Strong impact parameter dependence of pre-equilibrium particle emission in nucleus-nucleus reactions at intermediate energies

Charged particles and fragments emitted in reactions between 40 Ar at 45 and 65 MeV/u and an 27 Al target have been detected in a geometry close to 4π in the center of mass. A new global variable, the average parallel velocity, has been used to sort the events as a function of the impact parameter value. For particles with Z = 1 and 2, a pre-equilibrium component is present. Its multiplicity increases strongly when the impact parameter value decreases, and reaches 7 in head-on reactions.