1,836 research outputs found
Spinodal instability growth in new stochastic approaches
Are spinodal instabilities the leading mechanism in the fragmentation of a
fermionic system? Numerous experimental indications suggest such a scenario and
stimulated much effort in giving a suitable description, without being
finalised in a dedicated transport model.
On the one hand, the bulk character of spinodal behaviour requires an
accurate treatment of the one-body dynamics, in presence of mechanical
instabilities. On the other hand, pure mean-field implementations do not apply
to situations where instabilities, bifurcations and chaos are present. The
evolution of instabilities should be treated in a large-amplitude framework
requiring fluctuations of Langevin type.
We present new stochastic approaches constructed by requiring a thorough
description of the mean-field response in presence of instabilities. Their
particular relevance is an improved description of the spinodal fragmentation
mechanism at the threshold, where the instability growth is frustrated by the
mean-field resilience.Comment: Conf. proc. IWM2014-EC, Catania, 6-9 May 201
Self-consistent dynamical mean-field investigation of exotic structures in isospin-asymetric nuclear matter
The exotic structures expected in the outermost layer of neutron stars are investigated in a new approach. It is based on the DYnamical WAvelets in Nuclei (DYWAN) model of nuclear collisions. This microscopic dynamical approach is an Extended Time-Dependent Hartree-Fock description based on a wavelet representation. The model addresses the dynamical exploration of complex nuclear structures, beyond the Wigner-Seitz (WS) approximation and without any assumption on their final shapes. The present study focuses on exotic phases of cold matter evidenced dynamically at sub-saturation densities, currently within a pure mean field framework, before tackling the effects of the multi-particle correlations in a forthcoming study. Starting from inhomogeneous initial conditions provided by nuclei located on an initial crystalline lattice, the exotic structures result from a dynamical self-consistent treatment where, in principle, the nuclear system can freely self-organize, modify the lattice structure or even break the lattice and the initial matter distribution symmetries. In this work nuclei are initially slightly excited with low-lying collective modes. The system can then explore geometrical configurations with similar energies, without being trapped in the vicinity of a local minimum. In this quantum framework, different effects are analyzed, among them the sensitivity to the equation of state and to the proton fraction
Impact of Reaction Cross Section on the Unified Description of Fusion Excitation Function
International audienceA systematics of over 300 complete and incomplete fusion cross section data points covering energies beyond the barrier for fusion is presented. Owing to a usual reduction of the fusion cross sections by the total reaction cross sections and an original scaling of energy, a fusion excitation function common to all the data points is established. A universal description of the fusion exci-tation function relying on basic nuclear concepts is proposed and its dependence on the reaction cross section used for the cross section normalization is discussed. The pioneering empirical model proposed by Bass in 1974 to describe the complete fusion cross sections is rather successful for the incomplete fusion too and provides cross section predictions in satisfactory agreement with the observed universality of the fusion excitation function. The sophisticated microscopic transport DYWAN model not only reproduces the data but also predicts that fusion reaction mechanism disappears due to weakened nuclear stopping power around the Fermi energy
A dynamical description of neutron star crusts
Neutron Stars are natural laboratories where fundamental properties of matter
under extreme conditions can be explored. Modern nuclear physics input as well
as many-body theories are valuable tools which may allow us to improve our
understanding of the physics of those compact objects.
