Transition from Thermal to Rapid Expansion in Multifragmentation of Gold Induced by Light Relativistic Projectiles

Multiple emission of intermediate-mass fragments has been studied for the collisions of p, ${}^4$He and ${}^{12}$C on Au with the $4\pi$ setup FASA. In the case of ${}^{12}$C(22.4 GeV)+Au and ${}^4$He(14.6 GeV)+Au collisions, the deviations from a pure thermal break-up are seen in the energy spectra of the emitted fragments: the spectra are harder than calculated and than measured in p-induced collisions. This difference is attributed to a collective flow with the expansion velocity on the surface about 0.1 $c$ (for ${}^{12}$C+Au collisions).