Two-fragment reduced-velocity correlation functions of intermediate-mass fragments emitted from midrapidity component and quasiprojectile (QP) sources formed in Ni-58+ C-12 and Ni-58+ Au-197 reactions at 34.5 MeV/nucleon have been studied. For the midrapidity component, they show a stronger Coulomb suppression at low relative velocities than for the QP source, suggesting a shorter emission time for it than for the QP source. Comparing the experimental correlation functions with the prediction of many-body Coulomb trajectory code, the emission times of a QP source formed in both reactions were extracted as a function of the excitation energy. The variation of emission time of the QP source with the excitation energy is independent of the reaction system. It decreases monotonically with the excitation energy in the range of (2-6)A MeV from several hundred fm/c to about 100 fm/c. Above an excitation energy of 6A MeV, it becomes very short and saturates, suggesting that the QP source undergoes a multifragmentationlike breakup, The influence of the quasitarget fragment Coulomb interaction on QP emission time is also considered.

The formation and deexcitation of fusionlike events selected in events with a total charge equal or greater than 16 in Mg-24+C-12 system has been investigated at 25, 35, and 45 MeV/nucleon with a large multidetector array. Central single-source events are selected by use of the statistical discriminant analysis method applied to a set of 26 global variables. The fusion cross section has been extracted for the three bombarding energies and compared to other experimental data and to theoretical predictions. The total multiplicity is found to first increase to a maximum value and then decrease with increasing beam energy. It is shown that this behavior is connected to the opening of multifragmentation channels at 45 MeV/nucleon and the disappearance of channels with only light charged particles.

Complete events with at least 83% of the total charge of the 24Mg+12C system has been investigated at 45 A MeV with a large multidetector array. Central single-source events are selected by use of the Statistical Discriminant Analysis Method. Two-fragment reduced-velocity correlation functions are measured for the emission of intermediate-mass fragments from single-source events. Many-body trajectory calculations with a surface sequential model indicate an emission time of ≈400 fm/c suggesting that a large part of the single-source data can be described in a sequential decay picture. At the highest reconstructed excitation energy limit (≈8–10 A MeV), a mean emission time of ≈200 fm/c is deduced showing that part of the data may be explained by an instantaneous multifragmentation scenario.

Investigation of intermediate-velocity particle production is performed on entrance channel mass asymmetric collisions of 58Ni+C and 58Ni+Au at 34.5 MeV/nucleon. Distinctions between prompt pre-equilibrium ejections, multiple neck ruptures and an alternative phenomenon of delayed aligned asymmetric breakup is achieved using source reconstructed correlation observables and time-based cluster recognition in molecular dynamics simulations.

We have measured two-fragment correlation functions of the intermediate mass fragments emitted from quasiprojectile sources and midrapidity component formed in Ni-58+ Au-197 at 34.5 MeV/nucleon. The two-fragment correlation functions of midrapidity component show a stronger Coulomb suppression than the quasiprojectile source. This Coulomb suppression for midrapidity component changes very little with the excitation energy of the quasiprojectile source deduced event by event by calorimetry method. By comparing the experimental correlation functions with an N-body Coulomb trajectory code calculation, the emission time of quasiprojectile sources has been extracted as a function of the excitation energy. The emission time de creases monotonically with the excitation energy in the range of 2-6 A MeV from 550 fm/c to about 150 fm/c. Above excitation energy of 6A MeV, the emission time becomes shorter and constant, suggesting that prompt multifragmentation occurs in these quasiprojectile sources.

We have measured two-fragment correlation functions of the intermediate mass fragments emitted from quasiprojectile sources and midrapidity component formed in ${}^{58}\mathrm{Ni}{+}^{197}\mathrm{Au}$ at 34.5 MeV/nucleon. The two-fragment correlation functions of midrapidity component show a stronger Coulomb suppression than the quasiprojectile source. This Coulomb suppression for midrapidity component changes very little with the excitation energy of the quasiprojectile source deduced event by event by calorimetry method. By comparing the experimental correlation functions with an N-body Coulomb trajectory code calculation, the emission time of quasiprojectile sources has been extracted as a function of the excitation energy. The emission time decreases monotonically with the excitation energy in the range of 2--6 A MeV from 550 $\mathrm{fm}/c$ to about 150 $\mathrm{fm}/c.$ Above excitation energy of $6A$ MeV, the emission time becomes shorter and constant, suggesting that prompt multifragmentation occurs in these quasiprojectile sources.

