An experimental indication of negative heat capacity in excited nuclear systems is inferred from the event by event study of energy fluctuations in Au quasi-projectile sources formed in Au + Au collisions at 35 A.MeV. The excited source configuration is reconstructed through a calorimetric analysis of its de-excitation products. Fragment partitions show signs of a critical behavior at about 5 A.MeV excitation energy. In the same energy range the heat capacity shows a negative branch providing a direct evidence of a first order liquid gas phase transition.Phase transitions are the prototype of a complex system behavior which goes beyond the simple sum of individual properties [1]. In macroscopic systems the thermostatistical potential presents non analytical behaviors which unambiguously marks a phase transition. Non analytical behaviors of infinite systems originate from anomalies of the thermostatistical potentials in finite systems [2,3]. Specifically in microcanonical finite systems, the entropy is known to present a convex intruder in 1-st order phase transitions associated to a negative heat capacity between two poles. A 2-nd order phase transition is characterized by the merging of the two poles.The experimental study of phase transitions in finite systems has recently attracted a strong interest from various communities. Bose condensates with a small number of particles [4], melting of solid atomic clusters [5], vaporization of atomic nuclei [6] are examples of attempts to study phase transitions in finite systems. The problem usually encountered with these small systems is how to control the equilibrium and how to extract the thermostatistical variables from observable quantities in order to identify the possible phase transition. This is for instance the case in heavy ion reactions in which excited nuclear systems are formed. Comparing the observed decay channels with statistical models [2,7] it seems that a certain degree of equilibration is reached [8,9] but up to now it has not been possible to unambiguously identify the presence of the expected liquid-gas phase transition.It has recently been shown [3] that for a given total energy the average partial energy stored in a part of the system is a good microcanonical thermometer while the associated fluctuations can be used to construct the heat capacity. In the case of a phase transition anomalously large fluctuations are expected as a consequence of the divergence and of the possible negative branch of the heat capacity. Let us consider an equilibrated system which can be decomposed into two independent components so that the energy is simply the sum of the two partial energies E t = E 1 + E 2 and that the total level density W t ≡ exp(S t ) is the folding product of the two partial level densities W i ≡ exp(S i ).An example of such a decomposition is given by the kinetic and the potential energies in the absence of velocity dependent interactions.The probability distribution of the partial energy where: , 2) are the heat capacities calculated for th...

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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 (M.D'Agostino et al.-Phys. Lett. B 473 (2000) 219, N. Le Neindre et al.- contr. to the XXXVIII Bormio Winter Meeting on Nucl. Phys. (2001) 404) is a genuine signal of a first order phase transition in a finite system.Comment: 15 Postscript figures, submitted to NUCL. Phys. A on 24-apr-200

Nuclear stopping has been investigated in central nuclear collisions at intermediate energies by analyzing kinematically complete events recorded with the help of the 4π multidetector INDRA for a large variety of symmetric systems. It is found that the mean isotropy ratio defined as the ratio of transverse to parallel momenta (energies) reaches a minimum near the Fermi energy, saturates or slowly increases depending on the mass of the system as the beam energy increases, and then stays lower than unity, showing that significant stopping is not achieved even for the heavier systems. Close to and above the Fermi energy, experimental data show no effect of the isospin content of the interacting system. A comparison with transport model calculations reveals that the latter overestimates the stopping power at low energies.

The properties of fragments and light charged particles emitted in multifragmentation of single sources formed in central 36 A.MeV Gd+U collisions are reviewed. Most of the products are isotropically distributed in the reaction c.m. Fragment kinetic energies reveal the onset of radial collective energy. A bulk effect is experimentally evidenced from the similarity of the charge distribution with that from the lighter 32 A.MeV Xe+Sn system. Spinodal decomposition of finite nuclear matter exhibits the same property in simulated central collisions for the two systems, and appears therefore as a possible mechanism at the origin of multifragmentation in this incident energy domain.

Characteristics of the primary fragments produced in central collisions of129 Xe + nat Sn from 32 to 50 AMeV have been obtained. By using the correlation technique for the relative velocity between light charged particles (LCP) and fragments, we were able to extract the multiplicities and average kinetic energy of secondary evaporated LCP. We then reconstructed the size and excitation energy of the primary fragments. For each bombarding energy a constant value of the excitation energy per nucleon over the whole range of fragment charge has been found. This value saturates at 3 AMeV for beam energies 39 AMeV and above. The corresponding secondary evaporated LCP represent less than 40% of all produced particles and decreases down to 23% for 50 AMeV. The experimental characteristics of the primary fragments are compared to the predictions of statistical multifragmentation model (SMM) calculations. Reasonable agreement between the data and the calculation has been found for any given incident energy. However SMM fails to reproduce the trend of the excitation function of the primary fragment excitation energy and the amount of secondary evaporated LCP's.

Enhanced production of events with almost equal-sized fragments is experimentally revealed by charge correlations in the multifragmentation of a finite nuclear system selected in 129 Xe central collisions on nat Sn. The evolution of their weight with the incident energy: 32, 39, 45, 50 AMeV, is measured. Dynamical stochastic mean field simulations performed at 32 AMeV, in which spinodal instabilities are responsible for multifragmentation, exhibit a similar enhancement of this kind of events. The above experimental observation evidences the spinodal decomposition of hot finite nuclear matter as the origin of multifragmentation in the Fermi energy regime.

Multifragmentation of a "fused system" was observed for central collisions between 32 MeV/nucleon 129Xe and (nat)Sn. Most of the resulting charged products were well identified due to the high performances of the INDRA 4pi array. Experimental higher-order charge correlations for fragments show a weak but nonambiguous enhancement of events with nearly equal-sized fragments. Supported by dynamical calculations in which spinodal decomposition is simulated, this observed enhancement is interpreted as a "fossil" signal of spinodal instabilities in finite nuclear systems.