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...

No abstract

We report on the measurement of the ^{7}Be(n,p)^{7}Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the lithium yield of the big bang nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and they showed a large discrepancy between each other. The measurement was performed with a Si telescope and a high-purity sample produced by implantation of a ^{7}Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low energy, relative to current evaluations, in the region of BBN interest, the present results are consistent with the values inferred from the time-reversal ^{7}Li(p,n)^{7}Be reaction, thus yielding only a relatively minor improvement on the so-called cosmological lithium problem. The relevance of these results on the near-threshold neutron production in the p+^{7}Li reaction is also discussed.

No abstract

No abstract