437 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

### Constraining supernova equations of state with equilibrium constants from heavy-ion collisions

Cluster formation is a fundamental aspect of the equation of state (EOS) of
warm and dense nuclear matter such as can be found in supernovae (SNe). Similar
matter can be studied in heavy-ion collisions (HIC). We use the experimental
data of Qin et al. [Phys. Rev. Lett. 108, 172701 (2012)] to test calculations
of cluster formation and the role of in-medium modifications of cluster
properties in SN EOSs. For the comparison between theory and experiment we use
chemical equilibrium constants as the main observables. This reduces some of
the systematic uncertainties and allows deviations from ideal gas behavior to
be identified clearly. In the analysis, we carefully account for the
differences between matter in SNe and HICs. We find that, at the lowest
densities, the experiment and all theoretical models are consistent with the
ideal gas behavior. At higher densities ideal behavior is clearly ruled out and
interaction effects have to be considered. The contributions of continuum
correlations are of relevance in the virial expansion and remain a difficult
problem to solve at higher densities. We conclude that at the densities and
temperatures discussed mean-field interactions of nucleons, inclusion of all
relevant light clusters, and a suppression mechanism of clusters at high
densities have to be incorporated in the SN EOS.Comment: 20 pages, 15 figures, v2: matches published version, only minor
editorial correction

### 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 $^{242}$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 $^{241}$Pu($n_{\rm th}$,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 $6.7 \times 10^{-5}$ nucleons/fm$^3$ and
proton fraction $Y_p$ = 0.035 are found to provide a good representation of
yields of the ternary emitted light particles and clusters. In particular,
results for $Z= 1$ and 2 isotopes are presented. Isotopes with larger $Z$ are
discussed, and the roles of medium and continuum effects, even at very low
density are illustrated.Comment: 19 pages, 5 figure

### Liquid-gas phase transition in finite nuclei

In a finite temperature Thomas-Fermi framework, we calculate density
distributions of hot nuclei enclosed in a freeze-out volume of few times the
normal nuclear volume and then construct the caloric curve, with and without
inclusion of radial collective flow. In both cases, the calculated specific
heats $C_v$ show a peaked structure signalling a liquid-gas phase transition.
Without flow, the caloric curve indicates a continuous phase transition whereas
with inclusion of flow, the transition is very sharp. In the latter case, the
nucleus undergoes a shape change to a bubble from a diffuse sphere at the
transition temperature.Comment: Proc. of 6th Int. Conf. on N-N Collisions (Gatlinburg); Nuclear
Physics A (in press

### 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

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