Multifragment Disintegrations in Intermediate Energy Nucleus-Nucleus Collisions.

The possibility for the highly excited nuclear matter to break into multi-fragment final states has been predicted by various theoretical calculations. Experimental information about low-density nuclear equation of state may be obtainable from detailed studies of multi-fragment emission processes. A new low-threshold 4pi charged particle detector array, the MSU Miniball, has been constructed for the study of multi-fragmentation. Intermediate mass fragment (IMF) emission has been studied using beams of ^{14}N at E/A = 50 MeV and ^{36}Ar at E/A = 20, 35 MeV on ^{238}U and ^{197}Au targets. Most IMFs are emitted in central collisions characterized by large charged-particle multiplicities. Energy spectra and angular distributions indicate the non-equilibrium nature of IMF emission. Time-scales of multi-fragment emission, key in distinguishing the various reaction mechanisms, have been studied using two-fragment correlation functions. The experimental two-fragment correlation functions were found to depend mainly on the reduced relative velocity, rm v_{red} = v_{rel}/ sqrt{Z_1 + Z_2}, of the fragment pairs. It is therefore possible to sum over different pair combinations with little loss in resolution. From the comparisons of the inclusive experimental correlation functions with the classical expression of Koonin-Pratt formula, mean emission times of the order of 100-200 fm/c have been extracted with about 50% uncertainty. Three -body Coulomb trajectory calculations were also performed to estimate the uncertainty from the interaction with the heavy residue. A more detailed study indicates that fragment emission in central collisions begins at the very early stages of the reaction and continues throughout the later equilibrated stages.