J. Lubian

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.

New manifestation of the dispersion relation : Breakup threshold anomaly

It is pointed out that the usual threshold anomaly, found operative in the energy behavior of the imaginary and real parts of the optical potential representing the elastic scattering of tightly bound nuclei at near- and below-barrier energies, suffers a drastic qualitative change in the case of the elastic scattering of weakly bound nuclei. Owing to the strong coupling to the breakup channel even at sub-barrier energies, the imaginary potential strength seems to increase as the energy is lowered to below the natural barrier threshold; this is accompanied by a decrease in the real potential strength. This feature is consistent with the dispersion relation. The system {sup 6}Li+{sup 208}Pb is analyzed to illustrate this new phenomenon.

No evidence of break-up effects on the fusion of 9Be with medium-light nuclei

Abstract Fusion cross sections were measured for the 9 Be + 27 Al and 19 F + 9 Be , 12C systems, at energies above the Coulomb barrier, in order to investigate the possible effect of fusion hindrance due to the break-up of the weakly bound nuclei. Comparisons with one-dimensional barrier penetration models and with other similar systems, where no break-up is expected to occur, show no evidence of fusion hindrance.

Fusion, break-up and elastic scattering of weakly bound nuclei

We describe the behaviour of the fusion, break-up, reaction cross sections and elastic scattering of weakly bound nuclei, at near and above barrier energies. The total fusion cross sections are not affected by the break-up process at this energy regime. The elastic break-up cross sections are important at energies close and above the Coulomb barrier, even in systems with light targets, and increase the reaction cross sections. We also show that the break-up process at near and sub-barrier energies is responsible for the vanishing of the usual threshold anomaly of the optical potential and gives rise to a new type of anomaly, named by us as break-up threshold anomaly.

Searching for a polarization potential in the breakup of 8B

Continuum couplings are extremely important to describe the breakup of loosely bound nuclei. We explore the various methods of extracting a polarization potential, as an effective representation of these couplings in the breakup channel. Our application consists of 8B→7Be+p on 58Ni at near-barrier energies. We find that the standard ways of determining polarization potentials are not useful to describe the breakup cross section in terms of a one-step process. Our results indicate that continuum couplings are strongly non-local and the complexity of coupled channel methods may well be unavoidable.

Elastic, inelastic scatterings and transfer reactions for 16,18O on 58Ni described by the São Paulo potential

We have used a parameter - free potential, obtained from data analysis at sub-barrier energies, to explain our measurements of elastic and several inelastic scattering cross sections for the 16,18O + 58Ni systems at an energy above the barrier. The data were analyzed through extensive coupled-channel calculations. Transfer cross sections could also be explained by the same interaction, which is consistent with the more fundamental Sao Paulo potential.

Complete fusion of weakly bound nuclei applying the delayed X-ray technique: the 9Be + 144Sm system

The detection of delayed X-rays produced by the decay that follows electron capture in the residual nuclei has been used in the past for the determination of fusion cross sections of tightly bound nuclei. In this work we applied this technique to study the effect of the break-up of a weakly bound projectile, like in the case of the 9Be + 144Sm system. Preliminary results of the complete fusion in this system at near barrier energies are presented.

Does the break-up process influence the fusion cross section?

We give an overall picture of our present understanding of the effect of the break-up of stable weakly bound nuclei on their fusion cross section with light, medium and heavy mass nuclei, at energies above the Coulomb barrier. The discussion is based mostly on recent data obtained by our group in collaborative experiments with ANU, USP and Tandar Laboratories. We conclude that there is complete fusion suppression for heavy targets, due to the loss of flux, corresponding to the occurrence of incomplete fusion of one of the break-up fragments. For medium and light mass targets, the incomplete fusion is negligible and therefore is no complete fusion suppression.

The Fusion of Stable Weakly Bound Nuclei

We discuss our present understanding of the effect of the break-up of stable weakly bound nuclei on the fusion cross section of these projectiles with light, medium and heavy mass targets, at energies above the Coulomb barrier. The discussion is based mostly on data obtained by our group in collaborative experiments. We show that for heavy targets there is complete fusion suppression, corresponding to the occurrence of incomplete fusion of one of the break-up fragments, whereas for medium and light mass targets, there is no such effect, due to the negligible incomplete fusion process.

Influence of the break-up on the fusion and scattering processes

We present results of the study of the influence of the break-up process on the fusion, scattering and reaction cross sections, at near barrier energies. Most of the discussed reactions and scattering mechanisms are induced by the stable weakly bound nuclei 6,7Li and 9Be, although comparisons with reactions induced by strongly bound nuclei are also made. We give a picture of the break-up and fusion mechanisms at energies above the Coulomb barrier.