electric dipole response of neutron-rich calcium isotopes in relativistic quasiparticle time blocking approximation

“…Instead, each of them is fragmented and broadened due to Landau damping (occurring already on the R(Q)RPA level), damping to complex configurations (essentially taken into account in R(Q)TBA) and particle escape to continuum [54]. However, theoretical methods have a very convenient tool for an approximate separation of these two excitation modes, namely the proton and neutron transitions densities, which reveal a specific pattern characterizing neutron skin oscillation [82]: inphase oscillation of proton and neutron transition densities inside the nucleus with a visible dominance (correlated with the neutron excess) of the neutron component in the surface area. In contrast, GDR shows out of phase proton and neutron density oscillations.…”

Pygmy dipole resonance in208Pb

“…Another, more general problem is the experimental separation of the PDR from the GDR. While theoretical transition densities provide a signature in the model calculations [20], the experimentally determined reduced B(E1) strengths do not allow such a distinction. A possible experimental approach to the distinction of PDR and GDR is the use of isoscalar probes, and a pioneering (α, α ′ γ) experiment has been performed [21] demonstrating a large exhaustion of the isoscalar energy-weighted sum rule by lowenergy E1 transitions in 208 Pb.…”

“…In the models the properties of the PDR are very sensitive to the mean-field description and predicted strength might thus be partially shifted to higher excitation energies. One should rather analyze the theoretical transition densities [20,39] to select those with a dominant PDR character.…”

Neutrino-“pasta” scattering: The opacity of nonuniform neutron-rich matter

“…2) at subnuclear densities of the order of 10 13 g/cm 3 , the pasta resembles a collection of spherical neutron-rich nuclei embedded in a dilute neutron gas. Neutron-rich nuclei with large neutron skins have Pygmy giant resonances, involving collective oscillations of the neutron skin against the symmetric core [22,23]. We expect that the soft neutronrich pasta will have many low-energy collective oscillations.…”