Signals for a transition from surface to bulk emission in thermal multifragmentation

Excitation-energy-gated two-fragment correlation functions have been studie d between E*/A = (2-9)A MeV for equilibriumlike sources formed in 8-10 GeV/ c pi(-) and p + Au-197 reactions. Comparison with an N-body Coulomb-traject ory code shows an order of magnitude decrease in the fragment emission time in the interval E*/A = (2-5)A MeV, followed by a nearly constant breakup t ime at higher excitation energy. The decrease in emission time is strongly correlated with the onset of multifragmentation and thermally induced radia l expansion, consistent with a transition from surface-dominated to bulk em ission expected for spinodal decomposition.