C. Cabot

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.

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.

Onset of collective expansion in nucleus-nucleus collisions below 100 mev/u

In Ar-36 + Al-27 reactions at bombarding energies E/A from 55 to 95 MeV, the mass and excitation energy of the quasi-projectile have been reconstructed at all impact parameters. Up to excitation energies E*/A of similar to 5-6 MeV, the mean kinetic energies of Z = 1 to Z = 9 fragments are in agreement with values predicted by sequential statistical decay, Above, a collective expansion energy is needed that grows with E*/A, reaching 1 to 2.2 MeV/u, depending on the de-excitation process, at E*/A similar to 10 MeV. It decreases with the fragment charge, The dominant role of excitation energy supports the scenario of thermal expansion.

Directed collective flow and azimuthal distributions in Ar-36+Al-27 collisions from 55 to 95 MeV/u

A 4 pi charged particle detector array with a low velocity threshold has been used to detect the products from reactions induced by Ar-36 on Al-27 at energies ranging from 55 to 95 MeV/u. Well characterized events were selected and sorted as a function of the impact parameter. Two methods were used for sorting these events with respect to their impact parameters and three methods were compared to determine the reaction plane. The transverse momentum analysis has been found to be the best method to extract the direction of the reaction plane for this system and for the experimental set-up used here. The energy of vanishing flow for central collisions has been found to be around 90-95 MeV/u. The azimuthal distributions of mid-rapidity particles exhibit a preferential in-plane emission and no squeeze-out effect.

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 origins of intermediate velocity light particles in 64Zn+58Ni at 35–79 AMeV

The averaged projected momentum per nucleon and the velocity distribution of light particles are examined in coincidence with projectile‐like fragments. A dependence of the left‐right asymmetry on the incident energy is observed for the intermediate velocity proton emission. On the contrary the asymmetry of the emission of intermediate velocity α particles shows a similar trend for all incident energies. Possible dynamical models are discussed for the emission of intermediate velocity light particles.

Reaction mechanisms and multifragmentation processes in Zn-64 + Ni-58 at 35A-MeV to 79A-MeV

Reaction mechanisms and multifragmentation processes have been studied for Zn-64 + Ni-58 collisions at intermediate energies with the help of antisymmetrized molecular dynamics (AMD-V) model calculations. Experimental energy spectra, angular distributions, charge distributions, and isotope distributions, classified by their associated charged particle multiplicities, are compared with the results of the AMD-V calculations. In general the experimental results are reasonably well reproduced by the calculations. The multifragmentation observed experimentally at all incident energies is also reproduced by the AMD-V calculations. A detailed study of AMD-V events reveals that, in nucleon transport, the reaction shows some transparency, whereas in energy transport the reaction is much less transparent at all incident energies studied here. The transparency in the nucleon transport indicates that. even for central collisions, about 75% of the projectile nucleons appear in the forward direction. In energy transport about 80% of the initial kinetic energy of the projectile in the center-of-mass frame is dissipated. The detailed study of AMD-V events also elucidates the: dynamics of the multifragmentation process. The study suggests that, at 35A MeV, the semitransparency and thermal expansion are the dominant mechanisms for the multifragmentation process, whereas at 49A MeV and higher incident energies a nuclear compression occurs at an early stage of the reaction and plays an important role in the multifragmentation process in addition to that of the thermal expansion and the semitransparency.