Leandro Gasques

Relating Breakup and Incomplete Fusion of Weakly Bound Nuclei through a Classical Trajectory Model with Stochastic Breakup

A classical dynamical model that treats breakup stochastically is presented for low energy reactions of weakly bound nuclei. The three-dimensional model allows a consistent calculation of breakup, incomplete, and complete fusion cross sections. The model is assessed by comparing the breakup observables with continuum discretized coupled-channel quantum mechanical predictions, which are found to be in reasonable agreement. Through the model, it is demonstrated that the breakup probability of the projectile as a function of its distance from the target is of primary importance for understanding complete and incomplete fusion at energies near the Coulomb barrier.

Simple analytic model for astrophysicalSfactors

We propose a physically transparent analytic model of astrophysical S factors as a function of a center-of-mass energy E of colliding nuclei (below and above the Coulomb barrier) for nonresonant fusion reactions. For any given reaction, the S(E) model contains four parameters [two of which approximate the barrier potential, U(r)]. They are easily interpolated along many reactions involving isotopes of the same elements; they give accurate practical expressions for S(E) with only several input parameters for many reactions. The model reproduces the suppression of S(E) at low energies (of astrophysical importance) due to the shape of the low-r wing of U(r). The model can be used to reconstruct U(r) from computed or measured S(E). For illustration, we parametrize our recent calculations of S(E) (using the Sao Paulo potential and the barrier penetration formalism) for 946 reactions involving stable and unstable isotopes of C, O, Ne, and Mg (with nine parameters for all reactions involving many isotopes of the same elements, e.g., C+O). In addition, we analyze astrophysically important {sup 12}C+{sup 12}C reaction, compare theoretical models with experimental data, and discuss the problem of interpolating reliably known S(E) values to low energies (E < or approx. 2-3 MeV).

Effects of nuclear molecular configurations on the astrophysical S-factor for 16O+16O

Abstract The impact of nuclear molecular configurations on the astrophysical S -factor for 16 Oxa0+xa0 16 O is investigated within the realistic two-center shell model based on Woods–Saxon potentials. These molecular effects refer to the formation of a neck between the interacting nuclei and the radial dependent collective mass parameter. It is demonstrated that the former is crucial to explain the current experimental data with high accuracy and without any free parameter, whilst in addition the latter predicts a pronounced maximum in the S -factor. In contrast to very recent results by Jiang et al., the S -factor does not decline towards extremely low values as energy decreases.

{alpha}+{alpha} scattering reexamined in the context of the Sao Paulo potential

We have analyzed a large set of {alpha}+{alpha} elastic scattering data for bombarding energies ranging from 0.6 to 29.5 MeV. Because of the complete lack of open reaction channels, the optical interaction at these energies must have a vanishing imaginary part. Thus, this system is particularly important because the corresponding elastic scattering cross sections are very sensitive to the real part of the interaction. The data were analyzed in the context of the velocity-dependent Sao Paulo potential, which is a successful theoretical model for the description of heavy-ion reactions from sub-barrier to intermediate energies. We have verified that, even in this low-energy region, the velocity dependence of the model is quite important for describing the data of the {alpha}+{alpha} system.

Systematic study of the nuclear potential through high precision back-angle quasi-elastic scattering measurements of 16 O and 32 S on various targets

Elastic and inelastic scattering have proven an excellent method to probe the nuclear potential in the surface region [1]. A surface diffuseness parameter of around 0.63 fm is accepted as describing elastic and inelastic scattering data. A new method to determine the diffuseness was proposed [2] using high precision quasi‐elastic excitation functions at sub‐barrier energies, and at backward angles. In response to this proposal, we have made measurements for 32S+170Er, 186W, 197Au, 208Pb [3], and more recently for the reactions 16O+144,154Sm, 186W, 197Au, 208Pb [4].Both single channel and coupled‐channels calculations have been performed to extract the diffuseness parameter of the nuclear potential, assuming a Woods‐Saxon shape. For the reactions involving near‐spherical targets, both theoretical analyses give the same diffuseness parameter, as had been hoped, showing that the effect of couplings to intrinsic excited states in the target and projectile is negligible. On the other hand, for deformed systems...

Two Distinct Quasifission Modes in the {sup 32}S+{sup 232}Th Reaction

Comprehensive fission measurements, including mass-angle distributions, for the reaction of 32S with the prolate deformed nucleus 232Th at near-barrier energies show two distinct components in both mass and angle; surprisingly, both have characteristics of quasifission. Their relative probabilities vary rapidly with the ratio of the beam energy to the capture barrier, suggesting a relationship with deformation aligned (sub-barrier), or antialigned (above-barrier), configurations at contact.

A Unified Equation for the Reaction Rate in Dense Matter Stars

We analyze thermonuclear and pycnonuclear reaction rates in multi‐component dense stellar plasma. First we describe calculations of the astrophysical S‐factor at low energies using the Sao Paulo potential on the basis of the barrier penetration model. Then we present a simple phenomenological expression for a reaction rate. The expression contains several fit parameters which we adjust to reproduce the best microscopic calculations available in the literature.