M.V. Andrés

Long range absorption in the scattering of 6He on 208Pb and 197Au at 27 MeV

Quasi-elastic scattering of 6He at E_lab=27 MeV from 197Au has been measured in the angular range of 6-72 degrees in the laboratory system employing LEDA and LAMP detection systems. These data, along with previously analysed data of 6He + 208Pb at the same energy, are analyzed using Optical Model calculations. The role of Coulomb dipole polarizability has been investigated. Large imaginary diffuseness parameters are required to fit the data. This result is an evidence for long range absorption mechanisms in 6He induced reactions. Comment: 10 pages, 10 figures, minor corrections. To appear in Nucl. Phys. A

Dynamic polarization potential induced by dipole Coulomb excitation

Abstract A local L -independent polarization potential due to dipole Coulomb excitation is derived with the use of the semiclassical Coulomb excitation amplitudes. It is showed that an analytic form for the imaginary polarization potential can be obtained for arbitrary dipole excitation energy while the real polarization potential is related to the imaginary part through a dispersion relation. The relative importance of the real and imaginary parts of the polarization potential is illustrated for the case of the collision of 11 Li and 208 Pb at different projectile energies.

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.

α-particle production in the scattering of 6He by 208Pb at energies around the Coulomb barrier

New experimental data from the scattering of 6He + 208Pb at energies around and below the Coulomb barrier are presented. The yield of breakup products coming from projectile fragmentation is dominated by a strong group of α particles. The energy and angular distribution of this group have been analyzed and compared with theoretical calculations. This analysis indicates that the α particles emitted at backward angles in this reaction are mainly due to two-neutron transfer to weakly bound states of the final nucleus.

Dynamic polarization potential induced by dipole Coulomb excitation to break-up states in 11Li scattering

Abstract The polarization potential due to dipole Coulomb excitation to the continuum of break-up states is obtained. The real and imaginary parts of the polarization potential is evaluated for the collision of 11 Li and 208 Pb at different projectile energies. The potentials show a sharp energy dependence, characteristic of the threshold anomaly, at energies below 100 MeV. Scattering calculations show the importance of the polarization potential.

Effect of deformation on the elastic and quasielastic scattering of heavy ions near the Coulomb barrier

Abstract It is shown that elastic and quasielastic cross sections at large angles provide the same information on static deformation properties as does sub-barrier fusion. The results have their “classical” form even for coupling to a small number of excited states.

Microscopic description of Coulomb and nuclear excitation of multiphonon states in heavy ion collisions

Abstract We calculate the inelastic scattering cross sections to one- and two-phonon states in heavy ion collisions. Both Coulomb and nuclear excitations are included and their interplay is studied. Starting from a microscopic approach based on RPA, we go beyond it in order to treat anharmonicities and take explicitly into account the mixing of two-phonon states among themselves and with one-phonon states. Nonlinear terms in the exciting field are also introduced. These anharmonicities and nonlinearities are shown to have important effects on the cross sections both in the low energy part of the spectrum and in the energy region of the Double Giant Dipole Resonance.

Accidental degeneracy in a simple quantum system: A new symmetry group for a particle in an impenetrable square-well potential

The two-dimensional square-well potential is one of the simplest quantum-mechanical systems that exhibits accidental degeneracy. We show that the double degeneracy present is a consequence of a dynamical symmetry and derive a new symmetry group associated with the system.

Boson expansion methods applied to a two-level model in the study of multiple giant resonances

Abstract We apply boson expansion methods to an extended exactly solvable Lipkin-Meshkov-Glick model including anharmonicities in analogy with previous microscopic calculations. We study the effects of different approximations present in these calculations, among which the truncation of the Hamiltonian and of the space, in connection with the study of the properties of two-phonon states. By comparing the approximate results on the spectrum with the exact ones we conclude that the approximations made in the microscopic calculations on two-phonon states are well justified. We find also that a good agreement with the exact results for the three-phonon state is obtained by using a bosonic Hamiltonian truncated at the fourth order. This result makes us confident that such approximation can be used in future realistic calculations, thus allowing a theoretical study of triple excitations of giant resonances.