Entropy puzzle in small exploding systems

We use a simple hard-core gas model to study the dynamics of small exploding systems. The system is initially prepared in a thermalized state in a spherical container and then allowed to expand freely into the vacuum. We follow the expansion dynamics by recording the coordinates and velocities of all particles until their last collision points (freeze-out). We have found that the entropy per particle calculated for the ensemble of freeze-out points is very close to the initial value. This is in apparent contradiction with the Joule experiment in which the entropy grows when the gas expands irreversibly into a larger volume.Comment: 10 pages, 4 figures, accepted June 17 2003 for publication in Physics Letters

Nuclear liquid-gas phase transition within the lattice gas model

We study the nuclear liquid-gas phase transition on the basis of a two-component lattice gas model. A Metropolis type of sampling method is used to generate microscopic states in the canonical ensemble. The effective equation of state and fragment mass distributions are evaluated in a wide range of temperatures and densities. A definition of the phase coexistence region appropriate for mesoscopic systems is proposed. The caloric curve resulting from different types of freeze-out conditions are presented.Comment: 13 pages including 4 figure

On the free volume in nuclear multifragmentation

In many statistical multifragmentation models the volume available to the $N$ nonoverlapping fragments forming a given partition is a basic ingredient serving to the simplification of the density of states formula. One therefore needs accurate techniques for calculating this quantity. While the direct Monte-Carlo procedure consisting of randomly generating the fragments into the freeze-out volume and counting the events with no overlapped fragments is numerically affordable only for partitions with small $N$, the present paper proposes a Metropolis - type simulation which allows accurate evaluations of the free volume even for cases with large $N$. This procedure is used for calculating the available volume for various situations. Though globally this quantity has an exponential dependence on $N$, variations of orders of magnitude for partitions with the same $N$ may be identified. A parametrization based on the virial approximation adjusted with a calibration function, describing very well the variations of the free volume for different partitions having the same $N$ is proposed. This parametrization was successfully tested within the microcanonical multifragmentation model from [Al. H. Raduta and Ad. R. Raduta, Phys. Rev. C {\bf 55}, 1344 (1997); {\it ibid.}, {\bf 56}, 2059 (1997)]. Finally, it is proven that parametrizations of the free volume solely dependent on $N$ are rather inadequate for multifragmentation studies producing important deviations from the exact results.Comment: 20 pages, 9 figures, Nucl. Phys. A (in press

Thermodynamics of explosions

We present our first attempts to formulate a thermodynamics-like description of explosions. The motivation is partly a fundamental interest in non-equilibrium statistical physics, partly the resemblance of an explosion to the late stages of a heavy-ion collision. We perform numerical simulations on a microscopic model of interacting billiard-ball like particles, and we analyse the results of such simulations trying to identify collective variables describing the degree of equilibrium during the explosion.Comment: 6 pages. Talk presented at "Bologna 2000 - Structure of the nucleus" international conference, May 29 - June 3, Bologna, Italy. Shortened version, to appear in the Proceeding

Limits of complete equilibration of fragments produced in central Au on Au collisions at intermediate energies

Experimental data related to fragment production in central Au on Au collisions were analyzed in the framework of a modified statistical model which considers cluster production both prior and at the equilibrated stage. The analysis provides limits to the number of nucleons and to the temperature of the equilibrated source. The rather moderate temperatures obtained from experimental double-yield ratios of d,t,3He and 4He are in agreement with the model calculations. A phenomenological relation was established between the collective flow and the chemical temperature in these reactions. It was shown that dynamical mechanisms of fragment production, e.g. coalescence, dominate at high energies. It is demonstrated that coalescence may be consistent with chemical equilibrium between the produced fragments. The different meaning of chemical and kinetic temperatures is discussed.Comment: 13 pages, 10 figures, accepted at EPJ

Thermodynamics - a valuable approach to multifragmentation?

Since years it has been vividly debated whether multifragmentation is a thermal or a dynamical process. Recently it has been claimed \cite{toek1,po} that new data allow to decide this question. The conclusion, drawn in these papers, are, however, opposite. Whereas \cite{toek1} states that the behavior of different observables as a function of the fragment multiplicity excludes a thermal origin of the fragments in \cite{po} it has been argued that data show a first order phase transition between a liquid and a gaseous phase. It is the aim of this paper to show that both conclusions are premature. They are based on the salient assumption, that the system is sufficiently large to be susceptible to a canonical description. We will show that this is not the case. A micro canonical approach describes the data as good as dynamical calculations. Hence the quest for the physical origin of multifragmentation continues.Comment: 17 pages, 4 figures, completely revised, accepted for publication in NP

Effects of collective expansion on light cluster spectra in relativistic heavy ion collisions

We discuss the interplay between collective flow and density profiles, describing light cluster production in heavy ion collisions at very high energies. Calculations are performed within the coalescence model. We show how collective flow can explain some qualitative features of the measured deuteron spectra, provided a proper parametrization of the spatial dependence of the single particle phase space distribution is chosen.Comment: 11 pages Latex, 2 figures, to be published in Phys. Lett.

Generating new solutions for relativistic transverse flow at the softest point

Using the method of prolongation we generate new solutions from a simple particular solution for relativistic transverse flow with cylindrical symmetry in 1+3 dimensions. This is an extension of the longitudinal Bjorken flow ansatz and can be applied among others during first order phase transition in an expanding system. The prolongated solution allows for tracing back the flow profile from a transverse flow conjectured at the end of phase transition at CERN SPS heavy ion collisons.Comment: 11 pages LaTeX, 1 ps figur