Publications (189)91.84 Total impact
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Article: Constrained caloric curves and phase transition for hot nuclei
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ABSTRACT: 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.05/2013; -
Article: Isospin transport in 84Kr + 112,124Sn collisions at Fermi energies
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ABSTRACT: 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.01/2013; -
Article: Nuclear multifragmentation time-scale and fluctuations of largest fragment size
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ABSTRACT: 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.10/2012; -
Article: New isospin e ffects in central heavy-ion collisions at Fermi energies
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ABSTRACT: Isospin e ffects on multifragmentation properties were studied thanks to nuclear collisions between di fferent isotopes of xenon beams and tin targets. It is shown that, in central collisions leading to multifragmentation, the mean number of fragments and their mean kinetic energy increase with the neutron-richness of the total system. Comparisons with a stochastic transport model allow to attribute the multiplicity increase to the multifragmentation stage, before secondary decay. The total charge bound in fragments is proposed as an alternate variable to quantify preequilibrium emission and to investigate symmetry energy e ffects.09/2012; -
Article: Status and performances of the FAZIA project
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ABSTRACT: 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.09/2012; -
Article: The behaviour of constrained caloric curves as ultimate signature of a phase transition for hot nuclei
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ABSTRACT: Simulations based on experimental data obtained from multifragmenting quasifused nuclei produced in central 129Xe + natSn collisions have been used to deduce event by event freeze-out properties on the thermal excitation energy range 4-12 AMeV. From these properties and temperatures deduced from proton transverse momentum fluctuations constrained caloric curves have been built. At constant average volumes caloric curves exhibit a monotonous behaviour whereas for constrained pressures a backbending is observed. Such results support the existence of a first order phase transition for hot nuclei.07/2012; -
Article: Correlations between emission timescale of fragments and isospin dynamics in $^{124}$Sn+$^{64}$Ni and $^{112}$Sn+$^{58}$Ni reactions at 35 AMeV
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ABSTRACT: We present a new experimental method to correlate the isotopic composition of intermediate mass fragments (IMF) emitted at mid-rapidity in semi-peripheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of $<$N/Z$>$ isospin asymmetry, stronger angular anisotropies and reduced odd-even staggering effects in neutron to proton ratio $<$N/Z$>$ distributions than those produced in sequential statistical emission. All these effects support the concept of isospin "migration", that is sensitive to the density gradient between participant and quasi-spectator nuclear matter, in the so called neck fragmentation mechanism. By comparing the data to a Stochastic Mean Field (SMF) simulation we show that this method gives valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy.06/2012; -
Article: Asymmetric Fission in the 78Kr+40Ca reactions at 5.5 MeV/nucleon
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ABSTRACT: The cross section, kinetic energy distribution and angular distribution of fragments with atomic number 3 ≤ Z ≤ 28 emitted in the reaction 78Kr + 40Ca at the bombarding energy of 5.5 MeV/nucleon and coincidence between light charged particles and fragments were measured by means of the 4π-INDRA array to study the decay mechanism of medium mass excited nucleus. Global features indicate a high degree of relaxation and are compatible with a binary fission from compound nucleus. The mean value of the kinetic energy distributions of fragments indicates dominance of Coulomb interaction, while the width of the distribution signals large fluctuations. Inclusive cross-section distributions of fragments with charge 3 ≤ Z ≤ 28 are bell-shaped and a strong even-odd-staggering (o-es) is observed for 3 ≤ Z ≤ 12. Coincidence measurements suggest that the light partners in very asymmetric fission are emitted at excitation energies below the particle emission thresholds. Data were confronted to the predictions of statistical model describing the decay of compound nuclei by emission of light particles and fragments. Calculations assuming spherical fission fragments and finite-range liquid drop fission barriers are not able to explain the experimental features. Attempts have been made to improve the agreement with experimental data. The analysis indicates the strong influence of the shape parameterization of the potential energy surface in describing the fission process of intermediate mass compound nuclei.The European Physical Journal Conferences 01/2012; 21:02001. -
