Multifragmentation and Energy Dissipation in the 1.8-4.8 GEV HELIUM-3 + Silver-Nat GOLD-197 Reactions.

One of the signatures of highly excited finite nuclear matter is the multiple emission of intermediate -mass fragments (IMFs, rm3<=q Z _sp{~}{<} 20) --a process known as multifragmentation. The Indiana Silicon Sphere (ISiS) 4pi detector array has been constructed and used to study multifragmentation in light-ion induced nuclear reactions. The array consists of 162 ionization-chamber/Silicon/CsI(Tl) detector telescopes covering a solid angle of {~74} % of 4pi. The first experiment utilizing the full ISiS array was performed in late 1993 at the Laboratoire National Saturne, Saclay, France. Two systems were studied: the ^3He+ ^{nat}Ag reaction at projectile energies of 1.8, 3.6, and 4.8 GeV, and the ^3He+ ^{197}Au at 1.8 and 4.8 GeV. Total charge multiplicity, thermalized energy, and total charge were examined as gauges of excitation energy deposition. IMF multiplicity, thermalized energy and total charge distributions suggest that the excitation energy distributions of the emitting source are saturating above a projectile energy of about 3.6 GeV for the ^3He+ ^{nat }Ag system. Rapidity and moving source analysis of the IMF energy spectra are consistent with emission from a source that has obtained approximate kinetic equilibrium. An event shape analysis, demonstrated that the average sphericities are large ({~0.52}) and do not exhibit any abrupt changes with IMF multiplicity, in agreement with the simultaneous disassembly model FREESCO. For the most dissipative collisions, elemental energy spectra are indicative of emission from an expanded nuclear system. Good agreement is found between the data and predictions of a hybrid model (intranuclear cascade and Expanding Emitting Source) which includes volume emission for the most dissipative collisions. The model successfully describes the key features of the experimental IMF multiplicity distributions and the evolution of fragment spectral shapes.