Binary dissipative processes and formation of hot nuclei in reactions from 55 to 95 MeV/u

Abstract

A 4π multidetector array with a high efficiency has been used to detect the charged products from reactions induced by 36 Ar on 27 Al at energies ranging from 55 to 95 MeV/u. Well-characterized events were selected and sorted as a function of the impact parameter. Fusion or full stopping events have a very small cross section. Dissipative binary collisions accompanied by pre-equilibrium (mid-rapidity) emission, and followed by de-excitation of the quasi-projectile and quasi-target, were found to dominate for all impact parameters. The mass of the quasi-projectile has been reconstructed from its products and found to be slighty below the projectile mass. Isotropic emission in its rest frame has been observed at all energies and impact parameters, indicating that equilibrated nuclei may have been formed. The excitation energy of the quasi-projectile, determined via calorimetry, and the slope of kinetic energy spectra of α-particles were found to increase with incident energy and with decreasing impact parameter. Excitation energies per nucleon above 11 MeV are reached in a sizable cross section (70 mb). The mass distribution and kinetic energy spectra of emitted particles and fragments are essentially governed by the excitation energy, with a weak dependence on the impact parameter. The evolution of dissipative processes from deep inelastic collisions to the participant-spectator regime is discussed.