Signals for a Transition from Surface to Bulk Emission in Thermal Multifragmentation

Excitation-energy-gated two-fragment correlation functions have been studied between E*/A0ex0ex=0ex0ex(2--9)AMeV for equilibriumlike sources formed in 8--10GeV/c π^- and p+¹⁹⁷Au reactions. Comparison with an N-body Coulomb-trajectory code shows an order of magnitude decrease in the fragment emission time in the interval E*/A0ex0ex=0ex0ex(2--5)AMeV, followed by a nearly constant breakup time at higher excitation energy. The decrease in emission time is strongly correlated with the onset of multifragmentation and thermally induced radial expansion, consistent with a transition from surface-dominated to bulk emission expected for spinodal decomposition.