FORMATION AND DECAY OF HOT NUCLEI IN 475 MEV, 2 GEV PROTON-INDUCED AND 2 GEV HE-3-INDUCED REACTIONS ON AG, BI, AN, AND U

The formation and decay of hot nuclei generated in the interaction of light projectiles (475 MeV and 2 GeV protons and 2 GeV He-3) on a seri es of targets (Ag-107, Au-197, Bi-209, and U-238) are studied with an apparatus combining the efficient detection of neutrons in 4 pi sr and an accurate characterization of light charged particles, intermediate -mass fragments (IMF's), and fission fragments. A two-step approach wi th an intranuclear cascade process for modeling the initial off-equili brium phase of the collision followed by a classical step-by-step evap oration-including fission competition-is used to reproduce the data. I t is inferred from the model, which is found to reproduce several data sensitive to heat, that nuclei with temperatures exceeding T =5 MeV a re produced for a sizable part of the events, thus giving the opportun ity to study the behavior of hot nuclei free from strong collective ex citations which generally accompany nucleus-nucleus collisions. Most o f the observed features related to particle emission or more specifica lly to particle evaporation are rather well accounted for by the model calculation. The evaporationlike IMF emission is generally rather wea k, and does not show any rapid onset at the highest excitation energie s as would have been expected in a genuine thermal multifragmentation process. Binary fission of the U-like target is shown to be a fairly p robable channel at most excitation energies. Some of the characteristi cs of the fission channel are satisfactorily reproduced, but not all. [S0556-2813(98)02605-3].