Bimodalities : a survey of experimental data and models

Bimodal distributions of some chosen variables measured in nuclear collisions were recently proposed as a non ambiguous signature of a first order phase transition in nuclei. This section presents a compilation of both theoretical and experimental studies on bimodalities performed so far, in relation with the liquid-gas phase transition in nuclear matter.Comment: 12 pages, 18 figures, 1 table Appeared in European Physics Journal A as part of the Topical Volume "Dynamics and Thermodynamics with Nuclear Degrees of Freedom

CONSTRAINING THE DENSITY DEPENDENCE OF THE SYMMETRY ENERGY WITH EXPERIMENTAL RESULTS FROM HEAVY-ION COLLISIONS

The equation of state of asymmetric nuclear matter is still controversial, as predictions at subsaturation as well as above-normal density diverge widely. We discuss in this lecture several experimental observables measured in heavy-ion collisions in the energy range 20-400 AMeV. Estimates of the density dependence of the symmetry energy are derived from comparison of experimental results with those of transport codes with different implementatiosn of the potential part of the symmetry energy, or in statistical model frameworks

What Can be Learned Studying the Distribution of the Biggest Fragment ?

In the canonical formalism of statistical physics, a signature of a first order phase transition for finite systems is the bimodal distribution of an order parameter. Previous thermodynamical studies of nuclear sources produced in heavy-ion collisions provide information which support the existence of a phase transition in those finite nuclear systems. Some results suggest that the observable Z1 (charge of the biggest fragment) can be considered as a reliable order parameter of the transition. This talk will show how from peripheral collisions studied with the INDRA detector at GSI we can obtain this bimodal behaviour of Z1. Getting rid of the entrance channel effects and under the constraint of an equiprobable distribution of excitation energy (E*), we use the canonical description of a phase transition to link this bimodal behaviour with the residual convexity of the entropy. Theoretical (with and without phase transition) and experimental Z1-E* correlations are compared. This comparison allows us to rule out the case without transition. Moreover that quantitative comparison provides us with information about the coexistence region in the Z1-E* plane which is in good agreement with that obtained with the signal of abnormal uctuations of configurational energy (microcanonical negative heat capacity).Comment: 8 page

Ion stopping in dense plasma target for high energy density physics

The basic physics of nonrelativistic and electromagnetic ion stopping in hot and ionized plasma targets is thoroughly updated. Corresponding projectile-target interactions involve enhanced projectile ionization and coupling with target free electrons leading to significantly larger energy losses in hot targets when contrasted to their cold homologues. Standard stoppping formalism is framed around the most economical extrapolation of high velocity stopping in cold matter. Further elaborations pay attention to target electron coupling and nonlinearities due to enhanced projectile charge state, as well. Scaling rules are then used to optimize the enhanced stopping of MeV/amu ions in plasmas with electron linear densities nel ~ 10 18 -10 20 cm -2 . The synchronous firing of dense and strongly ionized plasmas with the time structure of bunched and energetic multicharged ion beam then allow to probe, for the first time, the long searched enhanced plasma stopping and projectile charge at target exit. Laser ablated plasmas (SPQR1) and dense linear plasma columns (SPQR2) show up as targets of choice in providing accurate and on line measurements of plasma parameters. Corresponding stopping results are of a central significance in asserting the validity of intense ion beam scenarios for driving thermonuclear pellets. Other applications of note feature thorium induced fission, novel ion sources and specific material processing through low energy ion beams. Last but not least, the given ion beam-plasma target interaction physics is likely to pave a way to the production and diagnostics of warm dense matter (WDM)

CONSTRAINING THE DENSITY DEPENDENCE OF THE SYMMETRY ENERGY WITH EXPERIMENTAL RESULTS FROM HEAVY-ION COLLISIONS

The equation of state of asymmetric nuclear matter is still controversial, as predictions at subsaturation as well as above-normal density diverge widely. We discuss in this lecture several experimental observables measured in heavy-ion collisions in the energy range 20-400 AMeV. Estimates of the density dependence of the symmetry energy are derived from comparison of experimental results with those of transport codes with different implementatiosn of the potential part of the symmetry energy, or in statistical model frameworks

