S. R. Souza

Scaling in fracture and refreezing of sea ice

Sea ice breaks up and regenerates rapidly during winter conditions in the Arctic. Analyzing satellite data from the Kara Sea, we find that the average ice floe size depends on weather conditions. Nevertheless, the frequency of floes of size A is a power law, N∼A−τ, where τ=1.6±0.2, for A less than approximately 100km2. This scale-invariant behavior suggests a competition between fracture due to strains in the ice field and refreezing of the fractures. A cellular model for this process gives results consistent with observations.

Isospin effects in nuclear multifragmentation

We develop an improved Statistical Multifragmentation Model that provides the capability to calculate calorimetric and isotopic observables with precision. With this new model we examine the influence of nuclear isospin on the fragment elemental and isotopic distributions. We show that the proposed improvements on the model are essential for studying isospin effects in nuclear multifragmentation. In particular, these calculations show that accurate comparisons to experimental data require that the nuclear masses, free energies and secondary decay must be handled with higher precision than many current models accord.

Mass parametrizations and predictions of isotopic observables

We discuss the accuracy of mass models for extrapolating to very asymmetric nuclei and the impact of such extrapolations on the predictions of isotopic observables in multifragmentation. We obtain improved mass predictions by incorporating measured masses and extrapolating to unmeasured masses with a mass formula that includes surface symmetry and Coulomb terms. We find that using accurate masses has a significant impact on the predicted isotopic observables.

THERMAL FLUCTUATIONS IN HEAVY-ION FUSION REACTIONS (I). One-dimensional models

Abstract We consider the effect of thermal fluctuations on the heavy-ion fusion probabilities calculated within different models of the nuclear potential and dissipation function. We show that the introduction of fluctuations has effects that depend considerably on the model utilized, and which may therefore help decide among the different theoretical descriptions now available.

Nuclear isotope thermometry

We discuss different aspects which might influence temperatures deduced from experimental isotopic yields in the multifragmentation process. It is shown that fluctuations due to the finite size of the system and distortions due to the decay of hot primary fragments conspire to blur the temperature determination in multifragmentation reactions. These facts suggest that caloric curves obtained through isotope thermometers, which were taken as evidence for a first-order phase transition in nuclear matter, should be investigated very carefully.

Probing the symmetry energy from the nuclear isoscaling

Using different parametrizations of the nuclear mass formula, we study the sensitivity of the isoscaling parameters to the mass formula employed in grand-canonical calculations. Previous works on isoscaling have suggested that the symmetry energy implied in such calculations is anomalously smaller than that suggested by fits to nuclear masses. We show that surface corrections to the symmetry energy naturally broadens the isotopic distribution thus allowing for values of the symmetry energy which more closely match those obtained from nuclear masses.

Comparison of statistical treatments for the equation of state for core-collapse supernovae

Neutrinos emitted during the collapse, bounce, and subsequent explosion provide information about supernova dynamics. The neutrino spectra are determined by weak interactions with nuclei and nucleons in the inner regions of the star, and thus the neutrino spectra are determined by the composition of matter. The composition of stellar matter at temperature ranging from T = 1-3 MeV and densities ranging from 10–5 to 0.1 times the saturation density is explored. We examine the single-nucleus approximation commonly used in describing dense matter in supernova simulations and show that while the approximation is accurate for predicting the energy and pressure at most densities, the predicted compositions are less accurate, varying by 50% or more at the largest densities. We find that as the temperature and density increase, the single nucleus approximation systematically overpredicts the mass number of nuclei that are actually present and underestimates the contribution from lighter nuclei which are present in significant amounts.

Fragment emission pattern in central and semi-central heavy-ion collisions

Abstract Event-by-event emission pattern of all charged fragments in central and semi-central, symmetric, semi-symmetric and asymmetric heavy-ion collisions at energies (50–220) A ·MeV have been studied in nuclear emulsions. The most central collisions, selected by momentum-tensor elongation and multiplicity criteria, exhibit one single fused source. Collective flow in this source is observed in symmetric (Kr + AgBr) collisions and weakly indicated in semi-symmetric (Ar + AgBr) collisions. Semi-central events exhibit quasi-target, quasi-projectile and participant or incomplete fusion sources from which no or very weak flow signals are observed. The emission from these sources is described by a Coulomb explosion model except for the fact that sub-Coulomb barrier fragments are frequent.

Statistical multifragmentation model with discretized energy and the generalized Fermi breakup: Formulation of the model

The generalized Fermi breakup model, recently demonstrated to be formally equivalent to the statistical multifragmentation model, if the contribution of excited states is included in the state densities of the former, is implemented. Because this treatment requires application of the statistical multifragmentation model repeatedly onhotfragmentsuntiltheyhavedecayedtotheirgroundstates,itbecomesextremelycomputationallydemanding, making its application to the systems of interest extremely difficult. Based on exact recursion formulas previously developed by Chase and Mekjian to calculate statistical weights very efficiently, we present an implementation which is efficient enough to allow it to be applied to large systems at high excitation energies. Comparison with the GEMINI++ sequential decay code and the Weisskopf-Ewing evaporation model shows that the predictions obtained with our treatment are fairly similar to those obtained with these more traditional models.

Emergence of scale-free networks from local connectivity and communication trade-offs

We suggest a mechanism of connectivity evolution in networks to account for the emergence of scale-free behavior. The mechanism works on a fixed set of nodes and promotes growth from a minimally connected initial topology by the addition of edges. A new edge is added between two nodes depending on the trade-off between a gain and a cost function of local connectivity and communication properties. We report on simulation results that indicate the appearance of power-law distributions of node degrees for selected parameter combinations.