Characterization of quasi-projectiles produced in symmetric collisions studied with INDRA Comparison with models

The characterization of hot quasi-projectiles produced in symmetric or quasi-symmetric reactions (Au + Au, Xe + Sn, Ni + Ni, Ar + KCl) at di erent incident energies are estimated by means of two di erent procedures. The advantages and disadvantages of each method are analyzed on the basis of simulations using events produced by two slightly di erent models: HIPSE and ELIE.Comment: A para\^itr

Bimodality as a signal of Liquid-Gas phase transition in nuclei?

We use the HIPSE (Heavy-Ion Phase-Space Exploration) Model to discuss the origin of the bimodality in charge asymmetry observed in nuclear reactions around the Fermi energy. We show that it may be related to the important angular momentum (spin) transferred into the quasi-projectile before secondary decay. As the spin overcomes the critical value, a sudden opening of decay channels is induced and leads to a bimodal distribution for the charge asymmetry. In the model, it is not assigned to a liquid-gas phase transition but to specific instabilities in nuclei with high spin. Therefore, we propose to use these reactions to study instabilities in rotating nuclear droplets.Comment: 4 pages, 4 figures Accepted to PR

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

Isospin Diffusion in 58^{58}Ni-Induced Reactions at Intermediate Energies

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$

Bimodality: a possible experimental signature of the liquid-gas phase transition of nuclear matter

We have observed a bimodal behaviour of the distribution of the asymmetry between the charges of the two heaviest products resulting from the decay of the quasi-projectile released in binary Xe+Sn and Au+Au collisions from 60 to 100 MeV/u. Event sorting has been achieved through the transverse energy of light charged particles emitted on the quasi-target side, thus avoiding artificial correlations between the bimodality signal and the sorting variable. Bimodality is observed for intermediate impact parameters for which the quasi-projectile is identified. A simulation shows that the deexcitation step rather than the geometry of the collision appears responsible for the bimodal behaviour. The influence of mid-rapidity emission has been verified. The two bumps of the bimodal distribution correspond to different excitation energies and similar temperatures. It is also shown that it is possible to correlate the bimodality signal with a change in the distribution of the heaviest fragment charge and a peak in potential energy fluctuations. All together, this set of data is coherent with what would be expected in a finite system if the corresponding system in the thermodynamic limit exhibits a first order phase transition.Comment: 30 pages, 31 figure

Coulomb chronometry to probe the decay mechanism of hot nuclei

In 129 Xe+ nat Sn central collisions from 8 to 25 MeV/A, the three-fragment exit channel occurs with a significant cross section. We show that these fragments arise from two successive binary splittings of a heavy composite system. The sequence of fragment production is determined. Strong Coulomb proximity effects are observed in the three-fragment final state. A comparison with Coulomb trajec-tory calculations shows that the time scale between the consecutive break-ups decreases with increasing bombarding energy, becoming quasi-simultaneous above excitation energy E * = 4.0$\pm$0.5 MeV/A. This transition from sequential to simultaneous break-up was interpreted as the signature of the onset of multifragmentation for the three-fragment exit channel in this system.Comment: 12 pages; 13 Figures; 4 Table; Accepted for publication in Physical Review

Transition from participant to spectator fragmentation in Au+Au reaction between 60 AMeV and 150 AMeV

Using the quantum molecular dynamics approach, we analyze the results of the recent INDRA Au+Au experiments at GSI in the energy range between 60 AMeV and 150 AMeV. It turns out that in this energy region the transition toward a participant-spectator scenario takes place. The large Au+Au system displays in the simulations as in the experiment simultaneously dynamical and statistical behavior which we analyze in detail: The composition of fragments close to midrapidity follows statistical laws and the system shows bi-modality, i.e. a sudden transition between different fragmentation pattern as a function of the centrality as expected for a phase transition. The fragment spectra at small and large rapidities, on the other hand, are determined by dynamics and the system as a whole does not come to equilibrium, an observation which is confirmed by FOPI experiments for the same system.Comment: published versio

Estimate of average freeze-out volume in multifragmentation events

An estimate of the average freeze-out volume for multifragmentation events is presented. Values of volumes are obtained by means of a simulation using the experimental charged product partitions measured by the 4pi multidetector INDRA for 129Xe central collisions on Sn at 32 AMeV incident energy. The input parameters of the simulation are tuned by means of the comparison between the experimental and simulated velocity (or energy) spectra of particles and fragments.Comment: To be published in Phys. Lett. B 12 pages, 5 figure