1,176 research outputs found
Dynamics of clusters and fragments in heavy-ion collisions
A review is given on the studies of formation of light clusters and heavier
fragments in heavy-ion collisions at incident energies from several tens of
MeV/nucleon to several hundred MeV/nucleon, focusing on dynamical aspects and
on microscopic theoretical descriptions. Existing experimental data already
clarify basic characteristics of expanding and fragmenting systems typically in
central collisions, where cluster correlations cannot be ignored. Cluster
correlations appear almost everywhere in excited low-density nuclear many-body
systems and nuclear matter in statistical equilibrium where the properties of a
cluster may be influenced by the medium. On the other hand, transport models to
solve the time evolution have been developed based on the single-nucleon
distribution function. Different types of transport models are reviewed putting
emphasis both on theoretical features and practical performances in the
description of fragmentation. A key concept to distinguish different models is
how to consistently handle single-nucleon motions in the mean field,
fluctuation or branching induced by two-nucleon collisions, and localization of
nucleons to form fragments and clusters. Some transport codes have been
extended to treat light clusters explicitly. Results indicate that cluster
correlations can have strong impacts on global collision dynamics and
correlations between light clusters should also be taken into account.Comment: review article, 64 pages, 27 figure
Multifragmentation and Symmetry Energy Studied with AMD
The antisymmetrized molecular dynamics (AMD) simulations suggest that the
isospin composition of fragments produced dynamically in multifragmentation
reactions is basically governed by the symmetry energy of low-density uniform
nuclear matter rather than the symmetry energy for the ground-state finite
nuclei. After the statistical secondary decay of the excited fragments, the
symmetry energy effect still remains in the fragment isospin composition,
though the effect in the isoscaling parameter seems a very delicate problem.Comment: Proceedings for VI Latin American Symposium on Nuclear Physics and
Applications, Iguazu, Argentina (2005). To be published in Acta Phys. Hung.
Improvement of the nucleon emission process and the statistical property in molecular dynamics
We propose to introduce a new stochastic process in molecular dynamics in
order to improve the description of the nucleon emission process from a hot
nucleus. We give momentum fluctuations originating from the momentum width of
the nucleon wave packet to the nucleon stochastically when it is being emitted
from the nucleus. We show by calculating the liquid gas phase equilibrium in
the case of antisymmetrized molecular dynamics, that with this improvement, we
can recover the quantum mechanical statistical property of the nucleus for the
particle emission process.Comment: 10 pages, LaTeX with revtex and epsf, uuenocded postscript figures,
postscript version available at http://pearl.scphys.kyoto-u.ac.jp/~ono
Probing neutron-proton dynamics by pions
In order to investigate the nuclear symmetry energy at high density, we study
the pion production in central collisions of neutron-rich nuclei
at 300 MeV/nucleon using a new
approach by combining the antisymmetrized molecular dynamics (AMD) and a
hadronic cascade model (JAM). The dynamics of neutrons and protons is solved by
AMD, and then pions and resonances in the reaction process are handled
by JAM. We see the mechanism how the resonance and pions are produced
reflecting the dynamics of neutrons and protons. We also investigate the
impacts of cluster correlations as well as of the high-density symmetry energy
on the nucleon dynamics and consequently on the pion ratio. We find that the
production ratio agrees very well with the
neutron-proton squared ratio in the high-density and high-momentum
region. We show quantitatively that production ratio, and therefore
, are directly reflected in the ratio, with modification
in the final stage of the reaction.Comment: 14 pages, 10 figures; Figures 3-8 are updated with corrected
numerical results. No change in the main conclusion
Viruses and Lipids
As obligatory intracellular pathogens, viruses exploit various cellular molecules and structures, such as cellular membranes, for their propagation. Enveloped viruses acquire lipid membranes as their outer coat through interactions with cellular membranes during morphogenesis within, and egress from, infected cells. In contrast, non-enveloped viruses typically exit cells by cell lysis, and lipid membranes are not part of the released virions. However, non-enveloped viruses also interact with lipid membranes at least during entry into target cells. Therefore, lipids, as part of cellular membranes, inevitably play some roles in life cycle of viruses. [...
