Dipole Resonance In 208Pb Research Articles

Constraining isovector nuclear interactions with giant resonances within a Bayesian approach

We put a stringent constraint on the isovector nuclear interactions in the Skyrme-Hartree-Fock model from the centroid energy E−1 of the isovector giant dipole resonance in 208Pb as well as its electric polarizability αD. Using the Bayesian analysis method, E−1 and αD are found to be mostly determined by the nuclear symmetry energy Esym at about ρ⋆=0.05 fm−3 and the isovector nucleon effective mass mv⋆ at the saturation density. At 90% confidence level, we obtain Esym(ρ⋆)=16.4−0.9+1.0 MeV and mv⋆/m=0.79−0.06+0.06.

Fine Structure and Collectivity of the Levels of the Pygmy Dipole Resonance in 208Pb in a Self-Consistent Model

A novel fully self-consistent microscopic approach based on the energy density functional method is employed to calculate the fine structure of the pygmy dipole resonance in 208Pb, i.e., the energies and reduced probabilities of E1 transitions for the states with energies below 10 MeV. The approach includes the random-phase approximation, quasiparticle–phonon interaction and the single-particle continuum. The theoretical results are compared to the available high-resolution data and found to agree with measured integral characteristics of the pygmy dipole resonance at energies above 5.7 MeV. Residual spin–spin forces are quantified, and their contribution is found to be significant at both low and high energies. A recently proposed criterion is employed to analyze the collectivity of the 1–states in 208Pb.

Radiative strength function and the pygmy dipole resonance in 208Pb and 70Ni

The pygmy-resonance parameters and the E1 strength function are derived for 208Pb using a fully self-consistent microscopic formalism recently developed for magic nuclei, which takes into account quasiparticle phonon interactions (or coupling to phonons) in addition to the random phase approximation. For the radiative strength function of 208Pb at energies above 5 MeV, the experimental data of the Oslo group are adequately described by our predictions, whereby the important role of coupling to phonons is confirmed. By comparing the measurements based on the (3He, 3He′γ) and (γ, γ′) reactions, we discuss the physical properties of the radiative strength function measured for 208Pb. For the neutron-rich 70Ni nucleus, predictions for the radiative strength function and the pygmy resonance are obtained using a partially self-consistent approach, which invokes the Skyrme forces in deriving the mean field, effective nucleon–nucleon interaction, and phonon characteristics.

Two-phonon giant dipole resonance in 208Pb

Excitation of the two-phonon isovector giant dipole resonance was observed in 208Pb projectiles incident on different target nuclei (C, Sn, Ho, Pb, U) at high energy (640 A·MeV). Evidence is found for a two-step electromagnetic excitation mechanism. An enhancement of 1.33(16) for the excitation cross section is observed relative to the harmonic approximation. Neutron- and γ-decay probabilities are derived as well.

The “other” giant dipole resonance: Evidence for the isoscalar giant dipole resonance in 208Pb from inelastic α scattering at 0°

The isoscalar giant dipole resonance (ISGDR) in 208Pb has been investigated using inelastic scattering of 200 MeV α particles at and near 0° where the angular distribution of the ISGDR can be clearly differentiated from other modes. The “difference of spectra” technique was employed to separate the contribution from the high-energy octupole resonance (HEOR). Preliminary angular distribution data provide clear evidence for the ISGDR adjacent to the HEOR.

Semiclassical estimate of the Coulomb excitation of the giant dipole resonance in 208Pb.

A semiclassical approach to study pure Coulomb excitation of [sup 208]Pb giant dipole isovector resonance is examined. We consider medium energy projectiles and assume the target excitation to be described by a simple Goldhaber-Teller model. It is shown that the main features concerning the angular distribution are obtained in the angular range described by the model and an estimate is made of the pure Coulomb dipole contribution to the measured cross sections.

Giant dipole resonance in 208Pb within the approach including 1p1h⊗ phonon configurations and continuum

Microscopic calculations of the giant E1 resonance in 208Pb have been performed in the 1p1h+2p2h+continuum approximation. A reasonable agreement with experiment, including the description of the resonance width, has been obtained.

Differences between dynamical theories of giant resonances

It is the objective of dynamical theories of collective excitations to describe the influence of particle-vibration coupling on the excitation energies of giant resonances. This yields dynamical corrections to the energies calculated in the random-phase approximation (RPA). The existing dynamical theories can be characterized by the effective particle-hole gap which they prescribe for RPA-type calculations of collective excitations. We investigate three dynamical theories in the framework of a schematic model for the nucleon self-energy. In the case of the giant dipole resonance in 208Pb, the microscopic dynamical model prescribes an effective p-h gap which is smaller than the experimental value; in contradistinction, the effective p-h gap is larger than the experimental value in the case of the isoscalar octupole surface vibration. These dynamical corrections are opposite to the corrections predicted by two other models which have been proposed. The origin of these differences is discussed.