L. C. Chamon

Toward a global description of the nucleus-nucleus interaction

Extensive systematizations of theoretical and experimental nuclear densities and of optical potential strengths extracted from heavy-ion elastic scattering data analyses at low and intermediate energies are presented. The energy dependence of the nuclear potential is accounted for within a model based on the nonlocal nature of the interaction. The systematics indicates that the heavy-ion nuclear potential can be described in a simple global way through a double-folding shape, which basically depends only on the density of nucleons of the partners in the collision. The possibility of extracting information about the nucleon-nucleon interaction from the heavy-ion potential is investigated.

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.

Nuclear fusion in dense matter: Reaction rate and carbon burning

In this paper we analyze the nuclear fusion rates among equal nuclei for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime). The rate is determined by Coulomb barrier penetration in dense environments and by the astrophysical S factor at low energies. We evaluate previous studies of the Coulomb barrier problem and propose a simple phenomenological formula for the reaction rate that covers all cases. The parameters of this formula can be varied to take into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the 12 C+ 12 C fusion reaction. This reaction is important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovas, and in accreting neutron stars. The S factor at stellar energies depends on a reliable fit and extrapolation of the experimental data. We calculate the energy dependence of the S factor by using a recently developed parameter-free model for the nuclear interaction, taking into account the effects of the Pauli nonlocality. For illustration, we analyze the efficiency of carbon burning in a wide range of densities and temperatures of stellar matter with the emphasis on carbon ignition at densities ρ > ∼ 10 9 gc m −3 .

Elastic scattering and total reaction cross section for the 6He + 27Al system

Abstract The elastic scattering of the radioactive halo nucleus 6 He on 27 Al target was measured at four energies close to the Coulomb barrier using the RIBRAS (Radioactive Ion Beams in Brazil) facility. The Sao Paulo Potential (SPP) was used and its diffuseness and imaginary strength were adjusted to fit the elastic scattering angular distributions. Reaction cross-sections were extracted from the optical model fits. The reduced reaction cross-sections of 6 He on 27 Al are similar to those for stable, weakly bound projectiles as 6,7 Li, 9 Be and larger than stable, tightly bound projectile as 16 O on 27 Al.

A parameter-free optical potential for the heavy-ion elastic scattering process☆

Abstract Thirty elastic scattering angular distributions for seven heavy-ion systems, in wide energy ranges, have been studied with the aim of systematizing the optical potential, real and imaginary parts, in a global way. The framework is: (i) an extensive systematization of nuclear densities, (ii) the energy dependence of the bare potential accounted by a model based on the nonlocal nature of the interaction, and (iii) the real and imaginary parts of the optical potential assumed to have the same radial shape.

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.

Experimental determination of the ion-ion potential in the N = 50 target region: A tool to probe ground-state nuclear densities

Abstract Precise elastic and inelastic differential cross sections have been measured for the 16 O + 88 Sr , 90,92 Zr, 92 Mo systems at sub-barrier energies. From a coupled channel data analysis, the corresponding “experimental” bare potentials have been determined. The comparison of these potentials with those derived from double-folding theoretical calculations and the high energy (96 Mev/nucleon) elastic scattering data analysis indicate that the method is a very sensitive probe of the ground-state nuclear densities in the surface region.

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.

Effect of the 18O nuclear density on the nuclear potentials of the 18O+58,60Ni systems

Abstract Quasi-elastic, inelastic, one- and two-neutron transfer differential cross sections have been measured for the 18 O+ 58,60 Ni systems at sub-barrier energies. The corresponding bare potentials have been determined at interaction distances larger than the respective barrier radii, and the results have been compared with those previously obtained for systems involving the 16 O as projectile. The detected difference between the 18 O and 16 O nuclear potentials has allowed the determination of the nuclear density that corresponds to the two extra neutrons of the 18 O nucleus.

The heavy-ion nuclear potential: determination of a systematic behavior at the region of surface interaction distances

Abstract Precise elastic scattering differential cross sections have been measured for the 16 O + 120 Sn , 138 Ba , 208 Pb systems at sub-barrier energies. The corresponding “experimental” nuclear potentials have been determined at interaction distances larger than the Coulomb barrier radii. These experimental potentials have been compared with our earlier results for other systems, and with theoretical calculations based on the double-folding and liquid-drop models. We have shown that the nuclear potentials have a systematic behavior at the surface region. The present results for the 16 O + 208 Pb system are used to extend earlier studies of the dispersion relation to sub-barrier energies.