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.Comment: 11 pages, Proceedings of IWND09, August 23-25, Shanghai (China

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

The STAR Silicon Strip Detector (SSD)

The STAR Silicon Strip Detector (SSD) completes the three layers of the Silicon Vertex Tracker (SVT) to make an inner tracking system located inside the Time Projection Chamber (TPC). This additional fourth layer provides two dimensional hit position and energy loss measurements for charged particles, improving the extrapolation of TPC tracks through SVT hits. To match the high multiplicity of central Au+Au collisions at RHIC the double sided silicon strip technology was chosen which makes the SSD a half million channels detector. Dedicated electronics have been designed for both readout and control. Also a novel technique of bonding, the Tape Automated Bonding (TAB), was used to fullfill the large number of bounds to be done. All aspects of the SSD are shortly described here and test performances of produced detection modules as well as simulated results on hit reconstruction are given.Comment: 11 pages, 8 figures, 1 tabl

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

Fragment properties of fragmenting heavy nuclei produced in central and semi-peripheral collisions

Fragment properties of hot fragmenting sources of similar sizes produced in central and semi-peripheral collisions are compared in the excitation energy range 5-10 AMeV. For semi-peripheral collisions a method for selecting compact quasi-projectiles sources in velocity space similar to those of fused systems (central collisions) is proposed. The two major results are related to collective energy. The weak radial collective energy observed for quasi-projectile sources is shown to originate from thermal pressure only. The larger fragment multiplicity observed for fused systems and their more symmetric fragmentation are related to the extra radial collective energy due to expansion following a compression phase during central collisions. A first attempt to locate where the different sources break in the phase diagram is proposed.Comment: 23 pages submitted to NP

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.Comment: 8 pages, 1 figure, paper presented at the First Workshop on "State of the Art in Nuclear Cluster Physics" 13-16 May, 2008, at Strasbourg, France (SOTANCP2008) and accepted for publication at International Journal of Modern Physics E (Special Issue), Proceedings of SOTANCP2008 (to be published

Multifragmentation and phase transition for hot nuclei

5 pages, Proceedings of NN2009, August 17-21, Beijing (China)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

Yield scaling, size hierarchy and fluctuations of observables in fragmentation of excited heavy nuclei

Multifragmentation properties measured with INDRA are studied for single sources produced in Xe+Sn reactions in the incident energy range 32-50 A MeV and quasiprojectiles from Au+Au collisions at 80 A MeV. A comparison for both types of sources is presented concerning Fisher scaling, Zipf law, fragment size and fluctuation observables. A Fisher scaling is observed for all the data. The pseudo-critical energies extracted from the Fisher scaling are consistent between Xe+Sn central collisions and Au quasi-projectiles. In the latter case it also corresponds to the energy region at which fluctuations are maximal. The critical energies deduced from the Zipf analysis are higher than those from the Fisher analysis.Comment: 30 pages, accepted for publication in Nuclear Physics A, references correcte

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

TAB Bonded SSD Module for the STAR and ALICE Trackers

Presentation made at LEB99, 20-24 September 1999A novel compact detector module has been produced by the "IReS"-"Subatech"-"Thomson-CSF-Detexis" collaboration. It includes a Double-Sided (DS) Silicon Strip Detector (SSD) and the related Front End Electronics (FEE) located on two hybrids, one for the N side and one for the P side. Bumpless Tape Automated Bonding (TAB) is used to connect the detector to the hybrids by means of microcables with neither wirebonding nor pitch adapter. Each of the six dedicated ALICE128C FE chip [1], located on the hybrid, is TABed on identical single layer microcables, which connect its inputs to the DS SSD and its outputs to the hybrid [2]. These microcables are bent in order to fold over the two hybrids on the DS SSD. This module meets the specifications of two experiments, ALICE (A Large Ion Collider Experiment) on the LHC accelerator at CERN [3] and STAR (Solenoid Tracker At Rhic) on the RHIC accelerator at BNL (Brookhaven National Laboratory)[4]. It can be used with air cooling (STAR) as well as with water cooling (ALICE)[5]. This mechanically self-consistent FE module has been tested on the SPS beam at CERN. Preliminary results are presented