A. Rahmani

A hot expanding source in 50 A MeV Xe+Sn central reactions

The INDRA multidetector has been used to study multifragmentation processes in central collisions for the Xe + Sn reaction at 50 A MeV. A single isotropic source formed at an excitation energy of 12 A MeV exhausting most of the emitted charged products has been isolated in such collisions. The fragment kinetic energy spectra indicate a fast disintegration of the system with a radial collective motion of about 2 A MeV. The light charged particle characteristics within this scenario are also discussed.

Surveying the nuclear caloric curve

Abstract The 4π array INDRA was used to detect nearly all charged products emitted in Ar + Ni collisions between 52 and 95 MeV/u. The charge, mass and excitation energy E ∗ of the quasi-projectiles have been reconstructed event by event. Excitation energies up to 25 MeV per nucleon are reached. Apparent temperatures obtained from several double isotopic yield ratios Tr0 show different dependences upon E ∗ . T6Li7Li3Heα0 yields the highest values, as well as the high energy slopes Ts of the kinetic energy spectra. Two statistical models, sequential evaporation and gas in complete equilibrium, taking into account side feeding and discrete excited states population, show that the data can be explained by a steady increase of the initial temperature with excitation energy without evidence for a liquid-gas phase transition.

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

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Vaporization events from binary dissipative collisions

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.

Kinematical properties and composition of vaporizing sources: is thermodynamical equilibrium achieved?

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.

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

Abstract Vaporization events detected with the 4π array INDRA, where all species have atomic numbers lower than 3 are studied. Binary dissipative collisions are found to be the dominant mechanism responsible for the production of such events. Due to large fluctuations (energy damping and mass of partners), the excitation energy of the vaporizing partners covers a very broad range from 6 to 28 A MeV.

Onset of vaporization for the Ar+Ni system

Abstract Kinematical properties and chemical composition of vaporizing sources observed in heavy-ion collisions are presented. Vaporizing sources detected with the 4π array INDRA, are defined as sources for which all detected species have atomic numbers lower than 3. The excitation energy of the sources covers a very broad range (6–28 A MeV). The occurrence of thermodynamical equilibrium is discussed by comparing the data with the results of two statistical models. The model describing a gas in thermal and chemical equilibrium reproduces rather well the data suggesting that thermodynamical equilibrium may have been reached for such sources.

High transverse momentum proton emission in Ar + Ta collisions at 94 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.

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

Abstract Using the 4π multidetector INDRA, collisions between 36Ar and 58Ni have been investigated over a broad bombarding energy range, from 32 to 95 AMeV. The onset for complete vaporization of the system into neutrons, H and He isotopes as well as the evolution with energy of the isotopic composition of the vaporization events were determined. Initial excitation energy needed for vaporization is discussed.