Non-equilibrium emission of complex fragments from p + Au collisions at 2.5 GeV proton beam energy

The energy and angular dependence of double differential cross sections d 2 σ/ ddE was measured for reactions induced by 2.5 GeV protons on Au target with isotopic identification of light products (H, He, Li, Be, and B) and with elemental identification of heavier intermediate mass fragments (C, N, O, F, Ne, Na, Mg, and Al). It was found that two different reaction mechanisms give comparable contributions to the cross sections. The intranuclear cascade of nucleon-nucleon collisions followed by evaporation from an equilibrated residuum describes the low energy part of the energy distributions whereas another reaction mechanism is responsible for the high energy part of the spectra of composite particles. A phenomenological model description of the differential cross sections by isotropic emission from two moving sources led to a very good description of all measured data. Values of the extracted parameters of the emitting sources are compatible with the hypothesis claiming that high energy particles emerge from preequilibrium processes consisting in a breakup of the target into three groups of nucleons; small, fast, and hot fireball of ∼8 nucleons, and two larger, excited prefragments, which emit light charged particles and intermediate mass fragments. The smaller of them contains ∼20 nucleons and moves with a velocity larger than the CM velocity of the proton projectile and the target. The heavier prefragment behaves similarly as the heavy residuum of the intranuclear cascade of nucleon-nucleon collisions.