2,895 research outputs found
Break-up mechanisms in heavy ion collisions at low energies
We investigate reaction mechanisms occurring in heavy ion collisions at low
energy (around 20 MeV/u). In particular, we focus on the competition between
fusion and break-up processes (Deep-Inelastic and fragmentation) in
semi-peripheral collisions, where the formation of excited systems in various
conditions of shape and angular momentum is observed. Adopting a Langevin
treatment for the dynamical evolution of the system configuration, described in
terms of shape observables such as quadrupole and octupole moments, we derive
fusion/fission probabilities, from which one can finally evaluate the
corresponding fusion and break-up cross sections. The dependence of the results
on shape, angular momentum and excitation energy is discussed.Comment: submitted to Physical Review
Investigation of collective radial expansion and stopping in heavy ion collisions at Fermi energies
We present an analysis of multifragmentation events observed in central Xe+Sn
reactions at Fermi energies. Performing a comparison between the predictions of
the Stochastic Mean Field (SMF) transport model and experimental data, we
investigate the impact of the compression-expansion dynamics on the properties
of the final reaction products. We show that the amount of radial collective
expansion, which characterizes the dynamical stage of the reaction, influences
directly the onset of multifragmentation and the kinematic properties of
multifragmentation events. For the same set of events we also undertake a shape
analysis in momentum space, looking at the degree of stopping reached in the
collision, as proposed in recent experimental studies. We show that full
stopping is achieved for the most central collisions at Fermi energies.
However, considering the same central event selection as in the experimental
data, we observe a similar behavior of the stopping power with the beam energy,
which can be associated with a change of the fragmentation mechanism, from
statistical to prompt fragment emission.Comment: 15 page
Spinodal instabilities within BUU approach
Using a recently developed method for the inclusion of fluctuation in the BUU
dynamics, we study the self-consistent propagation of inherent thermal noise of
unstable nuclear matter. The large time behaviour of the evolving system
exhibits synergism between fluctuation and non-linearities in a universal
manner which manifest in the appearance of macroscopic structure in the average
description.Comment: 12 pages Revtex. Two figures, uuencoded, are enclosed in a separate
fil
Nuclear collective dynamics within Vlasov approach
We discuss, in an investigation based on Vlasov equation, the properties of
the isovector modes in nuclear matter and atomic nuclei in relation with the
symmetry energy. We obtain numerically the dipole response and determine the
strength function for various systems, including a chain of Sn isotopes. We
consider for the symmetry energy three parametrizations with density providing
similar values at saturation but which manifest very different slopes around
this point. In this way we can explore how the slope affects the collective
response of finite nuclear systems. We focus first on the dipole polarizability
and show that while the model is able to describe the expected mass dependence,
A^{5/3}, it also demonstrates that this quantity is sensitive to the slope
parameter of the symmetry energy. Then, by considering the Sn isotopic chain,
we investigate the emergence of a collective mode, the Pygmy Dipole Resonance
(PDR), when the number of neutrons in excess increases. We show that the total
energy-weighted sum rule exhausted by this mode has a linear dependence with
the square of isospin I=(N-Z)/A, again sensitive to the slope of the symmetry
energy with density. Therefore the polarization effects in the isovector
density have to play an important role in the dynamics of PDR. These results
provide additional hints in the investigations aiming to extract the properties
of symmetry energy below saturation.Comment: 7 pages, 6 figure
Spinodal decomposition of expanding nuclear matter and multifragmentation
Density fluctuations of expanding nuclear matter are studied within a
mean-field model in which fluctuations are generated by an external stochastic
field. Fluctuations develop about a mean one-body phase-space density
corresponding to a hydrodinamic motion that describes a slow expansion of the
system. A fluctuation-dissipation relation suitable for a uniformly expanding
medium is obtained and used to constrain the strength of the stochastic field.
The distribution of the liquid domains in the spinodal decomposition is
derived. Comparison of the related distribution of the fragment size with
experimental data on the nuclear multifragmentation is quite satisfactory.Comment: 19 RevTex4 pages, 6 eps figures, to appear in Phys. Rev.
The Dynamical Dipole Mode in Fusion Reactions with Exotic Nuclear Beams
We report the properties of the prompt dipole radiation, produced via a
collective bremsstrahlung mechanism, in fusion reactions with exotic beams. We
show that the gamma yield is sensitive to the density dependence of the
symmetry energy below/around saturation. Moreover we find that the angular
distribution of the emitted photons from such fast collective mode can
represent a sensitive probe of its excitation mechanism and of fusion dynamics
in the entrance channel.Comment: 5 pages, 3 figures, to appear in Phys.Rev.
Isospin fluctuations in spinodal decomposition
We study the isospin dynamics in fragment formation within the framework of
an analytical model based on the spinodal decomposition scenario. We calculate
the probability to obtain fragments with given charge and neutron number,
focussing on the derivation of the width of the isotopic distributions. Within
our approach this is determined by the dispersion of N/Z among the leading
unstable modes, due to the competition between Coulomb and symmetry energy
effects, and by isovector-like fluctuations present in the matter that
undergoes the spinodal decomposition. Hence the widths exhibit a clear
dependence on the properties of the Equation of State. By comparing two systems
with different values of the charge asymmetry we find that the isotopic
distributions reproduce an isoscaling relationship.Comment: 18 RevTex4 pages, 6 eps figure
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