Isotopic yields of IMF's produced in the Ni-58+ C-12, Mg-24 reactions at 34.5A MeV are investigated. Analysis of experimental data from the CRL-Laval 4 pi multidetector array focuses on events where at least 75% (60%) of the charge and momentum were detected for the Ni-58+ (1)2(C) (Ni-58+ Mg-24) system. Averaged isospin ratios (N/Z) for IMF's with Z=3 and 4 are plotted as a function of emission angle and parallel velocity in the center-of-mass frame. Results from simulations with the statistical codes SMM and GEMINI, assuming an equilibrated source, are compared to the experimental ratios. The ratios seem to indicate the presence of a midrapidity necklike structure that would produce IMF's richer in neutrons than the two main emitters, even for very central collisions.

Isotopic yields of IMF's produced in the ${}^{58}\mathrm{Ni}{+}^{12}\mathrm{C}{,}^{24}\mathrm{Mg}$ reactions at $34.5A$ MeV are investigated. Analysis of experimental data from the CRL-Laval $4\ensuremath{\pi}$ multidetector array focuses on events where at least $75%$ $(60%)$ of the charge and momentum were detected for the ${}^{58}\mathrm{Ni}{+}^{12}\mathrm{C}$ ${(}^{58}\mathrm{Ni}{+}^{24}\mathrm{Mg})$ system. Averaged isospin ratios $(N/Z)$ for IMF's with $Z=3$ and 4 are plotted as a function of emission angle and parallel velocity in the center-of-mass frame. Results from simulations with the statistical codes SMM and GEMINI, assuming an equilibrated source, are compared to the experimental ratios. The ratios seem to indicate the presence of a midrapidity necklike structure that would produce IMF's richer in neutrons than the two main emitters, even for very central collisions.

The characteristics of the midrapidity and target sources (apparent temperatures, velocities, and neutron multiplicities) extracted from the neutron energy spectra, have been measured for various quasiprojectile (QP) excitation energies, reconstructed from charged particles of well defined peripheral events in the ${}^{35}{\mathrm{C}\mathrm{l}+}^{\mathrm{nat}}\mathrm{Ta}$ reaction at 43 MeV/nucleon. The reconstructed excitation energy of the QP is always smaller than the excitation energy calculated from its velocity, assuming pure dissipative binary collision. The latter observation combined with the neutron multiplicity at midrapidity and the apparent temperature suggests important preequilibrium and/or dynamical effects in the entrance channel. The midrapidity source moves at a velocity lower than the nucleon-nucleon center of mass velocity showing the importance of the attractive mean-field potential from the target even at 43 MeV/nucleon. The above picture is confirmed by comparison to Boltzman-Nordheim-Vlasov (BNV) simulations.

The characteristics of the midrapidity and target sources (apparent temperatures, velocities, and neutron multiplicities) extracted from the neutron energy spectra, have been measured for various quasiprojectile (QP) excitation energies, reconstructed from char ed particles of well defined peripheral events in the Cl-35 + Ta-nat reaction at 43 MeV/nucIeon. The reconstructed excitation energy of the QP is always smaller than the excitation energy calculated from its velocity, assuming pure dissipative binary collision. The latter observation combined with the neutron multiplicity at midrapidity and the apparent temperature suggests important preequilibrium and/or dynamical effects in the entrance channel. The midrapidity source moves at a velocity lower than the nucleon-nucleon center of mass velocity showing the importance of the attractive mean-field potential from the target even at 43 MeV/nucleon. The above picture is confirmed by comparison to Boltzman-Nordheim-Vlasov (BNV) simulations. [S0556-2813(99)50302-6].