Article: Correlations between emission timescale of fragments and isospin dynamics in Sn-124+Ni-64 and Sn-112+Ni-58 reactions at 35A MeV
Physical Review C. 01/2012; 86(1). -
Article: Decay of excited nuclei produced in $^{78,82}$Kr + $^{40}$Ca reactions at 5.5 MeV/nucleon
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ABSTRACT: Decay modes of excited nuclei are investigated in $^{78,82}$Kr + $^{40}$Ca reactions at 5.5 MeV/nucleon. Charged products were measured by means of the $4\pi$ INDRA array. Kinetic-energy spectra and angular distributions of fragments with atomic number 3 $\le Z \le$ 28 indicate a high degree of relaxation and are compatible with a fission-like phenomenon. Persistence of structure effects is evidenced from elemental cross-sections ($\sigma_{Z}$) as well as a strong odd-even-staggering (o-e-s) of the light-fragment yields. The magnitude of the staggering does not significantly depend on the neutron content of the emitting system. Fragment-particle coincidences suggest that the light partners in very asymmetric fission are emitted either cold or at excitation energies below the particle emission thresholds. The evaporation residue cross-section of the $^{78}$Kr + $^{40}$Ca reaction is slightly higher than the one measured in $^{82}$Kr + $^{40}$Ca reaction. The fission-like component is larger by $\sim$ 25% for the reaction having the lowest neutron-to-proton ratio. These experimental features are confronted to the predictions of theoretical models. The Hauser-Feshbach approach including the emission of fragments up to $Z$ = 14 in their ground states as well as excited states does not account for the main features of $\sigma_{Z}$. For both reactions, the transition-state formalism reasonably reproduces the $Z$-distribution of the fragments with charge 12 $\le Z \le$ 28. However, this model strongly overestimates the light-fragment cross-sections and does not explain the o-e-s of the yields for 6 $\le Z \le$ 10. The shape of the whole $Z$-distribution and the o-e-s of the light-fragment yields are satisfactorily reproduced within the dinuclear system framework which treats the competition between evaporation, fusion-fission and quasifission processes. The model suggests that heavy fragments come mainly from quasifission while light fragments are predominantly populated by fusion. An underestimation of the cross sections for 16 $\le Z \le$ 22 could signal a mechanism in addition to the capture process.04/2011; -
Article: New scalings in nuclear fragmentation.
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ABSTRACT: Fragment partitions of fragmenting hot nuclei produced in central and semiperipheral collisions have been compared in the excitation energy region 4-10 MeV per nucleon where radial collective expansion takes place. It is shown that, for a given total excitation energy per nucleon, the amount of radial collective energy fixes the mean fragment multiplicity. It is also shown that, at a given total excitation energy per nucleon, the different properties of fragment partitions are completely determined by the reduced fragment multiplicity (i.e., normalized to the source size). Freeze-out volumes seem to play a role in the scalings observed.Physical Review Letters 10/2010; 105(14):142701. · 7.37 Impact Factor -
Article: Study of nuclear stopping in central collisions at intermediate energies.
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ABSTRACT: Nuclear stopping has been investigated in central nuclear collisions at intermediate energies by analyzing kinematically complete events recorded with the help of the 4π multidetector INDRA for a large variety of symmetric systems. It is found that the mean isotropy ratio defined as the ratio of transverse to parallel momenta (energies) reaches a minimum near the Fermi energy, saturates or slowly increases depending on the mass of the system as the beam energy increases, and then stays lower than unity, showing that significant stopping is not achieved even for the heavier systems. Close to and above the Fermi energy, experimental data show no effect of the isospin content of the interacting system. A comparison with transport model calculations reveals that the latter overestimates the stopping power at low energies.Physical Review Letters 06/2010; 104(23):232701. · 7.37 Impact Factor -
Article: Strong enhancement of dynamical emission of heavy fragments in the neutron-rich 124Sn+64Ni reaction at 35A MeV
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ABSTRACT: A quantitative comparison is made between the absolute cross sections associated with statistical and dynamical emission of heavy fragments in the 124Sn+64Ni and 112Sn+58Ni collisions experimentally investigated at 35A MeV beam energy using the multidetector CHIMERA. The result shows that the dynamical process is about twice as probable in the neutron-rich 124Sn+64Ni system as in the 112Sn+58Ni neutron-poor one. This unexpected and significant difference indicates that the reaction mechanism is strongly dependent on the entrance-channel isospin (N/Z) content.Phys. Rev. C. 06/2010; 81(6). -
