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Multi-fragment decays of 129 Xe, 197 Au, and 238 U projectiles in collisions with Be, C, Al, Cu, In, Au, and U targets at energies between E/A = 400 MeV and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 SiCsI(Tl) telescopes the solid-angle coverage of the setup was extended to θ lab = 16 • . This permitted the complete detection of fragments from the projectile-spectator source.The dominant feature of the systematic set of data is the Z bound universality that is obeyed by the fragment multiplicities and correlations. These observables are invariant with respect to the entrance channel if plotted as a function of Z bound , where Z bound is the sum of the atomic numbers Z i of all projectile fragments with Z i ≥ 2. No significant dependence on the bombarding energy nor on the target mass is observed. The dependence of the fragment multiplicity on the projectile mass follows a linear scaling law.The reasons for and the limits of the observed universality of spectator fragmentation are explored within the realm of the available data and with model studies. It is found that the universal properties should persist up to much higher bombarding energies than explored in this work and that they are consistent with universal features exhibited by the intranuclear cascade and statistical multifragmentation models.

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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