M. Germain

Study of intermediate velocity products in the Ar+Ni collisions between 52 and 95 A.MeV

Abstract Intermediate velocity products in Ar+Ni collisions from 52 to 95 A.MeV are studied in an experiment performed at the GANIL facility with the 4 π multidetector INDRA. It is shown that these emissions cannot be explained by statistical decays of the quasi-projectile and the quasi-target in complete equilibrium. Three methods are used to isolate and characterize intermediate velocity products. The total mass of these products increases with the violence of the collision and reaches a large fraction of the system mass in mid-central collisions. This mass is found independent of the incident energy, but strongly dependent on the geometry of the collision. Finally it is shown that the kinematical characteristics of intermediate velocity products are weakly dependent on the experimental impact parameter, but strongly dependent on the incident energy. The observed trends are consistent with a participant–spectator-like scenario or with neck emissions and/or breakup.Intermediate velocity products in Ar+Ni collisions from 52 to 95 A.MeV are studied in an experiment performed at the GANIL facility with the 4

Multifragmentation in Xe(50 AMeV) + Sn: Confrontation of theory and data

pi

Measurements of sideward flow around the balance energy

multidetector INDRA. It is shown that these emissions cannot be explained by statistical decays of the quasi-projectile and the quasi-target in complete equilibrium. Three methods are used to isolate and characterize intermediate velocity products. The total mass of these products increases with the violence of the collision and reaches a large fraction of the system mass in mid-central collisions. This mass is found independent of the incident energy, but strongly dependent on the geometry of the collision. Finally it is shown that the kinematical characteristics of intermediate velocity products are weakly dependent on the experimental impact parameter, but strongly dependent on the incident energy. The observed trends are consistent with a participant-spectator like scenario or with neck emissions and/or break-up.

Emission time scale of light particles in the system Xe+Sn at 50 AMeV. A probe for dynamical emission?

Abstract We compare in detail central collisions Xe(50 AMeV) + Sn, recently measured by the INDRA collaboration, with the Quantum Molecular Dynamics (QMD) model in order to identify the reaction mechanism which leads to multifragmentation. We find that QMD describes most of the data quite well, in the projectile/target region as well as in the midrapidity zone where also statistical models can be and have been employed. The agreement between QMD and data allows us to use this dynamical model to investigate the reaction in detail. We arrive at the following observation: (a) the in-medium nucleon-nucleon cross section is not significantly different from the free cross section, (b) even the central collisions have a binary character, (c) most of the fragments are produced in the central collisions, (d) the simulations as well as the data show a strong attractive in-plane flow resembling deep inelastic collisions, and (e) at midrapidity the results from QMD and those from statistical model calculations agree for almost all of the observables with the exception of d 2 σ dZ dE . This renders it difficult to extract the reaction mechanism from midrapidity fragments only. According to the simulations the reaction shows a very early formation of fragments, even in central collisions, which pass through the reaction zone without being destroyed. The final transverse momentum of the fragments is very close to the initial one and due to the Fermi motion. A heating up of the systems is not observed and hence a thermal origin of the spectra cannot be confirmed. The disagreement between the simulations and the data for some observables is presumably due to a force range which is too large as compared to reality but necessary to keep the nuclei stable in these semiclassical approach.

High transverse momentum proton emission in Ar + Ta collisions at 94 MeV/ u

Sideward flow values have been determined with the INDRA multidetector for Ar+Ni, Ni+Ni and Xe+Sn systems studied at GANIL in the 30 to 100 A.MeV incident energy range. The balance energies found for Ar+Ni and Ni+Ni systems are in agreement with previous experimental results and theoretical calculations. Negative sideward flow values have been measured. The possible origins of such negative values are discussed. They could result from a more important contribution of evaporated particles with respect to the contribution of promptly emitted particles at mid-rapidity. But effects induced by the methods used to reconstruct the reaction plane cannot be totally excluded. Complete tests of these methods are presented and the origins of the ``auto-correlation effect have been traced back. For heavy fragments, the observed negative flow values seem to be mainly due to the reaction plane reconstruction methods. For light charged particles, these negative values could result from the dynamics of the collisions and from the reaction plane reconstruction methods as well. These effects have to be taken into account when comparisons with theoretical calculations are done.

