arXiv subjects

M. F. Rivet

Publications and source records attributed to M. F. Rivet.

At least 37 records · Page 2Linked to original sources

arXiv2013

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.

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arXiv2013

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.

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arXiv2012

The behaviour of constrained caloric curves as ultimate signature of a phase transition for hot nuclei

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.

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arXiv2012

Correlations between emission timescale of fragments and isospin dynamics in $^{124}$Sn+$^{64}$Ni and $^{112}$Sn+$^{58}$Ni reactions at 35 AMeV

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.

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arXiv2011

Evidence for $α$-particle condensation in nuclei from the Hoyle state deexcitation

The fragmentation of quasi-projectiles from the nuclear reaction $^{40}$Ca+$^{12}$C at 25 MeV/nucleon was used to produce excited states candidates to $α$-particle condensation. Complete kinematic characterization of individual decay events, made possible by a high-granularity 4$π$ charged particle multi-detector, reveals that 7.5$\pm$4.0% of the particle decays of the Hoyle state correspond to direct decays in three equal-energy $α$-particles.

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arXiv2011

Decay of excited nuclei produced in $^{78,82}$Kr + $^{40}$Ca reactions at 5.5 MeV/nucleon

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 $4π$ 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 ($σ_{Z}$) 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 $σ_{Z}$. 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.

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arXiv2010

Production of $α$-particle condensate states in heavy-ion collisions

The fragmentation of quasi-projectiles from the nuclear reaction $^{40}Ca$ + $^{12}C$ at 25 MeV/nucleon was used to produce excited states candidates to $α$-particle condensation. The experiment was performed at LNS-Catania using the CHIMERA multidetector. Accepting the emission simultaneity and equality among the $α$-particle kinetic energies as experimental criteria for deciding in favor of the condensate nature of an excited state, we analyze the $0_2^+$ and $2_2^+$ states of $^{12}$C and the $0_6^+$ state of $^{16}$O. A sub-class of events corresponding to the direct 3-$α$ decay of the Hoyle state is isolated.

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arXiv2010

New scalings in nuclear fragmentation

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 (fragment multiplicity normalized to the source size). Freeze-out volumes seem to play a role in the scalings observed.

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arXiv2010

Alpha-particle condensation in excited 12C

The fragmentation of quasi-projectiles from the nuclear reaction $^{40}Ca$+$^{12}C$ at 25 MeV/nucleon was used to produce excited states candidates to $α$-particle condensation. The methodology relies on high granularity 4$π$ detection coupled to correlation function techniques. Under the assumption that the equality among the kinetic energies of the emitted $α$-particles and the emission simultaneity constitutes a reliable fingerprint of $α$ condensation, we identify several tens of events corresponding to the deexcitation of the Hoyle state of $^{12}$C which fulfill the condition.

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arXiv2010

Multifragmentation and phase transition for hot nuclei

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.

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arXiv2010

Advancement in the understanding of multifragmentation and phase transition for hot nuclei

Recent advancement on the knowledge of multifragmentation and phase transition for hot nuclei is reported. It concerns i) the influence of radial collective energy on fragment partitions and the derivation of general properties of partitions in presence of such a collective energy, 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 charge distribution for fragmenting hot heavy quasi-projectiles which allows, for the first time, to estimate the latent heat of the phase transition.

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arXiv2010

The prominent role of the heaviest fragment in multifragmentation and phase transition for hot nuclei

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.

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arXiv2008

Nuclear multifragmentation and phase transition for hot nuclei

This review article is focused on the tremendous progress realized during the last fifteen years in the understanding of multifragmentation and its relationship to the liquid-gas phase diagram of nuclei and nuclear matter. The explosion of the whole nucleus, early predicted by Bohr [N. Bohr, Nature 137 (1936) 351], is a very complex and rich subject which continues to fascinate nuclear physicists as well as theoreticians who extend the thermodynamics of phase transitions to finite systems.

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arXiv2008

Isospin diffusion in semi-peripheral $^{58}Ni$ + $^{197}Au$ 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.

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arXiv2008

Isospin diffusion in semi-peripheral $^{58}Ni$ + $^{197}Au$ collisions at intermediate energies (II): Dynamical simulations

We study isospin effects in semi-peripheral collisions above the Fermi energy by considering the symmetric $^{58}Ni$ + $^{58}Ni$ and the asymmetric reactions $^{58}Ni$ + $^{197}Au$ over the incident energy range 52-74 A MeV. A microscopic transport model with two different parameterizations of the symmetry energy term is used to investigate the isotopic content of pre-equilibrium emission and the N/Z diffusion process. Simulations are also compared to experimental data obtained with the INDRA array and bring information on the degree of isospin equilibration observed in Ni + Au collisions. A better overall agreement between data and simulations is obtained when using a symmetry term which linearly increases with nuclear density.

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arXiv2008

Multi-particle correlation function to study short-lived nuclei

Unstable 10C nuclei are produced as quasi-projectiles in 12C+24Mg collisions at E/A = 53 and 95 MeV. The decay of their short-lived states is studied with the INDRA multidetector array via multi-particle correlation functions. The obtained results show that heavy-ion collisions can be used as a tool to access spectroscopic information of unbound states in exotic nuclei, such as their energies and the relative importance of different sequential decay widths.

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arXiv2008

Influence of Neutron Enrichment on Disintegration Modes of Compound Nuclei

Cross sections, kinetic energy and angular distributions of fragments with charge 6$\le$Z$\le$28 emitted in 78,82Kr+40C at 5.5 MeV/A reactions were measured at the GANIL facility using the INDRA apparatus. This experiment aims to investigate the influence of the neutron enrichment on the decay mechanism of excited nuclei. Data are discussed in comparison with predictions of transition state and Hauser-Feshbach models.

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arXiv2008

Comparison of fragment partitions production in peripheral and central collisions

Ensembles of single-source events, produced in peripheral and central collisions and correponding respectively to quasi-projectile and quasi-fusion sources, are analyzed. After selections on fragment kinematic properties, excitation energies of the sources are derived using the calorimetric method and the mean behaviour of fragments of the two ensembles are compared. Differences observed in their partitions, especially the charge asymmetry, can be related to collective energy deposited in the systems during the collisions.

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