Statistical properties of antisymmetrized molecular dynamics for non-nucleon-emission and nucleon-emission processes
Statistical properties of the antisymmetrized molecular dynamics (AMD) are
classical in the case of nucleon emission processes, while they are quantum
mechanical for the processes without nucleon emission. We first clarify that
there coexist mutually opposite two statistics in the AMD framework: One is the
classical statistics of the motion of wave packet centroids and the other is
the quantum statistics of the motion of wave packets which is described by the
AMD wave function. We prove the classical statistics of wave packet centroids
by using the framework of the microcanonical ensemble of the nuclear system. We
show that the quantum statistics of wave packets emerges from the classical
statistics of wave packet centroids. It is emphasized that the temperature of
the classical statistics of wave packet centroids is different from the
temperature of the quantum statistics of wave packets. We then explain that the
statistical properties of AMD for nucleon emission processes are classical
because nucleon emission processes in AMD are described by the motion of wave
packet centroids. When we improve the description of the nucleon emission
process so as to take into account the momentum fluctuation due to the wave
packet spread, the AMD statistical properties for nucleon emission processes
change drastically into quantum statistics. Our study of nucleon emission
processes can be conversely regarded as giving another kind of proof of the
fact that the statistics of wave packets is quantum mechanical while that of
wave packet centroids is classical.Comment: 20 pages, LaTeX with revtex and epsf, uuenocded postscript figures,
postscript version available at http://pearl.scphys.kyoto-u.ac.jp/~ono
Collision integral with momentum-dependent potentials and its impact on pion production in heavy-ion collisions
The momentum dependence of the nucleon mean-field potential in a wide
momentum range can be an important factor to determine the resonance
and pion production in intermediate-energy heavy-ion collisions. In particular,
in neutron-rich systems such as
collisions, we need to carefully treat the momentum dependence because the
neutron and proton potentials can have different momentum dependence, as
characterized at low momenta by effective masses. In the present work, we
rigorously calculate the collision terms of and
processes with the precise conservation of
energy and momentum under the presence of momentum-dependent potentials for the
initial and final particles of the process. The potentials affect not only the
threshold condition for the process but also the cross section in general as a
function of the momenta of the initial particles, which is treated in a natural
way in the present work. Calculations are performed by combining the nucleon
dynamics obtained by the antisymmetrized molecular dynamics (AMD) model with a
newly developed transport code which we call sJAM. The calculated results for
central collisions at 270 MeV/nucleon
clearly show that the momentum dependence of the neutron and proton potentials
has a significant impact on the process, and this information
is strongly reflected in the charged pion ratio (). We also
investigate the effects of the high-density symmetry energy and the isovector
part of the potential of resonances on pion production, which we find
are relatively small compared to the effect of the momentum dependence of the
neutron and proton potentials.Comment: 18 pages, 13 figures, 1 tabl
Relevance of equilibrium in multifragmentation
The relevance of equilibrium in a multifragmentation reaction of very central
collisions at 35 MeV/nucleon is investigated by using
simulations of Antisymmetrized Molecular Dynamics (AMD). Two types of ensembles
are compared. One is the reaction ensemble of the states at each reaction time
in collision events simulated by AMD, and the other is the equilibrium
ensemble prepared by solving the AMD equation of motion for a many-nucleon
system confined in a container for a long time. The comparison of the ensembles
is performed for the fragment charge distribution and the excitation energies.
Our calculations show that there exists an equilibrium ensemble which well
reproduces the reaction ensemble at each reaction time for the investigated
period fm/. However, there are some other observables
which show discrepancies between the reaction and equilibrium ensembles. These
may be interpreted as dynamical effects in the reaction. The usual static
equilibrium at each instant is not realized since any equilibrium ensemble with
the same volume as that of the reaction system cannot reproduce the fragment
observables.Comment: 13 pages and 12figures; added a few sentences and corrected typos,
accepted in Phys. Rev.
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