167 research outputs found
Looking for bimodal distributions in multi-fragmentation reactions
The presence of a phase transition in a finite system can be deduced,
together with its order, from the shape of the distribution of the order
parameter. This issue has been extensively studied in multifragmentation
experiments, with results that do not appear fully consistent. In this paper we
discuss the effect of the statistical ensemble or sorting conditions on the
shape of fragment distributions, and propose a new method, which can be easily
implemented experimentally, to discriminate between different fragmentation
scenarii. This method, based on a reweighting of the measured distribution to
account for the experimental constraints linked to the energy deposit, is
tested on different simple models, and appears to provide a powerful
discrimination.Comment: 11 pages, 7 figure
Bimodality and Coulomb effects with a canonical thermodynamic model
The effect of the Coulomb interaction on the phase diagram of finite nuclei
is studied within the Canonical Thermodynamic Model. If Coulomb effects are
artificially switched off, this model shows a phenomenology consistent with the
liquid-gas phase transition. The inclusion of Coulomb does not significantly
affect the phase diagram but it drastically modifies the nature and order
parameter of the transition. A clear understanding of the phenomenon can be
achieved looking at the distribution of the largest fragment produced in each
fragmentation event. Possible connections with experimental observations are
outlined.Comment: Submitted to NP
Experimental Signals of Phase Transition
The connection between the thermodynamics of charged finite nuclear systems
and the asymptotically measured partitions is presented. Some open questions,
concerning in particular equilibrium partitions are discussed. We show a
detailed comparison of the decay patterns in Au+ C,Cu,Au central collisions and
in Au quasi-projectile events. Observation of abnormally large fluctuations in
carefully selected samples of data is reported as an indication of a first
order phase transition (negative heat capacity) in the nuclear equation of
state.Comment: 8 pages, 8th International Conference on Nucleus-Nucleus Collisions,
Moscow 200
Isospin transport in 84Kr + 112,124Sn collisions at Fermi energies
Isotopically resolved fragments with Z<=20 have been studied with high
resolution telescopes in a test run for the FAZIA collaboration. The fragments
were produced by the collision of a 84Kr beam at 35 MeV/nucleon with a n-rich
(124Sn) and a n-poor (112Sn) target. The fragments, detected close to the
grazing angle, are mainly emitted from the phase-space region of the
projectile. The fragment isotopic content clearly depends on the n-richness of
the target and it is a direct evidence of isospin diffusion between projectile
and target. The observed enhanced neutron richness of light fragments emitted
from the phase-space region close to the center of mass of the system can be
interpreted as an effect of isospin drift in the diluted neck region.Comment: 8 pages, 7 figure
N and Z odd-even staggering in Kr + Sn collisions at Fermi energies
The odd-even staggering of the yield of final reaction products has been
studied as a function of proton (Z) and neutron (N) numbers for the collisions
84 Kr+112 Sn and 84 Kr+124 Sn at 35 MeV/nucleon, in a wide range of elements
(up to Z ~ 20). The experimental data show that staggering effects rapidly
decrease with increasing size of the fragments. Moreover the staggering in N is
definitely larger than the one in Z. Similar general features are qualitatively
reproduced by the GEMINI code. Concerning the comparison of the two systems,
the staggering in N is in general rather similar, being slightly larger only
for the lightest fragments produced in the n-rich system. In contrast the
staggering in Z, although smaller than that in N, is sizably larger for the
n-poor system with respect to the n-rich one.Comment: 6 pages, 5 figures, Revtex forma
Energy measurement and fragment identification using digital signals from partially depleted Si detectors
A study of identification properties of a Si-Si DE-E telescope exploiting an
underdepleted residual-energy detector has been performed. Five different bias
voltages have been used, one corresponding to full depletion, the others
associated with a depleted layer ranging from 90% to 60% of the detector
thickness. Fragment identification has been performed using either the DE-E
technique or Pulse Shape Analysis (PSA). Both detectors are reverse mounted:
particles enter from the low field side, to enhance the PSA performance. The
achieved charge and mass resolution has been quantitatively expressed using a
Figure of Merit (FoM). Charge collection efficiency has been evaluated and the
possibility of energy calibration corrections has been considered. We find that
the DE-E performance is not affected by incomplete depletion even when only 60%
of the wafer is depleted. Isotopic separation capability improves at lower bias
voltages with respect to full depletion, though charge identification
thresholds are higher than at full depletion. Good isotopic identification via
PSA has been obtained from a partially depleted detector whose doping
uniformity is not good enough for isotopic identification at full depletion.Comment: 13 pages, 10 figures 5 tables; submitted to European Physical Journal
SOPHIE velocimetry of Kepler transit candidates XII. KOI-1257 b: a highly eccentric three-month period transiting exoplanet
In this paper we report a new transiting warm giant planet: KOI-1257 b. It
was first detected in photometry as a planet-candidate by the
space telescope and then validated thanks to a radial velocity follow-up with
the SOPHIE spectrograph. It orbits its host star with a period of 86.647661 d
3 s and a high eccentricity of 0.772 0.045. The planet transits the
main star of a metal-rich, relatively old binary system with stars of mass of
0.99 0.05 Msun and 0.70 0.07 Msun for the primary and secondary,
respectively. This binary system is constrained thanks to a self-consistent
modelling of the transit light curve, the SOPHIE radial
velocities, line bisector and full-width half maximum (FWHM) variations, and
the spectral energy distribution. However, future observations are needed to
confirm it. The PASTIS fully-Bayesian software was used to validate the nature
of the planet and to determine which star of the binary system is the transit
host. By accounting for the dilution from the binary both in photometry and in
radial velocity, we find that the planet has a mass of 1.45 0.35 Mjup,
and a radius of 0.94 0.12 Rjup, and thus a bulk density of 2.1
1.2 g.cm. The planet has an equilibrium temperature of 511 50 K,
making it one of the few known members of the warm-jupiter population. The
HARPS-N spectrograph was also used to observe a transit of KOI-1257 b,
simultaneously with a joint amateur and professional photometric follow-up,
with the aim of constraining the orbital obliquity of the planet. However, the
Rossiter-McLaughlin effect was not clearly detected, resulting in poor
constraints on the orbital obliquity of the planet.Comment: 39 pages, 17 figures, accepted for publication in Astronomy &
Astrophysic
Projected Quasi-particle Perturbation theory
The BCS and/or HFB theories are extended by treating the effect of four
quasi-particle states perturbatively. The approach is tested on the pairing
hamiltonian, showing that it combines the advantage of standard perturbation
theory valid at low pairing strength and of non-perturbative approaches
breaking particle number valid at higher pairing strength. Including the
restoration of particle number, further improves the description of pairing
correlation. In the presented test, the agreement between the exact solution
and the combined perturbative + projection is almost perfect. The proposed
method scales friendly when the number of particles increases and provides a
simple alternative to other more complicated approaches
Isotope correlations as a probe for freeze-out characterization: central 124Sn+64Ni, 112Sn+58Ni collisions
124Sn+64Ni and 112Sn+58Ni reactions at 35 AMeV incident energy were studied
with the forward part of CHIMERA multi-detector. The most central collisions
were selected by means of a multidimensional analysis. The characteristics of
the source formed in the central collisions, as size, temperature and volume,
were inspected. The measured isotopes of light fragments (3 <= Z <=8) were used
to examine isotope yield ratios that provide information on the free neutron to
proton densities.Comment: 4 pages, Contribution to 8th International Conference on
Nucleus-Nucleus Collisions, Moscow 200
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