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

Geant4 hadronic physics status and validation for large HEP detectors

Optimal exploitation of hadronic final states played a key role in successes of all recent collider experiment in HEP, and the ability to use hadronic final states will continue to be one of the decisive issues during the analysis phase of the LHC experiments. Monte Carlo techniques facilitate the use of hadronic final states, and have been developed for many years. We will give a brief overview of the physics underlying hadronic shower simulation, discussing the three basic types of modeling; data driven, parametrization driven, and theory driven modeling at the example of Geant4. We will confront these different types of modeling with the stringent requirements posed by the LHC experiments on hadronic shower simulation, and report on the current status of the validation effort for large HEP applications. We will address robustness, and CPU and physics performance evaluations.Comment: Computing in High Energy and Nuclear Physics, La Jolla, California, March 24-28, 2003 1 tar fil

Multifragmentation of non-spherical nuclei

The shape influence of decaying thermalized source on various characteristics of multifragmentation as well as its interplay with effects of angular momentum and collective expansion are first studied and the most pertinent variables are proposed. The analysis is based on the extension of the statistical microcanonical multifragmentation model.Comment: 5 pages, 4 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

Calculation of the number of partitions with constraints on the fragment size

This article introduces recursive relations allowing the calculation of the number of partitions with constraints on the minimum and/or on the maximum fragment size

Influence of the Coulomb Interaction on the Chemical Equilibrium of Nuclear Systems at Break-Up

The importance of a Coulomb correction to the formalism proposed by Albergo et al. for determining the temperatures of nuclear systems at break-up and the ensities of free nucleon gases is discussed. While the proposed correction has no effect on the temperatures extracted based on double isotope ratios, it becomes non-negligible when such temperatures or densities of free nucleon gases are extracted based on multiplicities of heavier fragments of different atomic numbers

Mass Parameterizations and Predictions of Isotopic Observables

We discuss the accuracy of mass models for extrapolating to very asymmetric nuclei and the impact of such extrapolations on the predictions of isotopic observables in multifragmentation. We obtain improved mass predictions by incorporating measured masses and extrapolating to unmeasured masses with a mass formula that includes surface symmetry and Coulomb terms. We find that using accurate masses has a significant impact on the predicted isotopic observables.Comment: 12 pages, 4 figure

Searching for the statistically equilibrated systems formed in heavy ion collisions

Further improvements and refinements are brought to the microcanonical multifragmentation model [Al. H. Raduta and Ad. R. Raduta, Phys. Rev. C {\bf 55}, 1344 (1997); {\it ibid.} {\bf 61}, 034611 (2000)]. The new version of the model is tested on the recently published experimental data concerning the Xe+Sn at 32 MeV/u and Gd+U at 36 MeV/u reactions. A remarkable good simultaneous reproduction of fragment size observables and kinematic observables is to be noticed. It is shown that the equilibrated source can be unambiguously identified.Comment: Physical Review C, in pres

Isospin Effects in Nuclear Multifragmentation

We develop an improved Statistical Multifragmentation Model that provides the capability to calculate calorimetric and isotopic observables with precision. With this new model we examine the influence of nuclear isospin on the fragment elemental and isotopic distributions. We show that the proposed improvements on the model are essential for studying isospin effects in nuclear multifragmentation. In particular, these calculations show that accurate comparisons to experimental data require that the nuclear masses, free energies and secondary decay must be handled with higher precision than many current models accord.Comment: 46 pages, 16 figure