959 research outputs found
Study of thermometers for measuring a microcanonical phase transition in nuclear fragmentation
The aim of this work is to study how the thermodynamic temperature is related
to the known thermometers for nuclei especially in view of studying the
microcanonical phase transition. We find within the MMMC-model that the
"S-shape" of the caloric equation of state e^*(T) which is the signal of a
phase transition in a system with conserved energy, can be seen in the
experimentally accessible slope temperatures T_slope for different particle
types and also in the isotopic temperatures T_He-Li. The isotopic temperatures
T_H-He are weaker correlated to the shape of the thermodynamic temperature and
therefore are less favorable to study the signal of a microcanonical phase
transition. We also show that the signal is very sensitive to variations in
mass of the source
Isoscaling as a measure of Symmetry Energy in the Lattice Gas Model
The energetic properties of nuclear clusters inside a low-density,
finite-temperature medium are studied with a Lattice Gas Model including
isospin dependence and Coulomb forces. Important deviations are observed
respect to the Fisher approximation of an ideal gas of non-interacting
clusters, but the global energetics can still be approximately expressed in
terms of a simple modified energy-density functional. The multi-fragmentation
regime appears dominated by combinatorial effects in this model, but the
isoscaling of the largest fragment in low energy collisions appears a promising
observable for the experimental measurement of the symmetry energy.Comment: 4 pages, 3 figure, submitted to PR
Experimental and Theoretical Search for a Phase Transition in Nuclear Fragmentation
Phase transitions of small isolated systems are signaled by the shape of the
caloric equation of state e^*(T), the relationship between the excitation
energy per nucleon e^* and temperature. In this work we compare the
experimentally deduced e^*(T) to the theoretical predictions. The
experimentally accessible temperature was extracted from evaporation spectra
from incomplete fusion reactions leading to residue nuclei. The experimental
e^*(T) dependence exhibits the characteristic S-shape at e^* = 2-3 MeV/A. Such
behavior is expected for a finite system at a phase transition. The observed
dependence agrees with predictions of the MMMC-model, which simulates the total
accessible phase-space of fragmentation
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Investigation of collective radial expansion and stopping in heavy ion collisions at Fermi energies
We present an analysis of multifragmentation events observed in central Xe+Sn
reactions at Fermi energies. Performing a comparison between the predictions of
the Stochastic Mean Field (SMF) transport model and experimental data, we
investigate the impact of the compression-expansion dynamics on the properties
of the final reaction products. We show that the amount of radial collective
expansion, which characterizes the dynamical stage of the reaction, influences
directly the onset of multifragmentation and the kinematic properties of
multifragmentation events. For the same set of events we also undertake a shape
analysis in momentum space, looking at the degree of stopping reached in the
collision, as proposed in recent experimental studies. We show that full
stopping is achieved for the most central collisions at Fermi energies.
However, considering the same central event selection as in the experimental
data, we observe a similar behavior of the stopping power with the beam energy,
which can be associated with a change of the fragmentation mechanism, from
statistical to prompt fragment emission.Comment: 15 page
Evolution of the decay mechanisms in central collisions of + from = 8 to 29
Collisions of Xe+Sn at beam energies of = 8 to 29 and leading to
fusion-like heavy residues are studied using the INDRA multidetector.
The fusion cross section was measured and shows a maximum at = 18-20
. A decomposition into four exit-channels consisting of the number of
heavy fragments produced in central collisions has been made. Their relative
yields are measured as a function of the incident beam energy. The energy
spectra of light charged particles (LCP) in coincidence with the fragments of
each exit-channel have been analyzed. They reveal that a composite system is
formed, it is highly excited and first decays by emitting light particles and
then may breakup into 2- or many- fragments or survives as an evaporative
residue. A quantitative estimation of this primary emission is given and
compared to the secondary decay of the fragments. These analyses indicate that
most of the evaporative LCP precede not only fission but also breakup into
several fragments.Comment: Invited Talk given at the 11th International Conference on
Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1,
2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference
Series (JPCS
Pseudo-critical clusterization in nuclear multifragmentation
In this contribution we show that the biggest fragment charge distribution in
central collisions of Xe+Sn leading to multifragmentation is an admixture of
two asymptotic distributions observed for the lowest and highest bombarding
energies. The evolution of the relative weights of the two components with
bombarding energy is shown to be analogous to that observed as a function of
time for the largest cluster produced in irreversible aggregation for a finite
system. We infer that the size distribution of the largest fragment in nuclear
multifragmentation is also characteristic of the time scale of the process,
which is largely determined by the onset of radial expansion in this energy
range.Comment: 4 pages, 3 figures, Contribution to conference proceedings of the
25th International Nuclear Physics Conference (INPC 2013
Elaboration, par une approche sur VER, d’un modèle à champ moyen pour la prédiction de la refermeture de pores lors de la déformation à chaud de métaux
International audienceDans une pièce métallique, des pores résiduels non désirés sont toujours présents après les opérations de fonderie. Pour refermer ces pores de taille généralement négligeable par rapport à la taille de la pièce, les industriels utilisent des procédés à chaud tels que le forgeage libre ou le laminage. Dans la littérature, plusieurs modèles à champ moyen prédictifs en terme d'évolution du volume des pores traitent de cette problématique. Récemment un nouveau modèle à champ moyen, appelé Cicaporo4, a été élaboré et continue d'être amélioré afin de modéliser la refermeture des pores. Ce modèle se base sur une approche multi-échelles2 utilisant des simulations sur Volume Elémentaire Représentatif (VER). La plupart des modèles classiques étudient la refermeture en fonction de latriaxialité des contraintes et de la déformation plastique équivalente. Pour l'élaboration du modèle Cicaporo, les effets de la morphologie des pores et de leur orientation dans la matrice ont été investigués
Sequential fissions of heavy nuclear systems
In Xe+Sn central collisions from 12 to 20 MeV/A measured with the INDRA
4 multidetector, the three-fragment exit channel occurs with a significant
cross section. In this contribution, we show that these fragments arise from
two successive binary splittings of a heavy composite system. Strong Coulomb
proximity effects are observed in the three-fragment final state. By comparison
with Coulomb trajectory calculations, we show that the time scale between the
consecutive break-ups decreases with increasing bombarding energy, becoming
compatible with quasi-simultaneous multifragmentation above 18 MeV/A.Comment: 6 pages, 5 figures, contribution to conference proceedings of the
Fifth International Workshop on Nuclear fission and Fission-Product
Spectroscop
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