The peripheral and semiperipheral reactions in ${}^{35}\mathrm{Cl}{+}^{197}\mathrm{Au}$ have been studied at 30 and 43 MeV/nucleon. The nonequilibrium $\ensuremath{\alpha}$ and IMF components have been observed in the experiment. The fraction of nonequilibrium emission decreases with an increase in the atomic number of the projectilelike fragments but, for a given projectilelike fragment, it increases with the charge of the emitted particles. The characteristics of quasiprojectiles reconstructed from their decay products reveal several features reminiscent of damped reactions at lower bombarding energies. The atomic number and deflection angle of projectilelike fragments depend strongly on their kinetic energy or dissipated energy. At 30 MeV/nucleon, the experimental data can be explained by a deep inelastic transfer model. One-body dissipation is still the main mechanism for the energy and angular momentum dissipation. However, at 43 MeV/nucleon, deep inelastic transfer models can predict only the experimental tendency. Two-body dissipation plays a more important role at higher incident energies. The similarity observed in the decay product distributions, as a function of excitation energy, suggests that the excited quasiprojectiles formed in binary collisions might approach thermal equilibrium for both incident energies. The decay products have been analyzed with sequential-binary and simultaneous-disassembly statistical decay models. Both statistical models are able to provide good agreement with the experimental observables except for the mean kinetic energy of the products.

The experimental signature of the formation of a necklike structure, with a velocity between that of the projectilelike emitter and that of the targetlike emitter, is investigated with the same beam and experimental setup for targets lighter and heavier than the projectile. The reactions are ${}^{35}$Cl on ${}^{12}$C and on ${}^{197}$Au at 43 MeV/nucleon. Particle velocity distributions are compared with two-source statistical simulations and the presence of a necklike structure is inferred from the data. In the second part of the paper, dynamical model simulations with the formation of a necklike structure are presented for the ${}^{35}$Cl+${}^{12}$C system at 43 MeV/nucleon.

The experimental signature of the formation of a necklike structure, with a velocity between that of the projectilelike emitter and that of the targetlike emitter, is investigated with the same beam and experimental setup for targets lighter and heavier than the projectile. The reactions are Cl-35 on C-12 and on Au-197 at 43 MeV/nucleon. Particle velocity distributions are compared with two-source statistical simulations and the presence of a necklike structure is inferred from the data. In the second part of the paper, dynamical model simulations with the formation of a necklike structure are presented for the Cl-35+C-12 system at 43 MeV/nucleon.

The breakup of the projectile 24Mg, excited in peripheral collisions on a gold target, has been investigated at 25 and 35A MeV with a large scintillation-detector array allowing exclusive measurements. Absolute breakup cross sections were deduced and the projectile-like nucleus velocity and excitation energy have been reconstructed. The excitation energy partition between the projectile and the target is found to lay between the limits of equal excitation energy sharing and equal temperature with some evolution from one limit to the other. The statistical nature of the decay mechanism is inferred from global variables. Small-relative-angle analysis is applied to the six-alpha exit channel and the corresponding data were found to be consistent with a sequential evaporation decay mechanism, with some contribution from sequential fission at higher excitation energies. The time scale involved in the breakup of 24Mg projectiles into the 6α and the 5αHH channels has been investigated by examining distortions in the fragment velocity distributions due to the Coulomb field of the target. A decrease in the quasi-projectile lifetime is observed as the mean excitation energy increases from 3.4 to 4.5A MeV.

Multifragment disintegrations, measured for central Au + Au collisions at EA = 35 MeV, are analyzed with the Statistical Multifragmentation Model. Charge distributions, mean fragment energies, and two-fragment correlation functions are well reproduced by the statistical breakup of a large, diluted and thermalized system slightly above the multifragmentation threshold.

The partition of decay energy between the kinetic energy of reaction products and their Q-value of formation is obtained in a statistical derivation appropriate to highly excited nuclei, and is shown to be in a constant ratio. We measure the kinetic energy fraction, $R = \Sigma E_{kin}/(\Sigma E_{kin} + \Sigma Q_0)$, over a wide range of excitation energy for well-defined systems formed in the Cl + C reaction at 35A MeV. Relationships between excitation energy, charged-particle multiplicity, and intermediate-mass-fragment multiplicity, observed in this work and in recent experiments by a number of other groups, follow from the derivation of the average kinetic energies and Q-values.