N. Van Giai

On dipole compression modes in nuclei

Abstract Isoscalar dipole strength distributions in spherical medium- and heavy-mass nuclei are calculated within random phase approximation (RPA) or quasiparticle RPA. Different Skyrme-type interactions corresponding to incompressibilities in the range 200–250 MeV are used. The results are discussed in comparison with existing data on isoscalar giant dipole resonances. Two main issues are raised, firstly the calculated giant resonance energies are somewhat higher than the observed ones, and secondly a sizable fraction of strength is predicted below 20 MeV which needs to be experimentally confirmed.

High-lying two-phonon states in 40Ca

Abstract We have calculated the anharmonicities of the two-phonon spectrum of 40 Ca by using a boson expansion approach. The one-phonon states were constructed within the random phase approximation. The deviations from harmonicity are found to be very small, both for the low-lying phonons and giant resonances.

Folding model analysis of elastic and inelastic proton scattering on sulfur isotopes

Abstract The folding formalism for the nucleon–nucleus optical potential and inelastic form factor is applied to study elastic and inelastic proton scattering on 30–40S isotopes. A recently developed realistic density dependent M3Y interaction, well tested in the folding analysis of nucleus–nucleus elastic and inelastic scattering, is used as effective NN interaction. The nuclear ground state and transition densities (for the 2+ excitations in sulfur isotopes) are obtained in the Hartree–Fock-BCS and QRPA approaches, respectively. The best fit ratios of transition moments Mn2+/Mp2+ for the lowest 2+ states in sulfur isotopes are compared to those obtained earlier in the DWBA analysis of the same data using the same structure model and inelastic form factors obtained with the JLM effective interaction. Our folding + DWBA analysis has shown quite a strong isovector mixing in the elastic and inelastic scattering channels for the neutron rich 38,40S nuclei. In particular, the relative strength of the isovector part of the transition potential required by the inelastic p + 38 S data is significantly stronger than that obtained with the corresponding QRPA transition density.

Electromagnetic decay of two-phonon states

Abstract We study the electromagnetic decay of two-photon states corresponding to the multi-excitation of giant resonances. The calculations are performed within a boson expansion approach and the elementary modes are constructed in RPA. The rates for direct transition of two-phonon states to the ground states turn out to be not negligibly smaller than those from the (single) giant resonances. The former transitions are accompanied by a γ-ray whose energy is equal to the sum of the two-phonon energies. Thus the detection of such high energy γ-rays could provide a signature of the excitation of two-phonon states.

Coulomb displacement energies in relativistic and non-relativistic self-consistent models

Abstract Coulomb displacement energies in mirror nuclei are comparatively analyzed in Dirac-Hartree and Skyrme-Hartree-Fock models. Using a non-linear effective lagrangian fitted on ground-state properties of finite nuclei, it is found that the predictions of relativistic models are lower than those of Hartree-Fock calculations with a Skyrme force. The main sources of reduction are the kinetic energy and the Coulomb-nuclear interference potential. The discrepancy with the data is larger than in the Skyrme-Hartree-Fock case.

Comment on the width and the lifetime of multiphonon states

Abstract In this comment we discuss the relation between the width of a multiphonon state and the width of the single phonon. While it is clear that the inverse of the multiphonon lifetime 1 τ is an additive function of the inverse of the phonon lifetimes 1 τ i , it is pointed out that this is not necessarily true for the width of the multiphonon response function, and that the phonon widths can be quadratically additive.

From experimental monopole cross-sections to nuclear incompressibilities

Abstract In this letter we test carefully the procedure used to extract strength functions from experimental data. This test is performed on a set of pseudo-data generated from HF and RPA results for monopole resonances in 40Ca, 60Ni, 90Zr and 208Pb. We show how the scaling approximation used in the experimental analysis may induce systematic errors. We demonstrate, however, that whereas the discrepancy on the high energy component (4ħω) can be large, the distortion in the giant monopole resonance region (2ħω) is of the order of (10–30)% only. Moreover, we observe that when ratios of moments are considered, the errors partially compensate one another. From this we conclude that the systematic errors on the form factors have little influence on the extracted scaling energy from which nuclear incompressibilities are deduced.

Electronic properties of large metal clusters in jellium and pseudo-jellium models

The energy-density functional approach and jellium-like models are used to examine two important electronic properties of metal (Li, Na, K) clusters: their shell and supershell structures, and the behaviour of plasmon energies with increasing cluster sizes. A comparative study is made between predictions of the usual jellium model and those of the pseudo-jellium model where pseudohamiltonians are used.

Excited states of neutron-rich nuclei: mean field theory and beyond

Abstract Mean field calculations of the isovector dipole and low-lying 2 + , 3 − excitations, performed within the Skyrme-RPA and QRPA framework, are discussed. Examples are chosen among the neutron-rich nuclei of interest for present nuclear structure research. The relevance of the correlations beyond mean field, in particular of the contribution of the coupling with surface vibrations, is pointed out. We stress the importance of unifying the microscopic nuclear structure calculations and reaction models for a direct comparison of the theoretical predictions with the experimental data. In this context we show the effectiveness of the folding model, which can naturally accommodate the RPA transition densities.

Neutrino mean free path and in-medium nuclear interaction

Abstract Neutrinos produced during the collapse of a massive star are trapped in a nuclear medium (the proto-neutron star). Typically, neutrino energies (10–100 MeV) are of the order of nuclear giant resonances energies. Hence, neutrino propagation is modified by the possibility of coherent scattering on nucleons. We have compared the predictions of different nuclear interaction models. It turns out that their main discrepancies are related to the density dependence of the k-effective mass as well as to the onset of instabilities as density increases. This last point had led us to a systematic study of instabilities of infinite matter with effective Skyrme-type interactions. We have shown that for such interactions there is always a critical density, above which the system becomes unstable.