Multifragmentation of gold nuclei by light relativistic ions - thermal break-up versus dynamic disintegration

Multiple emission of intermediate-mass fragments has been studied for the collisions of p, 4He, and 12C + Au with the 4π setup FASA. The mean intermediate mass fragments (IMF) multiplicities saturate at a value of around 2 for incident energies above 6 GeV. An attempt to describe the observed IMF multiplicities in the two-stage scenario, a fast cascade followed by a statistical multifragmentation, fails. Agreement with the measured IMF multiplicities is obtained by introducing and intermediate expansion phase and modifying empirically the excitation energies and masses of the remnants. The angular distributions and energy spectra from the proton-induced collisions are in agreement with the scenario of thermal multifragmentation of a hot and expanded target-spectator. In the case of 12C + Au (22.4 GeV) and 4He (14.6 GeV) + Au collisions, deviations from a pure thermal break-up are seen in the fragment energy spectra, which are harder than those both from model calculations and from the measured ones for p-collisions. This difference is attributed to a collective flow with the expansion velocity at the surface of about 0.1 c (for 12C + Au collisions)