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

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

Sequential fissions of heavy nuclear systems

In Xe+Sn central collisions from 12 to 20 MeV/A measured with the INDRA 4$\pi$ 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

New opportunities in the study of in-medium nuclear properties with FAZIA

In this contribution we investigate the capabilities (resolution and efficiency) of the FAZIA demonstrator, with a particular emphasis on light cluster structure studies (excited state energy, width, and spin) and emitting source characterization (temperature and density), using multi-particle correlations. This study has been performed on simulated 32S+12C collisions from 25 to 80MeV/A

Reaction mechanisms leading to 3-body exit channels in central collisions of 129Xe +nat Sn at 12 MeV/u

We attempt to ascertain whether well-known sub-Fermi energy reaction mechanisms (fusion-fission, quasifission, deep-inelastic reactions) can explain the cross-sections and other characteristics recently observed for 3-fragment exit channels of central 129Xe + natSn collisions from 8 to 25MeV/u measured with INDRA. In a first step, we have simulated deep-inelastic and capture (fusion-fission and/or quasifission) reactions at 12MeV/u using the Deep Inelastic Transfers model coupled with GEMINI++, which we compare with data using a detailed software simulation of the INDRA array. Although minimum-bias measured and simulated reaction cross-sections are in good agreement, the cross-sections for 3-fragment exit channels are largely underestimated by the simulation. Moreover, 90% of simulated 3-body events are associated with mid-peripheral deep-inelastic collisions, whereas the measured coincident light charged-particle yields are consistent with central collisions leading to fusion or quasifission. However the observed 3-body yield seems beyond the reach of standard statistical decay of the primary (quasi)fission fragments

Constrained caloric curves and phase transition for hot nuclei

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.Comment: 14 pages, 5 figures, accepted in Physics Letters

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.Comment: Accepted for publication in Physical Review Letters on 8/4/201

Progresses in FAZIA detection system and preliminary results from the ISO-FAZIA experiment

In this contribution the status of the FAZIA project is presented, with a particular focus on the first experiment (identified as ISO-FAZIA) after the R&D phase. In this experiment four complete FAZIA blocks in a fully operating configuration were used. They were mounted in a planar “belt” geometry, symmetrically positioned with respect to the beam axis, covering the polar angles between 2.5◦ and 17.4◦ degrees. The investigated systems were 84Kr + 48,40Ca at 35AMeV. The main goal of the experiment was the study of the isospin transport phenomena, extending a previous analysis. This contribution will report on the isotopic identification capability of the FAZIA detector as well as preliminary results concerning the average isospin of the quasi-projectile produced in semiperipheral collisions as a function of the isospin of the target