M. Bruno

On the reliability of negative heat capacity measurements

Abstract A global protocol for the thermostatistical analysis of hot nuclear sources is discussed. Within our method of minimization of variances we show that the abnormal kinetic energy fluctuation signal recently reported in different experimental data from the MULTICS and INDRA collaborations is axa0genuine signal of axa0first-order phase transition in a finite system.

Isoscaling in the lattice gas model

A good quality scaling of the cluster size distributions is obtained for the Lattice Gas Model using the Fishers ansatz for the scaling function. This scaling identifies a pseudo-critical line in the phase diagram of the model that spans the whole (subcritical to supercritical) density range. The independent cluster hypothesis of the Fisher approach is shown to describe correctly the thermodynamics of the lattice only far away from the critical point.

Critical-like behaviours in central and peripheral collisions: a comparative analysis

Quasi-projectile events from peripheral 35 A MeV Au + Au collisions are compared with 25 A MeV Au+ C, 25 and 35 A MeV Au+ Cu and 35 A MeV Au+ Au central events. By fitting the measured charge yields with the Fisher droplet model technique, all the different data sets coherently point to a value E ∗ ≈ 4.5 A MeV as the excitation energy where size distribution are best fitted by a power law. The physical parameters extracted from the fit strongly depend on the detailed shape assumed for the scaling function with the exception of τ and σ which are consistent with the critical exponents of the liquid-gas phase transition. Possible implications concerning the observation of a phase transition in multifragmentation experiments are discussed. uf6d9 2003 Elsevier B.V. All rights reserved.

Signals of bimodality in the fragmentation of Au quasi-projectiles

Signals of bimodality have been investigated in experimental data of quasi-projectile decay produced in Au+Au collisions at 35 AMeV. This same data set was already shown to provide several signals characteristic of a first order, liquid-gas-like phase transition. Different event sortings proposed in the recent literature are analyzed. A sudden change in the fragmentation pattern is revealed by the distribution of the charge of the largest fragment, compatible with a bimodal behavior.

Probing the Statistical Decay and α-clustering effects in 12C + 12C and 14N + 10B reactions

An experimental campaign has been undertaken at Laboratori Nazionali di Legnaro (LNL INFN), Italy, in order to progress in our understanding of the statistical properties of light nuclei at excitation energies above particle emission threshold, by mea- suring exclusive data from fusion-evaporation reactions. On the experimental side, a first reaction: 12 C+ 12 C at 95 MeV beam energy has been measured, using the GARFIELD+ Ring Counter (RCo) apparatuses. Fusion-evaporation events have been exclusively se- lected out of the entire data set. The comparison to a dedicated Hauser-Feshbach calcu- lation allows us to give constraints on the nuclear level density at high excitation energy for light systems ranging from C up to Mg. Out-of-equilibriumemission has been evidenced and attributed both to an entrance channel effect (favoured by the cluster na- ture of reaction partners), and, in more dissipative events, to the persistence of cluster correlations well above the 24 Mg threshold for 6 �s decay. In order to study the same 24 Mg compound nucleus at similar excitation energy with respect to this first reaction a new measurement, 14 N + 10 B at 5.7 A.MeV, was performed at LNL laboratories with the same experimental setup. The comparison between the two systems would allow us to further constrain the level density of light nuclei in the mass-excitation energy range of interest. In this perspective, deviations from a statistical behaviour can be used as a tool to get information on nuclear clustering, both in the ground-state for projectile and target and in the hot source formed in the collision.

Pre-equilibrium emission and clustering in medium-mass nuclei: 46Ti from 16O + 30Si, 18O + 28Si, 19F + 27Al

The study of nuclear cluster states bound by valence neutrons is a field of recent large interest. In particular, it is interesting to study the pre-formation of α-clusters in α-conjugate nuclei and the dynamical condensation of clusters during nuclear reactions. The NUCL-EX collaboration (INFN, Italy) is carrying out a research campaign studying pre-equilibrium emission of light charged particles and cluster properties of light and medium-mass nuclei. For this purpose, a comparative study of the three nuclear reactions: 16O + 30Si, 18O + 28Si and 19F + 27Al, has been recently carried out using the GARFIELD+RCo 4π setup. After a general introduction on the experimental campaign, the preliminary results for the three systems are presented.

Clustering effects in fusion evaporation reactions with light even-even N=Z nuclei

In the recent years, cluster structures have been evidenced in many ground and excited states of light nuclei [1, 2]. In the currently experimental campaign, the NUCL-EX collaboration has measured the12C+12C and14N+10B reactions at 95 MeV and 80 MeV respectively. The experimental data corresponding to complete fusion of target and projectile into an excited24Mg nucleus was compared to the results of a pure statistical model [3, 4]. In addition, data from12C+12C have been analyzed to investigate the decay of the Hoyle state of12C* [12] obtained as an intermediate step in the 6α decay channel of the24Mg* formed in central events.

Clustering effects in fusion evaporation reactions with light even-even N = Z nuclei. The 24Mg and 28Si cases

In the recent years, cluster structures have been evidenced in many ground and excited states of light nuclei [1, 2]. Within the currently ongoing experimental campaign by the NUCL-EX collaboration we have measured the 12C+12C and 14N+10B reactions at 95 MeV and 80 MeV respectively, and compared experimental data corresponding to complete fusion of target and projectile into an excited 24Mg nucleus to the results of a pure statistical model[3, 4].We found clear deviations from the statstical model in the decay pattern: emission channels involving multiple α particles are more probable than expected from a purely statistical behavior. To continue the investigation on light systems, we have recentely measured the 16O+12C reaction at three different beam energies, namely Ebeam = 90, 110 and 130 MeV.

Isospin transport and reaction mechanism in nuclear reactions in the range 20–40 MeV/n

In recent years, many efforts have been devoted to the investigation of the isospin degree of freedom in nuclear reactions. Comparing systems involving partners with different N/Z, it has been possible to investigate the isospin transport process and its influence on the final products population. This can be then related to the symmetry energy term of the nuclear EOS. From the experimental point of view, this task requires detectors able to measure both charge and mass of the emitted products, in the widest possible range of energy and size of the fragments. With this objective, the FAZIA and GARFIELD+RCo apparatus have been used with success in some recent experiments.

Isospin transport in 84Kr+112,124Sn reactions at Fermi energies

Isospin transport phenomena in dissipative heavy ion collisions have been investigated at Fermi energies with a beam of 84Kr at 35AMeV. A comparison of the /Z of light and medium products forward-emitted in the centre of mass frame when the beam impinges on a n-poor 112Sn and a n-rich 124Sn targets is presented. Data were collected by means of a three-layer telescope with very good performances in terms of mass identification (full isotopic resolution up to Z about 20 for ions punching through the first detector layer) built by the FAZIA Collaboration and located just beyond the grazing angle for both reactions. The /Z of the decay products emitted when the n-rich target is used is always higher than that associated to the n-poor one. Since the detector was able to measure only fragments coming from the QuasiProjectile decay and/or neck emission, the observed behaviour can be ascribed to the isospin diffusion, driven by the isospin gradient between QuasiProjectile and QuasiTarget. Moreover, for light fragments the /Z as a function of the lab velocity of the fragment increases when we move from the QuasiProjectile velocity to the centre of mass (neck zone). This effect can be interpreted as an evidence of isospin drift driven by the density gradient between the QuasiProjectile zone (at normal density) and the more diluted neck zone.