3,534 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
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
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.
Heavy Ion Dynamics and Neutron Stars
Some considerations are reported, freely inspired from the presentations and
discussions during the Beijing Normal University Workshop on the above Subject,
held in July 2007. Of course this cannot be a complete summary but just a
collection of personal thougths aroused during the meeting.Comment: 11 pages, no figures, Summary Talk, Int.Workshop on "Nuclear Dynamics
in Heavy Ion Collisions and Neutron Stars", Beijing Normal Univ. July 07, to
appear in Int.Journ.Modern Physics E (2008
Symmetry Energy Effects on the Mixed Hadron-Quark Phase at High Baryon Density
The phase transition of hadronic to quark matter at high baryon and isospin
density is analyzed. Relativistic mean field models are used to describe
hadronic matter, and the MIT bag model is adopted for quark matter. The
boundaries of the mixed phase and the related critical points for symmetric and
asymmetric matter are obtained. Due to the different symmetry term in the two
phases, isospin effects appear to be rather significant. With increasing
isospin asymmetry 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 are suggested, in particular an "Isospin Distillation" mechanism with a
more isospin asymmetric quark phase, to be seen in charged meson yield ratios,
and an onset of quark number scaling of the meson/baryon elliptic flows. The
presented isospin effects on the mixed phase appear to be robust with respect
to even large variations of the poorly known symmetry term at high baryon
density in the hadron phase. The dependence of the results on a suitable
treatment of isospin contributions in effective QCD Lagrangian approaches, at
the level of explicit isovector parts and/or quark condensates, is finally
discussed.Comment: 14 two column pages, 14 figures, new results with other hadron EoS.
Accepted for publication in Phys.Rev.
Isospin effects on sub-threshold kaon production at intermediate energies
We show that in collisions with neutron rich heavy ions at energies around
the production threshold K^0 and K^+ yields might probe the isospin dependent
part of the nuclearEquation of State (EoS) at high baryon densities. In
particular we suggest the K^0/K^+ ratio as a promising observable. Results
obtained in a fully covariant relativistic transport approach are presented for
central Au+Au collisions in the beam energy range 0.8-1.8~AGeV. The focus is
put on the EoS influence which goes beyond the "collision-cascade" picture. The
isovector part of the in-medium interaction affects the kaon multiplicities via
two mechanisms: i) a "symmetry potential" effect, i.e. a larger neutron
repulsion in n-rich systems (isospin fractionation); ii) a "threshold" effect,
due to the change in the self-energies of the particles involved in inelastic
processes. Genuine relativistic contributions are revealed, that could allow to
directly ``measure'' the Lorentz structure of the effective isovector
interaction.Comment: 5 pages, 2 figures, revtex
Application of density dependent parametrization models to asymmetric nuclear matter
Density dependent parametrization models of the nucleon-meson effective
couplings, including the isovector scalar \delta-field, are applied to
asymmetric nuclear matter. The nuclear equation of state and the neutron star
properties are studied in an effective Lagrangian density approach, using the
relativistic mean field hadron theory. It is known that the introduction of a
\delta-meson in the constant coupling scheme leads to an increase of the
symmetry energy at high density and so to larger neutron star masses, in a pure
nucleon-lepton scheme. We use here a more microscopic density dependent model
of the nucleon-meson couplings to study the properties of neutron star matter
and to re-examine the \delta-field effects in asymmetric nuclear matter. Our
calculations show that, due to the increase of the effective \delta coupling at
high density, with density dependent couplings the neutron star masses in fact
can be even reduced.Comment: 5 pages, 4 figure
Static stress drop associated with brittle slip events on exhumed faults
We estimate the static stress drop on small exhumed strike-slip faults in the Lake Edison granodiorite of the central Sierra Nevada (California). The sub-vertical strike-slip faults were exhumed from 4-15 km depth, and were chosen because they are exposed in outcrop along their entire tip-to-tip lengths of 8-12 m. Slip nucleated on joints and accumulated by ductile shearing (forming quartz mylonites from early quartz vein filling in joints) and successive brittle faulting (forming epidote-bearing cataclasites). The occurrence of thin, < 1 mm wide, pseudotachylytes along some small faults throughout the study area suggests that some portion of the brittle slip was seismic. We suggest that the contribution of seismic slip to the total slip along the studied cataclasite-bearing small faults may be estimated by the length of epidote-filled, rhombohedral dilatational jogs (rhombochasms) distributed semi-periodically along the length of the faults. The interpretation that slip recorded by rhombochasms occurred in single events is based on evidence that: 1) epidote crystals are randomly oriented and undeformed within the rhombochasm; and 2) cataclasite structure in principal slip zones does not include clasts of previous cataclasite. We thereby constrain both the rupture length and slip. Based on these measurements, we calculate stress drops ranging over 90-250 MPa, i.e., one to two orders of magnitude larger than typical seismological estimates for earthquakes, but similar in magnitude to recent observations of small (< M2) earthquakes from the San Andreas Fault Observatory at Depth (SAFOD). These inferred seismic ruptures occurred along small, deep-seated faults, and, given the calculated stress drops and observations that brittle faults exploited joints sealed by quartz-bearing mylonite, we conclude that these were “strong” faults
Collective Dipole Bremsstrahlung in Fusion Reactions
We estimate the dipole radiation emitted in fusion processes. We show that a
classical bremsstrahlung approach can account for both the preequilibrium and
the thermal photon emission. We give an absolute evaluation of the
pre-equilibrium component due to the charge asymmetry in the entrance channel
and we study the energy and mass dependence in order to optimize the
observation. This dynamical dipole radiation could be a relevant cooling
mechanism in the fusion path. We stress the interest in experiments with the
new available radioactive beams.Comment: 4 pages (LATEX), 4 Postscript figures, minor text modification
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