2,741 research outputs found
Mean-field instabilities and cluster formation in nuclear reactions
We review recent results on intermediate mass cluster production in heavy ion
collisions at Fermi energy and in spallation reactions. Our studies are based
on modern transport theories, employing effective interactions for the nuclear
mean-field and incorporating two-body correlations and fluctuations. Namely we
will consider the Stochastic Mean Field (SMF) approach and the recently
developed Boltzmann-Langevin One Body (BLOB) model. We focus on cluster
production emerging from the possible occurrence of low-density mean-field
instabilities in heavy ion reactions. Within such a framework, the respective
role of one and two-body effects, in the two models considered, will be
carefully analysed. We will discuss, in particular, fragment production in
central and semi-peripheral heavy ion collisions, which is the object of many
recent experimental investigations. Moreover, in the context of spallation
reactions, we will show how thermal expansion may trigger the development of
mean-field instabilities, leading to a cluster formation process which competes
with important re-aggregation effects
Probing the Nuclear Symmetry Energy with Heavy Ion Collisions
Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium
nuclear interaction in regions away from saturation. In this report we present
a selection of new reaction observables in dissipative collisions particularly
sensitive to the symmetry term of the nuclear Equation of State (). We
will first discuss the Isospin Equilibration Dynamics. At low energies this
manifests via the recently observed Dynamical Dipole Radiation, due to a
collective neutron-proton oscillation with the symmetry term acting as a
restoring force. At higher beam energies Iso-EoS effects will be seen in an
Isospin Diffusion mechanism, via Imbalance Ratio Measurements, in particular
from correlations to the total kinetic energy loss. For fragmentation reactions
in central events we suggest to look at the coupling between isospin
distillation and radial flow. In Neck Fragmentation reactions important Iso-EoS
information can be obtained from fragment isospin content, velocity and
alignement correlations. The high density symmetry term can be probed from
isospin effects on heavy ion reactions at relativistic energies (few AGeV
range), in particular for high transverse momentum selections of the reaction
products. Rather isospin sensitive observables are proposed from
nucleon/cluster emissions, collective flows and meson production. The
possibility to shed light on the controversial neutron/proton effective mass
splitting in asymmetric matter is also suggested.
A large symmetry repulsion at high baryon density will also lead to an
"earlier" hadron-deconfinement transition in n-rich matter. The binodal
transition line of the (T,\rho_B) diagram is lowered to a region accessible
through heavy ion collisions in the energy range of the new planned facilities,
e.g. the FAIR/NICA projects. Some observable effects of the formation of a
Mixed Phase are suggested, in particular a Neutron Trapping mechanism. The
dependence of the results on a suitable treatment of the isovector part of the
interaction in effective QCD Lagrangian approaches is critically discussed. We
stress the interest of this study in nuclear astrophysics, in particular for
supernovae explosions and neutron star structure, where the knowledge of the
Iso-EoS is important at low as well as at high baryon density.Comment: 52 pages, 28 figures, topical review submitted to J. Phys. G: Nucl.
Phys (IOP Latex
Reaction Dynamics with Exotic Beams
We review the new possibilities offered by the reaction dynamics of
asymmetric heavy ion collisions, using stable and unstable beams. We show that
it represents a rather unique tool to probe regions of highly Asymmetric
Nuclear Matter () in compressed as well as dilute phases, and to test the
in-medium isovector interaction for high momentum nucleons. The focus is on a
detailed study of the symmetry term of the nuclear Equation of State () in
regions far away from saturation conditions but always under laboratory
controlled conditions.
Thermodynamic properties of are surveyed starting from nonrelativistic
and relativistic effective interactions. In the relativistic case the role of
the isovector scalar -meson is stressed. The qualitative new features
of the liquid-gas phase transition, "diffusive" instability and isospin
distillation, are discussed. The results of ab-initio simulations of n-rich,
n-poor, heavy ion collisions, using stochastic isospin dependent transport
equations, are analysed as a function of beam energy and centrality. The
isospin dynamics plays an important role in all steps of the reaction, from
prompt nucleon emissions to the final fragments. The isospin diffusion is also
of large interest, due to the interplay of asymmetry and density gradients. In
relativistic collisions, the possibility of a direct study of the covariant
structure of the effective nucleon interaction is shown. Results are discussed
for particle production, collective flows and iso-transparency.
Perspectives of further developments of the field, in theory as well as in
experiment, are presented.Comment: 167+5 pages, 77 figures, general revie
Isospin in fragment production
Based on a general approach to binary systems we show that in the low density
region asymmetric nuclear matter (ANM) is unstable only against isoscalarlike
fluctuations. The physical meaning of the thermodynamical chemical and
mechanical instabilities is related to the inequality relations verified by the
strength of interaction among different components. Relevance of these results
in bulk and neck fragmentation is discussed.Comment: 8 pages, 5 Postscript figures, talk at Cortona 2002 Conference,
Oct.7-Oct.12, Italy, World Scientific (in press
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
Isospin emission and flows at high baryon density: a test of the symmetry potential
High energy Heavy Ion Collisions (HIC) are studied in order to access nuclear
matter properties at high density. Particular attention is paid to the
selection of observables sensitive to the poorly known symmetry energy at high
baryon density, of large fundamental interest, even for the astrophysics
implications. Using fully consistent transport simulations built on effective
theories we test isospin observables ranging from nucleon/cluster emissions to
collective flows (in particular the elliptic, squeeze out, part). The effects
of the competition between stiffness and momentum dependence of the Symmetry
Potential on the reaction dynamics are thoroughly analyzed. In this way we try
to shed light on the controversial neutron/proton effective mass splitting at
high baryon and isospin densities. New, more exclusive, experiments are
suggested.Comment: 10 pages, 16 figures, new figure added, accepted for publication in
Phys.Rev.
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