E.G. Lanza

Effect of large neutron excess on the dipole response in the region of the giant dipole resonance

Abstract The evolution of the dipole response in nuclei with strong neutron excess is studied in the Hartree-Fock plus random phase approximation with Skyrme forces. We find that the neutron excess increases the fragmentation of the isovector giant dipole resonance, while pushing the centroid of the distribution to lower energies beyond the mass dependence predicted by the collective models. The radial separation of proton and neutron densities associated with a large neutron excess leads to non-vanishing isoscalar transition densities to the GDR states, which are therefore predicted to be excited also by isoscalar nuclear probes. The evolution of the isoscalar compression dipole mode as a function of the neutron excess is finally studied. We find that the large neutron excess leads to a strong concentration of the strength associated with the isoscalar dipole operator ∑ i r i 3 Y 10 , that mainly originates from uncorrelated excitations of the neutrons of the skin.

Role of anharmonicities and nonlinearities in heavy ion collisions A microscopic approach

Abstract Using a microscopic approach beyond RPA to treat anharmonicities, we mix two-phonon states among themselves and with one-phonon states. We also introduce nonlinear terms in the external field. These nonlinear terms and the anharmonicities are not taken into account in the “standard” multiphonon picture. Within this framework we calculate Coulomb excitation of 208Pb and 40Ca by a 208Pb nucleus at 641 and 1000 MeV/A. We show with different examples the importance of the nonlinearities and anharmonicities for the excitation cross section. We find an increase of 10% for 208Pb and 20% for 40Ca of the excitation cross section corresponding to the energy region of the double giant dipole resonance with respect to the “standard” calculation. We also find important effects in the low-energy region. The predicted cross section in the DGDR region is found to be rather close to the experimental observation.

Anharmonicities and non-linearities in the excitation of double giant resonances

Abstract We investigate the non-linear response of a quantum anharmonic oscillator as a model for the excitation of giant resonances in heavy ion collisions. We show that the introduction of small anharmonicities and non-linearities can double the predicted cross section for the excitation of the two-phonon states. These findings suggest that such ingredients must be included in future more complete calculations in order to reduce the huge discrepancy between the previous theoretical predictions and the experimental cross section of double giant resonance states.

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 low-lying 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.

Nuclear excitations as coupled one and two random-phase-approximation modes

We present an extension of the random--phase approximation (RPA) where the RPA phonons are used as building blocks to construct the excited states. In our model, that we call double RPA (DRPA), we include up to two RPA phonons. This is an approximate and simplified way, with respect to the full second random--phase approximation (SRPA), to extend the RPA by including two particle--two hole configurations. Some limitations of the standard SRPA model, related to the violation of the stability condition, are not encountered in the DRPA. We also verify in this work that the energy--weighted sum rules are satisfied. The DRPA is applied to low--energy modes and giant resonances in the nucleus

Chaos in heavy-ion dynamics at low energy

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Microscopic calculations of double and triple giant resonance excitations in heavy ion collisions

O. We show that the model (i) produces a global downwards shift of the energies with respect to the RPA spectra; (ii) provides a shift that is however strongly reduced compared to that generated by the standard SRPA. This model represents an alternative way of correcting for the SRPA anomalous energy shift, compared to a recently developed extension of the SRPA, where a subtraction procedure is applied. The DRPA provides results in good agreeement with the experimental energies, with the exception of those low--lying states that have a dominant two particle--two hole nature. For describing such states, higher--order calculations are needed.

Microscopic description of multiphonon states excitation in heavy ion collisions

Abstract We discuss the quantum analog of classical chaotic scattering in the case of a collision between a deformed and a spherical nucleus. In agreement with previous investigations on model potentials, a firm correspondence between classical and quantal scattering is established; when wild oscillations in the classical deflection function are predicted, sharp irregular quantal fluctuations in the excitation functions and angular distributions appear. This behaviour occurs around the centrifugal barrier and for deformed light nuclei. Both in classical and quantum dynamics coexistence of regular and irregular motion is obtained. Chaotic scattering could be the natural explanation of those fluctuations observed in the experimental cross sections as a function of energy and scattering angle for light heavy-ion systems.

Are giant resonances harmonic vibrations

We perform microscopic calculations of the inelastic cross sections for the double and triple excitation of giant resonances induced by heavy-ion probes within a semiclassical coupled-channels formalism. The channels are defined as eigenstates of a bosonic quartic Hamiltonian constructed in terms of collective random-phase approximation phonons. Therefore, they are superpositions of several multiphonon states, also with different numbers of phonons, and the spectrum is anharmonic. The inclusion of (n+1) phonon configurations affects the states whose main component is a n-phonon one and leads to an appreacible lowering of their energies. We check the effects of such further anharmonicities on the previously published results for the cross section for the double excitation of giant resonances (GR). We find that the only effect is a shift of the peaks toward lower energies, the double GR cross section being unmodified by the explicity inclusion of the three-phonon channels in the dynamical calculations. The latter provide an important contribution to the cross section in the triple GR energy region, which, however, is still smaller than the experimental available data. The inclusion of four-phonon configurations in the structure calculations does not modify the results.

Importance of Giant-Resonance Excitation for the Surface Properties of the Heavy-Ion Optical Potential

Starting from a microscopic approach based on RPA, we go beyond it by introducing anharmonicity by taking into account the mixing of two-phonon states among themselves and with one-phonon states. By introducing also non linear terms in the external field we show that these new ingredients give an explanation of the puzzle of the excitation cross section of the double giant dipole resonance (DGDR) for the 208 Pb + 208 Pb system at 641 MeV/A.