Voir bouillir les noyaux atomiques avec INDRA

En 1993 le multi-détecteur INDRA entrait en fonctionnement auprès du Grand accélérateur national d’ions lourds (Ganil) à Caen. Pendant une vingtaine d’années les physiciens utilisant cet outil d’exception allaient produire des avancées scientifiques dans le domaine des collisions noyau-noyau. La transition de phase des noyaux chauds, comparable au passage d’un fluide de l’état liquide à l’état gazeux, était un des sujets étudiés.In 1993 the Indra array started operating at the Grand Accélérateur National d’Ions Lourds (GANIL) in Caen (France). For around twenty years the physicists using this outstanding instrument gave rise to scientific advances in the comprehension of nucleus-nucleus collisions. Phase transition of hot nuclei, similar to the liquid-gas transition, was one of the subjects studied

Effets d'isospin et noyaux chauds

La décomposition rapide (t<10-21 seconde) du noyau en plusieurs fragments, appelée multifragmentation, est théoriquement associée à une transition de phase de type liquide-gaz. Depuis plusieurs années, les physiciens tentent d'observer une preuve expérimentale de ce phénomène. Afin d'y parvenir, il a été suggéré d'étudier une signature propre du mécanisme par lequel une telle transition procéderait: la décomposition spinodale. La signature de ce mécanisme est la production de fragments de tailles égales. La méthode proposée pour observer cette signature est celle des corrélations en charge à l'aide du calcul des probabilités intrinsèques. La 5ième campagne d'expériences du multidétecteur INDRA a été réalisée au GANIL afin d'obtenir la statistique nécessaire pour avoir un signal positif avec un niveau de confiance de 5 . Les systèmes 124Xe+112Sn et 136Xe+124Sn ont été étudiés à 32 et 45 AMeV afin d'étudier l'influence du ratio N/Z sur la transition de phase dans les noyaux. La présence d'événements avec une distribution en charge très étroite, conformément à l'hypothèse d'une décomposition spinodale a été confirmée. L'impact de la densité neutronique sur la configuration de la voie de sortie des fragments, dans les collisions centrales a été mis en évidence : un système initialement riche en neutrons produit davantage de fragments et moins de particules qu'un système initialement pauvre. Finalement, une étude du ratio N/Z des fragments en fonction de leur énergie cinétique, dans le but d'obtenir une contrainte expérimentale sur l'énergie de symétrie est réalisée.The rapid decomposition (t<10-21 seconds) of a nucleus into multiple fragments, named multifragmentation, is associated to a liquid-gas phase transition. For many years, physicists have tried to obtain an experimental proof of this behaviour. It has been suggested that, to achieve this, one could observe the particular signature of the mechanism of such a transition: the spinodal decomposition, through the production of equal size fragments. A method is that of charge correlation using intrinsic probabilities. The 5th campaign for the INDRA multidetector which took place at GANIL was aimed at obtaining a high number of events, in order to sign the spinodal decomposition with a high confidence level. The chosen systems were 124Xe+112Sn and 136Xe+124Sn at beam energies of 32 and 45 AMeV. The acquired statistics allowed to confirm the existence of events with very narrow charge distributions, which agrees with the spinodal decomposition hypothesis. The study of 124,136Xe+112,124Sn at 32 and 45 AMeV shows the impact of neutron density on the exit channel: a neutron-rich system produces more fragments and fewer particles than a system initially neutron-poor. Finally, the modification on 27 modules from the INDRA multidetector allows the isotopic resolution of fragments up to oxygen (Z=8). This provides the information required for the study of the N/Z ratio of fragments versus their kinetic energy with the goal of obtaining an experimental constraint on the symmetry term of the equation of state.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Correlations between signals of the liquid-gas phase transition in nuclei

Finite systems such as atomic nuclei present at phase transition specific features different from those observed at the thermodynamic limit. Several characteristic signals were found in samples of events resulting from heavy ion collisions at and above the Fermi energy. The concomitant observation of different signatures of a liquid-gas phase transition in nuclei on a given sample strongly supports the occurrence of this transition

Bimodal behavior of the heaviest fragment distribution in projectile fragmentation

5 pages, submitted to PRLInternational audienceThe charge distribution of the heaviest fragment detected in the decay of quasi-projectiles 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 non-extensive systems. In this paper 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