Methane anomalies over TAG hydrothermal field on Mid Atlantic Ridge

Black smoker type hydrothermal venting was discovered in July 1985 at the TAG hydrothermal field in the rift valley of the Mid Atlantic Ridge near 26N. We present here, the first methane profiles in the seawater column over this black smoker along with methane profiles away from the black smokers for comparison. The CH4 plume, spreading over 400 m above the seafloor, shows a maximum concentration of 105 nl/l at 3450 m (238 m above the bottom), correlated with a positive temperature anomaly (0.02°C). CH4 concentrations up to 2422 nl/l, correlated with a huge positive increase of in situ temperature (0.349°C), have been measured in samples taken 5 m above the sea floor, showing an important CH4 input in this slow spreading site which is similar to inputs from fast spreading sites like the East Pacific Rise

Thermodynamical features of multifragmentation in peripheral Au + Au Collisions at 35 A.MeV

The distribution of fragments produced in events involving the multifragmentation of excited sources is studied for peripheral Au + Au reactions at 35 A.MeV. The Quasi-Projectile has been reconstructed from its de-excitation products. An isotropic emission in its rest frame has been observed, indicating that an equilibrated system has been formed. The excitation energy of the Quasi-Projectile has been determined via calorimetry. A new event by event effective thermometer is proposed based on the energy balance. A peak in the energy fluctuations is observed related to the heat capacity, suggesting that the system undergoes a liquid-gas type phase transition at an excitation energy about 5 A.MeV and a temperature 4 - 6 MeV, dependent on the freeze-out hypothesis. By analyzing different regions of the Campi-plot, the events associated with the liquid and gas phases as well as the critical region are thermodynamically characterized. The critical exponents, tau, beta,gamma, extracted from the high moments of the charge distribution are consistent with a liquid-gas type phase transition.Comment: 44 pages, 16 Postscript figures, Fig14_nucl-ex.eps in colors, to be published in Nucl.Phys.A (1999

Nuclear multifragmentation time-scale and fluctuations of largest fragment size

Distributions of the largest fragment charge, Zmax, in multifragmentation reactions around the Fermi energy can be decomposed into a sum of a Gaussian and a Gumbel distribution, whereas at much higher or lower energies one or the other distribution is asymptotically dominant. We demonstrate the same generic behavior for the largest cluster size in critical aggregation models for small systems, in or out of equilibrium, around the critical point. By analogy with the time-dependent irreversible aggregation model, we infer that Zmax distributions are characteristic of the multifragmentation time-scale, which is largely determined by the onset of radial expansion in this energy range.Comment: Accepted for publication in Physical Review Letters on 8/4/201

Topology of event distribution as a generalized definition of phase transitions in finite systems

We propose a definition of phase transitions in finite systems based on topology anomalies of the event distribution in the space of observations. This generalizes all the definitions based on the curvature anomalies of thermodynamical potentials and provides a natural definition of order parameters. The proposed definition is directly operational from the experimental point of view. It allows to study phase transitions in Gibbs equilibria as well as in other ensembles such as the Tsallis ensemble.Comment: 4 pages, 3 figure

Constrained caloric curves and phase transition for hot nuclei

Simulations based on experimental data obtained from multifragmenting quasi-fused nuclei produced in central $^{129}$Xe + $^{nat}$Sn collisions have been used to deduce event by event freeze-out properties in the thermal excitation energy range 4-12 AMeV [Nucl. Phys. A809 (2008) 111]. From these properties and the temperatures deduced from proton transverse momentum fluctuations, constrained caloric curves have been built. At constant average volumes caloric curves exhibit a monotonic behaviour whereas for constrained pressures a backbending is observed. Such results support the existence of a first order phase transition for hot nuclei.Comment: 14 pages, 5 figures, accepted in Physics Letters

Isospin diffusion in semi-peripheral 58Ni^{58}Ni + 197Au^{197}Au collisions at intermediate energies (I): Experimental results

Isospin diffusion in semi-peripheral collisions is probed as a function of the dissipated energy by studying two systems $^{58}Ni$ + $^{58}Ni$ and $^{58}Ni$ + $^{197}Au$, over the incident energy range 52-74\AM. A close examination of the multiplicities of light products in the forward part of phase space clearly shows an influence of the isospin of the target on the neutron richness of these products. A progressive isospin diffusion is observed when collisions become more central, in connection with the interaction time

Isospin Diffusion in 58^{58}Ni-Induced Reactions at Intermediate Energies

Isospin diffusion is probed as a function of the dissipated energy by studying two systems $^{58}$Ni+$^{58}$Ni and $^{58}$Ni+$^{197}$Au, over the incident energy range 52-74\AM. Experimental data are compared with the results of a microscopic transport model with two different parameterizations of the symmetry energy term. A better overall agreement between data and simulations is obtained when using a symmetry term with a potential part linearly increasing with nuclear density. The isospin equilibration time at 52 \AM{} is estimated to 130$\pm$10 fm/$c$

Status and performances of the FAZIA project

FAZIA is designed for detailed studies of the isospin degree of freedom, extending to the limits the isotopic identification of charged products from nuclear collisions when using silicon detectors and CsI(Tl) scintillators. We show that the FAZIA telescopes give isotopic identification up to Z$\sim$25 with a $\Delta$E-E technique. Digital Pulse Shape Analysis makes possible elemental identification up to Z=55 and isotopic identification for Z=1-10 when using the response of a single silicon detector. The project is now in the phase of building a demonstrator comprising about 200 telescopes