Exclusive study of nucleus-nucleus reactions at intermediate energies: Impact parameter dependence of pre-equilibrium emission, Collective flow and hot nuclei formation

Abstract

Charged particles and fragments emitted in reactions between 40Ar at energies ranging from 25 to 85 MeV/u and an 27Al target have been detected in a geometry close to 4π in the center of mass with the 4π array MUR + TONNEAU. A new global variable, the average parallel velocity, has been used to sort the events as a function of the impact parameter value. The multiplicity of particles emitted from the interaction region increases strongly when the impact parameter value decreases, and reaches 7 in head-on reactions. The flow of these particles is attributed to scattering at negative angles. When the energy increases, compression gradually opposes this negative scattering, until the flow falls to zero. This is obtained at a beam energy in the range 70–80 MeV/u for impact parameters below 2 fm and increases with the impact parameter. This study as a function of the impact parameter and the energy should allow information both on the nucleon-nucleon cross section in medium and the EOS of nuclear matter to be obtained. In central reactions, incomplete fusion nuclei are formed at all incident energies. Their excitation energy increases with the incident energy. Above 36 MeV/u no heavy residue is left, the multiplicity of final products increases as well as the emission probability of several heavy fragments.