Article: The prominent role of the heaviest fragment in multifragmentation and phase transition for hot nuclei
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ABSTRACT: The role played by the heaviest fragment in partitions of multifragmenting hot nuclei is emphasized. Its size/charge distribution (mean value, fluctuations and shape) gives information on properties of fragmenting nuclei and on the associated phase transition.03/2010; -
Article: Multifragmentation and phase transition for hot nuclei
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ABSTRACT: Recent important progress on the knowledge of multifragmentation and phase transition for hot nuclei, thanks to the high detection quality of the INDRA array, is reported. It concerns i) the radial collective energies involved in hot fragmenting nuclei/sources produced in central and semi- peripheral collisions and their influence on the observed fragment partitions, ii) a better knowledge of freeze-out properties obtained by means of a simulation based on all the available experimental information and iii) the quantitative study of the bimodal behaviour of the heaviest fragment distribution for fragmenting hot heavy quasi-projectiles which allows the extraction, for the first time, of an estimate of the latent heat of the phase transition. Comment: 5 pages, Proceedings of NN2009, August 17-21, Beijing (China)03/2010; -
Article: Isospin Diffusion in $^{58}$Ni-Induced Reactions at Intermediate Energies
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ABSTRACT: 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$.03/2010; -
Article: Multifragmentation and phase transition for hot nuclei: recent progress
Nuclear Physics A 03/2010; 834:535. · 1.54 Impact Factor -
Article: Strong enhancement of dynamical emission of heavy fragments in the neutron-rich Sn124+Ni64 reaction at 35A MeV
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ABSTRACT: A quantitative comparison is made between the absolute cross sections associated with statistical and dynamical emission of heavy fragments in the Sn124+Ni64 and Sn112+Ni58 collisions experimentally investigated at 35A MeV beam energy using the multidetector CHIMERA. The result shows that the dynamical process is about twice as probable in the neutron-rich Sn124+Ni64 system as in the Sn112+Ni58 neutron-poor one. This unexpected and significant difference indicates that the reaction mechanism is strongly dependent on the entrance-channel isospin (N/Z) content.Physical Review C - PHYS REV C. 01/2010; 81(6). -
Article: Influence Of The Neutron Richness On Binary Decays
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ABSTRACT: The influence of the neutron richness on binary decays is investigated in 78,82Kr+ 40Ca reactions at 5.5 MeV/A incident energy. Kinetic energy distributions and angular distributions of fragments with atomic number 6 ≤ Z ≤ 28 were measured using the 4π-INDRA array. Global features are compatible with an emission from a long-lived system. The yields around the symmetric splitting are about 30The persistence of strong structural effects is evidenced from elemental cross-sections of the light fragments. The cross-sections for odd-Z fragments are higher for the neutron rich CN while cross-sections for even-Z fragments are higher for the neutron poor CN. Calculations assuming two different potential energy surfaces are presented.EPJ Web of Conferences. 01/2010; -
Article: Bimodal behavior of the heaviest fragment distribution in projectile fragmentation.
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ABSTRACT: The charge distribution of the heaviest fragment detected in the decay of quasiprojectiles produced in intermediate energy heavy-ion collisions has been observed to be bimodal. This feature is expected as a generic signal of phase transition in nonextensive systems. In this Letter, we present new analyses of experimental data from Au on Au collisions at 60, 80, and 100 MeV/nucleon showing that bimodality is largely independent of the data selection procedure and of entrance channel effects. An estimate of the latent heat of the transition is extracted.Physical Review Letters 08/2009; 103(7):072701. · 7.37 Impact Factor
Top co-authors
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Institutions
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2009
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Université Paris-Sud 11
- Institut de Physique Nucléaire (IPN)
Paris, Ile-de-France, France
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2006
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Université de Caen Basse-Normandie
Caen, Basse-Normandie, France
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2003
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Institute of Nuclear and Particle Physics
Agía Paraskeví, Voreio Aigaio, Greece
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2000
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Conservatoire National des Arts et Métiers
Paris, Ile-de-France, France
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