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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 series 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 with an intranuclear cascade process for modeling the initial off-equilibrium phase of the collision followed by a classical step-by-step evaporation-including fission competition-is used to reproduce the data. It is inferred from the model, which is found to reproduce several data sensitive to heat, that nuclei with temperatures exceeding T =5 MeV are produced for a sizable part of the events, thus giving the opportunity to study the behavior of hot nuclei free from strong collective excitations which generally accompany nucleus-nucleus collisions. Most of the observed features related to particle emission or more specifically to particle evaporation are rather well accounted for by the model calculation. The evaporationlike IMF emission is generally rather weak, and does not show any rapid onset at the highest excitation energies as would have been expected in a genuine thermal multifragmentation process. Binary fission of the U-like target is shown to be a fairly probable channel at most excitation energies. Some of the characteristics of the fission channel are satisfactorily reproduced, but not all. [S0556-2813(98)02605-3].

Formation and decay of hot nuclei have been studied in Zn-64 + Ti-nat collisions between 35 and 79 MeV/nucleon. The mass and excitation energy of excited quasiprojectiles are reconstructed from the kinematical characteristics of their decay products. In central collisions, excitation energies larger than 10 MeV/nucleon are reached. Comparisons with theoretical predictions indicate that a fraction of the excitation energy is associated with an isotropic radial flow.

Neutral pion production has been studied in the $^{36}\mathrm{Ar}$${+}^{27}$Al reaction at 95 MeV/nucleon with the aim to get a quantitative estimate of its impact-parameter dependence. A near 4\ensuremath{\pi} multidetector has been used to detect both the gamma rays originating from the ${\mathrm{\ensuremath{\pi}}}^{0}$ decay and the associated charged particles. The charged particle multiplicity has been used in the present analysis as a global variable to extract the impact parameter scale. A comparison with a Boltzmann-Nordheim-Vlasov calculation, whcih takes into account the effect of pion reabsorption in the nuclear medium, has been performed.

The production of neutral pions in the reaction [sup 16]O+[sup 27]Al at 94 MeV/nucleon was studied with a multidetector, which includes 180 BaF[sub 2] modules. Kinetic energy spectra for several laboratory angles were measured. The total cross section for neutral pion production was deduced. Results were compared with previous findings on charged pions from the same reaction at the same energy and with the prediction of a dynamical model based on the numerical solution of the Boltzmann-Nordheim-Vlasov equation.

Reaction mechanisms and multifragmentation processes have been studied for Zn-64 + Ni-58 collisions at intermediate energies with the help of antisymmetrized molecular dynamics (AMD-V) model calculations. Experimental energy spectra, angular distributions, charge distributions, and isotope distributions, classified by their associated charged particle multiplicities, are compared with the results of the AMD-V calculations. In general the experimental results are reasonably well reproduced by the calculations. The multifragmentation observed experimentally at all incident energies is also reproduced by the AMD-V calculations. A detailed study of AMD-V events reveals that, in nucleon transport, the reaction shows some transparency, whereas in energy transport the reaction is much less transparent at all incident energies studied here. The transparency in the nucleon transport indicates that. even for central collisions, about 75% of the projectile nucleons appear in the forward direction. In energy transport about 80% of the initial kinetic energy of the projectile in the center-of-mass frame is dissipated. The detailed study of AMD-V events also elucidates the: dynamics of the multifragmentation process. The study suggests that, at 35A MeV, the semitransparency and thermal expansion are the dominant mechanisms for the multifragmentation process, whereas at 49A MeV and higher incident energies a nuclear compression occurs at an early stage of the reaction and plays an important role in the multifragmentation process in addition to that of the thermal expansion and the semitransparency.

An elaborated investigation was done into the reaction mechanism of heavy ion collisions at intermediate energies of small nuclear systems. Experimental data measured with the Huygens detectors and isospin quantum molecular dynamics (IQMD) simulations of ${}^{24}{\mathrm{M}\mathrm{g}+}^{27}\mathrm{Al}$ at 45 and $95A\mathrm{MeV}$ were analyzed with respect to the impact-parameter selection, the in-plane flow and the IMF production at midrapidity. It was found that only the midcentral impact-parameter class could be identified effectively. The event-plane reconstruction and flow measurements were studied with IQMD and a GEANT detector simulation. This showed that the azimuthal-correlation method gives the best reconstruction. The experimentally measured in-plane flow was consistent with the balancing energy derived from literature of $114\ifmmode\pm\else\textpm\fi{}10A\mathrm{MeV}.$ The azimuthal correlations and polar-angle distributions of IMF's emitted at midrapidity were used to distinguish between a dynamic scenario, i.e., IQMD, and a statistical scenario described by a simple decaying-source model. The IQMD simulations were in good agreement with the experimental data while a single decaying-source simulation fails to describe the data which shows that multifragmentation in these small systems is consistent with a dynamic process.

Two-particle correlations at small relative momenta were measured for /sup 16/O-induced reactions on /sup 197/Au at E/A = 94 MeV, providing information about the space-time localization and excitation energy density of the emitting system. Two-particle correlation functions suggest that emission occurs from a region which is slightly larger than observed for /sup 14/N-induced reactions at E/A = 35 MeV, but significantly smaller than for /sup 40/Ar-induced reactions at E/A = 60 MeV. In contrast, the relative populations of widely separated particle unbound states are found to be surprisingly independent of incident energy and projectile mass.

Two-particle correlations at small relative momenta provide information about the space-time extent and the excitation energy density of the emitting system. Detailed measurements of two-particle correlations were performed for $^{40}\mathrm{induced}$ reactions on $^{197}\mathrm{Au}$ at E/A=60 MeV. In general, more pronounced correlations are measured for particles of higher outgoing energy. From the relative populations of excited states, a mean emission temperature of T\ensuremath{\approxeq}5.5 MeV is extracted. Within the framework of the quantum statistical model, our findings are consistent with an expanding nuclear system which disintegrates at low temperature (T\ensuremath{\approxeq}5.5 MeV) and rather low density (\ensuremath{\rho}\ensuremath{\le}0.1${\ensuremath{\rho}}_{0}$).

Correlations between coincident alpha particles and deuterons emitted in /sup 40/Ar-induced reactions on /sup 197/Au at E/A = 60 MeV were measured. The relative populations of particle-unstable states of /sup 6/Li were extracted. The measured spectra are consistent with a mean nuclear temperature at emission of T = 5 MeV. Limitations of temperature measurements via the population of excited states are discussed.