Evidence for dynamical proton emission in peripheral Xe+Sn collisions at 50 MeV/u

Abstract: Proton and deuteron correlation functions have been investigated with both impact parameter and emission source selections. The correlations of the system 129Xe+NatSn at 50 AMeV have been measured with the 4π INDRA which provides a complete kinematical description of each event. The emission time scale analyzed with a quantum model reveals the time sequence of the light particles emitted by the projectile-like fragment. The short and constant emission time of the proton, independent of the impact parameter, can be attributed to a preequilibrium process.

Apparent Temperatures in Hot Quasi-Projectiles and the Caloric Curve

Abstract Energy spectra of fast protons arising from Ar + Ta collisions at 94 MeV/ u have been measured at large angles. These data are analysed in the framework of a one-body transport theory simulated by the BNV code. The low cross section measured for very energetic protons rules out a possible mechanism of kaon production via incoherent nucleon collisions at such low beam energies.

IS REDUCIBILITY IN NUCLEAR MULTIFRAGMENTATION RELATED TO THERMAL SCALING

Abstract Relative angle correlation functions for mid-rapidity protons built up from 50 MeV/u Xe+Sn data recorded by the INDRA multidetector exhibit the characteristics of a non-equilibrated emission: they show an energy dependent anisotropy which cannot be accounted for by standard evaporation processes; a possible explanation lies in the dynamical origin of these protons as suggested by a Landau–Vlasov simulation.

Experimental determination of fragment excitation energies in multifragmentation events

The dependence of nuclear temperature upon excitation energy has been experimentally studied with increasing values of excitation energy over the years. At excitation energies per nucleon, E*/A, lower than 6 MeV the temperatures deduced from the kinetic properties of the emitted particles and clusters follow the Fermi gas law : E* = (A/k).T 2. The value of the inverse level density parameter k was found to be in the range 8 to 13[1]. When excitation energies up to 10 MeV per nucleon were reached, temperatures obtained from the relative populations of excited levels in the emitted light nuclei did not overcome 5–6 MeV[2], but this limitation could be explained by side-feeding effects. Such hot nuclei were formed in fusion or deep inelastic reactions. At incident energies above 40–50 MeV/u, binary dissipative collisions dominate and the quasi-projectiles reach excitation energies per nuclEon and kinetic temperatures above 10 MeV[4, 5]. The study of projectile “spectators” in reactions at several hundreds of MeV/u made it possible to reach similar excitation energies[3]. In this Aladin experiment at GSI, the temperature was obtained via the relative abundances of two isotope pairs[6]. The relation between this temperature Tr 0 and E*/A was interpreted as indicating a phase transition, with the nuclear gas regime dominating above E*/A = 10 MeV, as predicted[7]. However, a monotonic increase of the temperature with excitation energy was observed in similar conditions[8] and questions were raised about the significance of these caloric curves[9, 10, 11, 12], about the role played by the mass dependence of the decaying nucleus upon E*/A [13], as well as the strong effects of side-feeding, especially at high temperatures [14]. This point will be discussed by Xi Hong Fei at this meeting.[15].

New method for the discrimination of single-source events in heavy-ion collisions

Abstract Thermal scaling (Arrhenius law for an “elementary” probability p of binomial function) and reducibility in intermediate mass fragments (IMFs) production are examined for data of the reaction 129Xe+ natSn at 50 MeV/u. The study of the longitudinal velocities and of the average transverse energies of the IMFs contradicts the assumption that the total transverse energy of all detected particles Et is related to a well defined temperature. The separation of Et into the total transverse energy of light charged particles (Z=1,2) and that of IMFs elucidates the algorithm which induces a linear behavior of log(1/p) versus 1/ E t . Even in the case of a single thermalized source, calculations based on a sequential statistical model show that the Arrhenius law cannot be observed if Et is taken as an estimation of the thermal energy.