733 research outputs found
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 Xe + 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 + 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 + and
+ , 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 Ni-Induced Reactions at Intermediate Energies
Isospin diffusion is probed as a function of the dissipated energy by
studying two systems Ni+Ni and Ni+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 13010 fm/
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 Z25
with a 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
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