M.F. Rivet

Study of a fast collective mode in deep inelastic reactions: Quantal fluctuations

We have measured for the 430 MeV86Kr+92.98Mo systems the triple differential cross sectiond3σ/dEdAdZ (E=kinetic energy,A=mass andZ=atomic number) of the reaction products in the angular range (θ=25, 45°). We focussed attention on a fast collective mode in deep inelastic reactions: the neutron excess or charge equilibration. This mode is shown to relax very quickly to equilibrium within a time scale of the order of 10−22s. It is shown that this collective degree of freedom exhibit a quantal behavior which can be seen in the observation of quantal fluctuations. The experimental results are discussed in terms of a simple equilibrium model. The giant dipole resonance of the composite system could be closely connected to this neutron excess mode.

Deeply inelastic collisions as a source of intermediate mass fragments at E/A=27 MeV☆

Abstract Intermediate-mass fragments detected in coincidence with heavy residues were measured in 40 Ar induced reactions on Ag at E / A =27 MeV. From the observed characteristics, it is inferred that intermediate-mass fragments associated with the so-called intermediate-velocity source come mainly from deeply inelastic collisions occuring after or at the same time as preequilibrium particle emission.

Coulomb instability in collisions between very heavy nuclei around 30 MeV per nucleon

Abstract Simulations, based on the Landau-Vlasov model, of central collisions ( b =0−3 fm) between very heavy nuclei around 30 MeV per nucleon reveal the occurence of the Coulomb instability as already predicted by static calculations of hot nuclei a few years ago. The Coulomb instability shows itself in the formation of unstable bubbles.

Light fragments produced in40Ar+natAg reactions at 27MeV/u

Angular and energy distributions have been measured for products of 4≦Z≦12 from reaction of 1,095 MeV40Ar+natAg. In addition to sources located near the projectile and fusion system velocities, the data show the presence of a source located aroundVbeam/2 which cannot be explained by any existing model. A possible interpretation is proposed.

Dynamical aspects of violent collisions in Ar+Ag reactions at EA=27 MeV

Abstract In a recent study of the Ar+Ag reaction at E A =27 MeV , involving intermediate-mass fragments, am overview of the reaction dynamics of violent collisions was attained. A comparison is made with semi-classical calculations (Landau-Vlasov) which reproduce experimental observations well and sustain the persistence of deeply inelastic processes at high incident relative velocity (∼0.2 c ).

Dynamics and thermalization in violent collisions between40Ar and Ag at 27 MeV/nucleon

For the violent collisions of 27 MeV/nucleon40Ar with Ag, coincidence measurements have been made between heavy residues and intermediate mass fragments (3≦Z≦14) or light charged particles. From the analysis of the correlation between heavy residues (mass and velocity) and intermediate mass fragments, the main characteristics of the dominant mechanisms, fusion and partially damped collisions preceded or accompanied by a preequilibrium emission, are presented. Balances concerning mean values of parallel linear momentum, mass and atomic number, are established and confirm that a complete description of violent collisions was obtained. Then thermalization is discussed, first in terms of excitation energies derived from kinematics between heavy residues and intermediate mass fragments, and secondly in terms of initial temperature estimates derived from light charged particle spectra. Very hot nuclei (T⋍5.7–6.6 MeV) are produced over a large impact parameter range from very central collisions to medium peripheral ones. Various experimental results are compared to predictions obtained with semi-classical calculations (Landau-Vlasov equation). From their good agreement one may conclude that, depending on the impact parameter, thermal equilibrium is achieved within 4–10×10−22 s.

Excitation energy partition in deeply inelastic collisions between40Ar and Ag at 27 MeV per nucleon

The dynamics of the two partners produced in dissipative collisions has been experimentally studied for the system40Ar + Ag at 27 MeV per nucleon. Primary masses of the fragments can then be calculated; the excitation energy partition between the two fragments is derived from the number of particles evaporated by each fragment. We found that this division evolves from equipartition to a repartition close to thermal equilibrium in the excitation energy range 300–350 MeV or interaction times 5-10×l0−22 s.

Dynamical analysis of dissipative collisions between Ar and Ag nuclei in the Fermi energy domain

Abstract40Ar+107 Ag reactions at 27 AMeV and 44AMeV have been studied within the Landau Vlasov microscopic transport model. In this framework the main characteristics of primary partners of binary collisions are shown to be well described, and information aboutσnn can be extracted as well as the angular momentum associated with the heavy fragment. Different contributions to the total energy are studied as a function of the reaction time. Our numerical method is compared with the results obtained using experimental techniques like proton spectra. Their impact parameter and energy dependences have been studied and the estimated “temperature parameter” is in agreement with the experimental results.