L. F. Canto

Disentangling static and dynamic effects of low breakup threshold in fusion reactions

A new technique to analyze fusion data is developed. From experimental cross sections and results of coupled-channel calculations a dimensionless function is constructed. In collisions of strongly bound nuclei this quantity is very close to a universal function of a variable related to the collision energy, whereas for weakly bound projectiles the effects of breakup coupling are measured by the deviations with respect to this universal function. This technique is applied to collisions of stable and unstable weakly bound isotopes.

Theory of multiphonon excitation in heavy-ion collisions

We study the effects of channel coupling in the excitation dynamics of giant resonances in relativistic heavy ions collisions. For this purpose, we use a semiclassical approximation to the coupled-channels problem and separate the Coulomb and the nuclear parts of the coupling into their main multipole components. In order to assess the importance of multistep processes, we neglect the resonance widths and solve the set of coupled equations exactly. Finite widths are then considered. In this case, we handle the coupling of the ground state with the dominant giant dipole resonance exactly and study the excitation of the remaining resonances within the coupled-channels Born approximation. A comparison with recent experimental data is made. Relativistic Coulomb excitation ~RCE! is a well established tool to unravel interesting aspects of nuclear structure @1#. Examples are the studies of multiphonon resonances in the SIS accelerator at the GSI facility, in Darmstadt, Germany @2,3#. Important properties of nuclei far from stability @4# have also been studied with this method. The RCE induced by large-Z projectiles and/or targets, often yields large cross sections in grazing colisions. This results from the large nuclear response ~in the region of the giant resonances! to the acting electromagnetic fields. As a consequence, a strong coupling between the excited states is expected This coupling might be responsible for the large discrepancies between experimental data of RCE and the calculations based on first-order perturbation theory @1‐3#, or the harmonic oscillator model. In the present paper, we apply a semiclassical method @5# to the coupled-channels ~CC! problem and study RCE in several collisions between heavy ions. In this method, the projectile-target relative motion is approximated by a classical trajectory and the excitation of the giant resonances is treated quantum mechanically @6,7#. The use of this method is justified due to the small wavelengths associated with the relative motion. In Sec. II, we neglect the resonance widths and introduce the semiclassical CC equations for relativistic Coulomb excitation. The time-dependent matrix elements of the main multipole components of the Coulomb ~Sec. II A! and nuclear ~Sec. II B! parts of the coupling interaction are calculated. The CC equations are then solved in some limiting cases. Section III is devoted to the excitation of resonances of finite widths. Generalizing the schematic treatment of Ref. @6#, we present an ‘‘exact’’ solution for the coupling between the ground state ~g.s.! and the dominant GDR. The excitation of the weaker resonances are then evaluated through the coupled-channels Born approximation ~CCBA!, from the g.s. and GDR amplitudes. In Sec. IV we apply the results of the previous sections to specific cases and make a comparison with recent experimental data. Finally, in Sec. V, we summarize our results and present the conclusions of this work.

Continuum-continuum coupling and polarization potentials for weakly bound systems

We investigate the influence of couplings among continuum states in collisions of weakly bound nuclei. For this purpose, we compare cross sections for complete fusion, breakup, and elastic scattering evaluated by continuum discretized coupled channel (CDCC) calculations, including and not including these couplings. In our study, we discuss this influence in terms of the polarization potentials that reproduces the elastic wave function of the coupled channel method in single channel calculations. We find that the inclusion of couplings among continuum states renders the real part of the polarization potential more repulsive, whereas it leads to weaker absorption to the breakup channel. We show that the noninclusion of continuum-continuum couplings in CDCC calculations may lead to qualitative and quantitative wrong conclusions.

A coupled-channels study of 11Be Coulomb excitation

Abstract We study the effects of channel coupling in the excitation of 11 Be projectiles incident on heavy targets. The contribution to the excitation from the Coulomb and the nuclear fields in peripheral collisions are considered. Our results are compared with recent data on the excitation of the 1 2 − state in 11 Be projectiles. We show that the experimental results cannot be explained, unless very unusual parameters are used.

Eikonal form of the dynamic polarization potential and its application to the scattering of exotic nuclei

Abstract The eikonal theory of the dynamic polarization potential (DDP) is developed. Application to the scattering of loosely bound exotic nuclei is made. In particular, the effect of our DPP on the scattering of 11 Li + 12 C at 85 A · MeV is discussed.

Liquid-drop model description of heavy ion fusion at sub-barrier energies

Abstract The enhancement of the heavy ion fusion cross section at sub-barrier energies is studied in the liquid-drop model approach. The shape of the system is described by two spheres smoothly connected by a neck, and the kinetic and potential energies are calculated within this parametrization. Underbarrier fusion cross sections for symmetric projectile-target combinations are calculated in the WKB approximation and a comparison with the available data is made. The agreement is quite satisfactory, except for those systems in which the reaction is strongly affected by the details of the nuclear structure of the collision partners.

Dipole polarizability of neutron-rich nuclei☆

Abstract We calculate the dynamic polarization potential for neutron-rich nuclei arising from the Coulomb coupling of the elastic channel to: (i) the break-up channel; (ii) the low-lying dipole mode. These potentials are found to be different only by a numerical factor. The effect of these polarization potentials on the elastic scattering of 11 Li + 208 Pb at E CM = 50 MeV is calculated.

The nuclear two-neutron removal cross-section of 11Li: A dynamic polarization potential approach

Abstract We determine the dynamic polarization potential associated to the two-neutron removal process from 11 Li. This potential is then employed to calculate the nuclear break-up cross section of 11 Li when it collides with different targets, as a function of its bombarding energy. An analytic expression for this cross section is obtained and shown to be in good agreement with the calculation based on the exact expression. We also discuss the relevance of this potential in the study of the fusion of 11 Li with a heavy target.

On the possibility of observing experimentally diabolic pair transfer in rotating nuclei

Abstract We investigate the cross section for two-nucleon transfer reactions to rotational states by heavy projectiles. In particular we study the influence of the recently predicted unexpected behavior of the diabolic pair transfer amplitude. Dramatic reduction of the probabilities for two-nucleon transfer in connection with inelastic Coulomb excitation are found in the angular momentum region, where the pair transfer matrix element changes its sign.

A nuclear squid: Diabolic pair transfer in rotating nuclei☆

Abstract A new unexpected behavior of pair transfer matrix elements in superfluid rotating nuclei is predicted. With increasing angular velocity they drop to zero, change their sign and in some cases even oscillate between positive and negative values. This effect is related to diabolical points in rotating quasiparticle spectra and is closely analogous to the DC-Josephson effect in superconductors in the presence of a magnetic field.