The Emission of Complex Fragments from Highly Excited Nuclear Systems.

The emission of complex fragments is investigated for the following reactions: ^{12} C + ^{197}Au at E/A = 15 and 30 MeV, ^{12}C + Ag at E/A = 15 and 30 MeV, ^{14} N + Ag at E/A = 35 MeV, and ^{32 }S + Ag at E/A = 22.5 MeV. In all reactions, intermediate mass fragments, A_{rm f}>4, are produced with single particle inclusive cross sections which generally decrease with increasing element number. The energy spectra and angular distributions indicate substantial non-equilibrium contributions, particularly for lighter fragments. Measurements of light particles and target-like residues coincident with intermediate mass fragments provide information about the dynamical configuration of the fragmenting system. From these measurements it appears that intermediate mass fragments are emitted in highly damped collisions. The average fragment multiplicities are low, of the order of one. Intermediate mass fragments are typically accompanied by a significant number of nucleons, approximately 10 in the form of light particles alone, emitted prior to the attainment of full statistical equilibrium. As a consequence, only about 0.7 of the beam momentum is transferred to the fragment-residue system. Angular correlations between light particles and intermediate mass fragments indicate enhanced emission in the entrance channel reaction plane and, therefore, that dynamical effects are important in the fragment emission mechanism. Furthermore, the angular correlations indicate that fragment emission is not restricted to central collisions. The target residue angular distributions and the mass, isotopic, and excited state distributions indicate substantial emission of intermediate mass fragments in excited states. The effects of the sequential decay of particle-unstable states are studied in schematic statistical calculations in which the states of nuclei are assumed to be populated according to a thermal equilibrium distribution. These calculations indicate that the characteristic structures in the fragment mass spectra and the apparent suppression of excited state emission are the result of the decays of particle-unstable states. A model is discussed in which particles are emitted at statistical rates from a source which is in the process of equilibrium with the target system. The resulting calculations are discussed and compared to measured fragment multiplicities, momentum transfers, and energy spectra. The comparison indicates that descriptions of dynamical evolution and the fragment emission mechanism must be integrated in a realistic model of nuclear reactions.