In this work the occurrence of exotic structures in the outermost layers of
neutron stars is investigated within the framework of a microscopic model. In
this approach the nucleonic dynamics is described by a time-dependent mean
field approach at around zero temperature. Starting from an initial crystalline
lattice of nuclei at subnuclear densities the system evolves toward a manifold
of self-organized structures with different shapes and similar energies. These
structures are studied in terms of a phase diagram in density and the
corresponding sensitivity to the isospin-dependent part of the equation of
state and to the isotopic composition is investigated.Comment: 8 pages, 5 figures, conference NN201
Stopping power as a signature of dissipative processes in heavy-ion collisions
In heavy-ions collisions different observables have been studied in order to get an insight about dissipative processes taking place in the excited nuclear system. Among them, we can focus on the ratio between the transverse and longitudinal kinetic energy components, or stopping power RE. A substancial reduction of this quantity has been recently evidenced by the INDRA collaboration at incident energies between 32 and 100 A MeV, for various symmetric systems. In this work, the impact of σnn on the stoping power RE is studied in the framework of the microscopic DYWAN model. Calculations have been performed in Xe+Sn central collisions at incident energies between 45 and 100 A MeV, where the theoretical values are shown to be more sensitive to σnn. They are compared with experimental data and with the results of the semiclassical Landau-Vlasov model
Fusion excitation function revisited
We report on a comprehensive systematics of fusion-evaporation and/or
fusion-fission cross sections for a very large variety of systems over an
energy range 4-155 A.MeV. Scaled by the reaction cross sections, fusion cross
sections do not show a universal behavior valid for all systems although a high
degree of correlation is present when data are ordered by the system mass
asymmetry.For the rather light and close to mass-symmetric systems the main
characteristics of the complete and incomplete fusion excitation functions can
be precisely determined. Despite an evident lack of data above 15A.MeV for all
heavy systems the available data suggests that geometrical effects could
explain the persistence of incomplete fusion at incident energies as high as
155A.MeV.Comment: 8 pages, 5 figures, contribution to the NN2012 Proceeding
Nuclear Flow Excitation Function
We consider the dependence of collective flow on the nuclear surface
thickness in a Boltzmann--Uehling--Uhlenbeck transport model of heavy ion
collisions. Well defined surfaces are introduced by giving test particles a
Gaussian density profile of constant width. Zeros of the flow excitation
function are as much influenced by the surface thickness as the nuclear
equation of state, and the dependence of this effect is understood in terms of
a simple potential scattering model. Realistic calculations must also take into
account medium effects for the nucleon--nucleon cross section, and impact
parameter averaging. We find that balance energy scales with the mass number as
, where has a numerical value between 0.35 and 0.5, depending on
the assumptions about the in-medium nucleon-nucleon cross section.Comment: 11 pages (LaTeX), 7 figures (not included), MSUCL-884, WSU-NP-93-
Momentum--dependent nuclear mean fields and collective flow in heavy ion collisions
We use the Boltzmann-Uehling-Uhlenbeck model to simulate the dynamical
evolution of heavy ion collisions and to compare the effects of two
parametrizations of the momentum--dependent nuclear mean field that have
identical properties in cold nuclear matter. We compare with recent data on
nuclear flow, as characterized by transverse momentum distributions and flow
() variables for symmetric and asymmetric systems. We find that the precise
functional dependence of the nuclear mean field on the particle momentum is
important. With our approach, we also confirm that the difference between
symmetric and asymmetric systems can be used to pin down the density and
momentum dependence of the nuclear self consistent one--body potential,
independently. All the data can be reproduced very well with a
momentum--dependent interaction with compressibility K = 210 MeV.Comment: 15 pages in ReVTeX 3.0; 12 postscript figures uuencoded; McGill/94-1
Isospin dependence of collective flow in heavy-ion collisions at intermediate energies
Within the framework of an isospin-dependent Boltzmann-Uehling-Uhlenbeck
(BUU) model using initial proton and neutron densities calculated from the
nonlinear relativistic mean-field (RMF) theory, we compare the strength of
transverse collective flow in reactions and
, which have the same mass number but different neutron/proton
ratios. The neutron-rich system () is found to show
significantly stronger negative deflection and consequently has a higher
balance energy, especially in peripheral collisions. NOTE ADDED IN PROOF: The
new phenomenon predicted in this work has just been confirmed by an experiment
done by G.D. Westfall et al. using the NSCL/MSU radioactive beam facility and a
spartan soccer. A paper by R. Pak et al. is submitted to PRL to report the
experimental result.Comment: Latex file, 9 pages, 4 figures availabe upon request; Phys. Rev.
Lett. (June 3, 1996) in pres
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