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

Employing ternary fission of 242^{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

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

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

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

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 $^{129}Xe$ 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 ($A_{max}$) 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

Limiting Temperatures and the Equation of State of Nuclear Matter

From experimental observations of limiting temperatures in heavy ion collisions we derive Tc, the critical temperature of infinite nuclear matter. The critical temperature is 16.6 +- 0.86 MeV. Theoretical model correlations between Tc, the compressibility modulus, K the effective mass, $m^*$ and the saturation density, rho_s, are exploited to derive the quantity (K/m^*)**1/2*rho_s^{-1/3}$. This quantity together with calculations employing Skyrme and Gogny interactions indicates a nuclear matter incompressibility in moderately excited nuclei that is in excellent agreement with the value determined from Giant Monopole Resonance data. This technique of extraction of K may prove particularly useful in investigations of very neutron rich systems using radioactive beams.Comment: 4 pages, 5 figure