Francesca Gulminelli

Thermodynamical features of multifragmentation in peripheral Au + Au Collisions at 35 A.MeV

Abstract The distribution of fragments produced in events involving the multifragmentation of excited sources is studied for peripheral Au + Au reactions at 35 A MeV. The Quasi-Projectile has been reconstructed from its de-excitation products. An isotropic emission in its rest frame has been observed, indicating that an equilibrated system has been formed. The excitation energy of the Quasi-Projectile has been determined via calorimetry. A new event by event effective thermometer is proposed based on the energy balance. A peak in the energy fluctuations is observed related to the heat capacity suggesting that the system undergoes a liquid-gas type phase transition at an excitation energy ∼ 5 A MeV and a temperature 4–6 MeV, dependent on the freeze-out hypothesis. By analyzing different regions of the Campiplot, the events associated with the liquid and gas phases as well as the critical region are thermodynamically characterized. The critical exponents, τ, β, γ, extracted from the high moments of the charge distribution are consistent with a liquid-gas type phase transition.

Surveying the nuclear caloric curve

Abstract The 4π array INDRA was used to detect nearly all charged products emitted in Ar + Ni collisions between 52 and 95 MeV/u. The charge, mass and excitation energy E ∗ of the quasi-projectiles have been reconstructed event by event. Excitation energies up to 25 MeV per nucleon are reached. Apparent temperatures obtained from several double isotopic yield ratios Tr0 show different dependences upon E ∗ . T6Li7Liue5f83Heα0 yields the highest values, as well as the high energy slopes Ts of the kinetic energy spectra. Two statistical models, sequential evaporation and gas in complete equilibrium, taking into account side feeding and discrete excited states population, show that the data can be explained by a steady increase of the initial temperature with excitation energy without evidence for a liquid-gas phase transition.

Bimodal behavior of the heaviest fragment distribution in projectile fragmentation.

The charge distribution of the heaviest fragment detected in the decay of quasiprojectiles produced in intermediate energy heavy-ion collisions has been observed to be bimodal. This feature is expected as a generic signal of phase transition in nonextensive systems. In this Letter, we present new analyses of experimental data from Au on Au collisions at 60, 80, and 100 MeV/nucleon showing that bimodality is largely independent of the data selection procedure and of entrance channel effects. An estimate of the latent heat of the transition is extracted.

Theoretical comparison of different thermometers for the determination of the nuclear caloric curve

Abstract The problem of the experimental determination of a nuclear temperature in heavy ion collisions is addressed within an extended Quantum Statistical Model which includes secondary evaporation from continuum states and an effective excluded volume interaction in the freeze-out configuration. The comparison to experimental data from the INDRA Collaboration suggests that thermodynamical equilibration can actually be reached even at excitation energies as high as 25 MeV/u, however final state interactions and side feeding can strongly deform the response of the different thermometers.

Distribution of the largest fragment in the lattice gas model

The distribution of the largest fragment is studied in different regions of the lattice gas model phase diagram. We show that first- and second-order transitions can be clearly distinguished in the grancanonical ensemble, while signals typical of a continuous transition are seen inside the coexistence region if a mass conservation constraint is applied. Some possible implications of these findings for heavy-ion multifragmentation experiments are discussed.

Microscopic description of dissipative fragmentation

Abstract A method to calculate intermediate mass fragment production in intermediate energy heavy ion collisions with a self-consistent kinetic equation approach is presented. Inclusive mass distributions, energy spectra and mass correlations are compared to experimental data.

Phase Transitions in Finite Systems

In this series of lectures we will first review the general theory of phase transition in the framework of information theory and briefly address some of the well known mean field solutions of three dimensional problems. The theory of phase transitions in finite systems will then be discussed, with a special emphasis to the conceptual problems linked to a thermodynamical description for small, short-lived, open systems as metal clusters and data samples coming from nuclear collisions. The concept of negative heat capacity developed in the early seventies in the context of self-gravitating systems will be reinterpreted in the general framework of convexity anomalies of thermostatistical potentials. The connection with the distribution of the order parameter will lead us to a definition of first order phase transitions in finite systems based on topology anomalies of the event distribution in the space of observations. Finally a careful study of the thermodynamical limit will provide a bridge with the standard theory of phase transitions and show that in a wide class of physical situations the different statistical ensembles are irreducibly inequivalent.

Kinematical properties and composition of vaporizing sources: is thermodynamical equilibrium achieved?

Abstract Kinematical properties and chemical composition of vaporizing sources observed in heavy-ion collisions are presented. Vaporizing sources detected with the 4π array INDRA, are defined as sources for which all detected species have atomic numbers lower than 3. The excitation energy of the sources covers a very broad range (6–28 A MeV). The occurrence of thermodynamical equilibrium is discussed by comparing the data with the results of two statistical models. The model describing a gas in thermal and chemical equilibrium reproduces rather well the data suggesting that thermodynamical equilibrium may have been reached for such sources.

Isospin-dependent clusterization of neutron-star matter

Abstract Because of the presence of a liquid-gas phase transition in nuclear matter, compact-star matter can present a region of instability against the formation of clusters. We investigate this phase separation in a matter composed of neutrons, protons and electrons, within a Skyrme–Lyon mean-field approach. Matter instability and phase properties are characterized through the study of the free-energy curvature. The effect of β -equilibrium is also analyzed in detail, and we show that the opacity to neutrinos has an influence on the presence of clusterized matter in finite-temperature proto-neutron stars.

Equation of state for dense nucleonic matter from metamodeling. I. Foundational aspects

A metamodeling for the nucleonic equation of state (EOS), inspired from a Taylor expansion around the saturation density of symmetric nuclear matter, is proposed and parameterized in terms of the empirical parameters. The present knowledge of nuclear empirical parameters is first reviewed in order to estimate their average values and associated uncertainties, and thus defining the parameter space of the metamodeling. They are divided into isoscalar and isovector type, and ordered according to their power in the density expansion. The goodness of the metamodeling is analyzed against the predictions of the original models. In addition, since no correlation among the empirical parameters is assumed a priori, all arbitrary density dependences can be explored, which might not be accessible in existing functionals. Spurious correlations due to the assumed functional form are also removed. This meta-EOS allows direct relations between the uncertainties on the empirical parameters and the density dependence of the nuclear equation of state and its derivatives, and the mapping between the two can be done with standard Bayesian techniques. A sensitivity analysis shows that the more influential empirical parameters are the isovector parameters