Exclusive measurements of neutrons and charged products have been performed using a combination of 4/r neutron and 4n charged-particle detectors. The maximum observed energy dissipation corresponds to only approximately one-half of the available kinetic energy. For any degree of dissipation, the velocity distributions of charged particles are characteristic of sequential emission following binary collisions. The data imply that central collisions also lead to bimodal emission patterns or that they are not sufficiently well isolated by the requirement of high particle multiplicities. PACS numbers: 25.70.Lm During the last few years, considerable effort [1] has been devoted to the search for novel reaction mechanisms and nuclear decay modes that have been postulated for the Fermi energy domain [2-5]. Such novel reaction scenarios include the formation of inherently unstable, hot, and possibly compressed mononuclei that are predicted either to expand and eventually disintegrate into multiple fragments of intermediate mass (IMFs) or to vaporize into a multitude of light particles. Features similar to those expected for such processes have been observed in heavy-ion reactions at intermediate bombarding energies [6-11], but their interpretation is subject to considerable ambiguities. In order to resolve some of these ambiguities, the strategy adopted in the present work was to explore how the binary reaction dynamics characteristic of weakly dissipative, peripheral heavy-ion collisions evolves with increasing energy dissipation for the very heavy system 209 Bi+ ,36 Xe at 28.2 MeV per nucleon, i.e., at the lower boundary of the Fermi energy domain. As suggested by systematics established at lower bombarding energies [12], such a heavy system is likely to exhibit a gradual evolution of a rather uniform dissipative reaction mechanism over a significant range of impact parameters, which would simplify the detection of the onset of a new, competing process.The present paper reports on the first experiment of its kind in which almost full solid-angle coverage for both neutrons and charged particles was achieved. The experiment was performed at the National Superconducting Cyclotron Laboratory of the Michigan State University. A beam of 28.2 MeV per nucleon l36 Xe ions from the K1200 cyclotron was focused on a self-supporting 209 Bi target (1.5 mg/cm 2 ) placed in the center of the internal scattering chamber of the Rochester neutron multiplicity meter (NMM). The NMM, providing information on multiplicity and summed neutron energy, has an outer

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Multiplicity distributions of neutrons and energy spectra of light charged particles have been measured in coincidence with evaporation residues from the S + ' Sn fusion reactions at Estab --130-180 Mev. The experimental data are found to be in agreement with the results of statistical-model calculations, provided a = A./12 MeV is assumed for the efFective level-density parameter for all nuclides in the decay path. Simulation calculations have further shown that such a low value of the level-density parameter cannot be explained as a result of a possible population of superdeformed bands, unaccounted for by the statistical-decay code.PACS number(s): 25.70.

Neutron spectra from the inverse-kinematics reaction '"Ni+ "Pb at F~"s/A =6.65 Me.V have been measured in coincidence with nickel-like fragments.Neutron emission patterns for net pickup and stripping channels have been analyzed in terms of sequentia1 evaporation from fully accelerated projectilelike and targetlike fragments. As at higher energies, these patterns suggest an absence of appreciable correlations between net mass transfer and excitation energy division for strongly damped collisions, at the present near-barrier energy. The overall multiplicities, as well as energy spectra and angular distributions of neutrons, are well reproduced by simulation calculations, assuming an energy division always in favor of the heavy fragment.

Neutrons from the fusion reactions of 120 -150 MeV Si with Sn and Sn target nuclei have been measured in coincidence with evaporation residues, in two series of complementary experiments using either a 4' neutron multiplicity meter or a neutron time-of-flight spectrometer. Both the energy spectra and multiplicity distributions reveal significant quantitative di8'erences in the decay patterns of the compound nuclei Gd and Gd formed in the two reactions studied. It is shown that these differences cannot be understood in terms of decay cascades proceeding through states of enhanced collective energy, such as the superdeformed states, suggested in earlier studies. Instead, they can be explained consistently within the framework of a statistical decay model, if diferent effective level density parameters are allowed for the evaporation chains of the two composite systems. PACS number(s): 25.70.Jj, 25.70.Gh