387 research outputs found
The equation of state and symmetry energy of low density nuclear matter
The symmetry energy of nuclear matter is a fundamental ingredient in the
investigation of exotic nuclei, heavy-ion collisions and astrophysical
phenomena. A recently developed quantum statistical (QS) approach that takes
the formation of clusters into account predicts low density symmetry energies
far above the usually quoted mean field limits. A consistent description of the
symmetry energy has been developed that joins the correct low-density limit
with values calculated from quasi-particle approaches valid near the saturation
density. The results are confronted with experimental values for free symmetry
energies and internal symmetry energies, determined at sub-saturation densities
and temperatures below 10 MeV using data from heavy-ion collisions. There is
very good agreement between the experimental symmetry energy values and those
calculated in the QS approachComment: 16 pages, 10 figures. arXiv admin note: text overlap with
arXiv:0908.234
The Role of Surface Entropy in Statistical Emission of Massive Fragments from Equilibrated Nuclear Systems
Statistical fragment emission from excited nuclear systems is studied within
the framework of a schematic Fermi-gas model combined with Weisskopf's detailed
balance approach. The formalism considers thermal expansion of finite nuclear
systems and pays special attention to the role of the diffuse surface region in
the decay of hot equilibrated systems. It is found that with increasing
excitation energy, effects of surface entropy lead to a systematic and
significant reduction of effective emission barriers for fragments and,
eventually, to the vanishing of these barriers. The formalism provides a
natural explanation for the occurrence of negative nuclear heat capacities
reported in the literature. It also accounts for the observed linearity of
pseudo-Arrhenius plots of the logarithm of the fragment emission probability
{\it versus} the inverse square-root of the excitation energy, but does not
predict true Arrhenius behavior of these emission probabilities
Continuous phase transition and negative specific heat in finite nuclei
The liquid-gas phase transition in finite nuclei is studied in a heated
liquid-drop model where the nuclear drop is assumed to be in thermodynamic
equilibrium with its own evaporated nucleonic vapor conserving the total baryon
number and isospin of the system. It is found that in the liquid-vapor
coexistence region the pressure is not a constant on an isotherm indicating
that the transition is continuous. At constant pressure, the caloric curve
shows some anomalies, namely, the systems studied exhibit negative heat
capacity in a small temperature domain. The dependence of this specific feature
on the mass and isospin of the nucleus, Coulomb interaction and the chosen
pressure is studied. The effects of the presence of clusters in the vapor phase
on specific heat have also been explored.Comment: 18 pages, 13 figures; Phys. Rev. C (in press
Employing ternary fission of Pu as a probe of very neutron rich matter
Detailed assessments of the ability of recent theoretical approaches to
modeling existing experimental data for ternary fission confirm earlier
indications that the dominant mode of cluster formation in ternary fission is
clusterization in very neutron rich, very low density, essentially chemically
equilibrated, nucleonic matter. An extended study and comparison of these
approaches applied to ternary fission yields in the thermal neutron induced
reaction Pu(,f) has been undertaken to refine the
characterization of the source matter. The resonance gas approximation has been
improved taking in-medium effects on the binding energies into account. A
temperature of 1.29 MeV, density of nucleons/fm and
proton fraction = 0.035 are found to provide a good representation of
yields of the ternary emitted light particles and clusters. In particular,
results for and 2 isotopes are presented. Isotopes with larger are
discussed, and the roles of medium and continuum effects, even at very low
density are illustrated.Comment: 19 pages, 5 figure
Nucleation and cluster formation in low-density nucleonic matter: A mechanism for ternary fission
Ternary fission yields in the reaction 241Pu(nth,f) are calculated using a
new model which assumes a nucleation-time moderated chemical equilibrium in the
low density matter which constitutes the neck region of the scissioning system.
The temperature, density, proton fraction and fission time required to fit the
experimental data are derived and discussed. A reasonably good fit to the
experimental data is obtained. This model provides a natural explanation for
the observed yields of heavier isotopes relative to those of the lighter
isotopes, the observation of low proton yields relative to 2H and 3H yields and
the non-observation of 3He, all features which are shared by similar thermal
neutron induced and spontaneous fissioning systems.Comment: 6 pages, 3 figure
Liquid-Gas Coexistence and Critical Behavior in Boxed Pseudo-Fermi Matter
A schematic model is presented that allows one to study the behavior of
interacting pseudo-Fermi matter locked in a thermostatic box. As a function of
the box volume and temperature, the matter is seen to show all of the familiar
charactersitics of a Van der Waals gas, which include the coexistence of two
phases under certain circumstances and the presence of a critical point
Cluster emission and phase transition behaviours in nuclear disassembly
The features of the emissions of light particles (LP), charged particles
(CP), intermediate mass fragments (IMF) and the largest fragment (MAX) are
investigated for as functions of temperature and 'freeze-out'
density in the frameworks of the isospin-dependent lattice gas model and the
classical molecular dynamics model. Definite turning points for the slopes of
average multiplicity of LP, CP and IMF, and of the mean mass of the largest
fragment () are shown around a liquid-gas phase transition temperature
and while the largest variances of the distributions of LP, CP, IMF and MAX
appear there. It indicates that the cluster emission rate can be taken as a
probe of nuclear liquid--gas phase transition. Furthermore, the largest
fluctuation is simultaneously accompanied at the point of the phase transition
as can be noted by investigating both the variances of their cluster
multiplicity or mass distributions and the Campi scatter plots within the
lattice gas model and the molecular dynamics model, which is consistent with
the result of the traditional thermodynamical theory when a phase transition
occurs.Comment: replace nucl-th/0103009 due to the technique problem to access old
versio
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