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Semi-peripheral collisions of ^{64}Ni + ^{124}Sn in direct and inverse kinematics, at 35 A MeV incident energy, are studied to characterize the dynamical origin and align-ments properties of fragments emission at midrapidity. This emission is partly due to the fragmentation of a transient neck joining the projectile and target in the early stages of the reaction. Recently we exploited observable from neck fragmentation to constraint the density dependence of the symmetry energy. In this contribution we focus on the density evolution of the mid-rapidity source. An experimental survey is suggested to check if data are consistent with the formation of neck fragments in a dilute density region in contact with the projectile-like and target-like spectators at normal saturation density.

Matière Nucléaire NIM; Semiperipheral collisions in the $^{124}$Sn+${64}Ni reaction at 35 MeV/nucleon were studied using the forward part of the Charged Heavy Ion Mass and Energy Resolving Array. Nearly completely determined ternary events involving projectilelike fragments (PLF), targetlike fragments (TLF), and intermediate mass fragments (IMF) were selected. A new method of studying the reaction mechanism, focusing on the analysis of the correlations between relative velocities in the IMF+PLF and IMF+TLF subsystems, is proposed. The relative velocity correlations provide information on the time sequence and time scale of the neck fragmentation processes leading to production of IMFs. It is shown that the majority of light IMFs are produced within 40–80 fm/c after the system starts to reseparate. Heavy IMFs are formed at times of about 120 fm/c or later and can be viewed as resulting from two-step (sequential) neck rupture processes.

Gas ionization chambers of a special shape adapted for 4π detector systems are presented. Different anode wire configurations are discussed for three working gases: Ar + methane(10%), isobutane, and tetrafluoromethane.

Populations of excited 5 Li and 6 Li fragments have been measured for the 3 He + Ag reaction at 200 MeV. At forward angles, fragments emitted via preequilibrium mechanisms display population probabilities that are consistent with emission temperatures of about 4 MeV, comparable to other measurements of preequilibrium processes. In contrasts, fragments emitted to backward angles display much smaller population probabilities consistent with emission temperatures of about 1 MeV. Such low temperatures are consistent with residue excitation energies predicted by molecular dynamics calculations.

Intermediate-mass fragments formed in reactions of $^{3}\mathrm{He}$ ions with $^{\mathrm{nat}}\mathrm{Ag}$ and $^{197}\mathrm{Au}$ targets have been studied at five energies between 0.48 and 3.6 GeV. Inclusive measurements show that as the bombarding energy increases, there is a strong enhancement in fragment cross sections and a trend toward isotropic angular distributions. Between 0.90 and 1.8 GeV, a change in the emission mechanism is suggested by (1) kinetic energy spectra with high-energy tails that become distinctly flatter, (2) a broadening of the spectral Coulomb peaks toward lower energies, and (3) charge distributions that become constant, exhibiting a power-law exponent \ensuremath{\tau}\ensuremath{\approxeq}2.0. Exclusive studies of the $^{3}\mathrm{He}$${+}^{\mathrm{nat}}$Ag system at 0.90 and 3.6 GeV detected multifragment events with multiplicities up to four. The probability for high-multiplicity events increases about 40-fold between 0.90 and 3.6 GeV. At both energies, the kinetic energy spectra depend on multiplicity, especially when triggering on backward-emitted fragments. For multiplicity three events, a rapidity analysis of the data at 3.6 GeV is consistent with a single, relatively low source velocity, ${\mathit{v}}_{\mathit{S}}$\ensuremath{\approxeq}0.4 cm/ns. The data are compared with predictions of a coplanarity-sphericity calculation, the sequential statistical decay code GEMINI, and a hybrid intranuclear cascade/percolation model.

Classical dynamical calculations of the heavy ion induced fission processes have been performed for the reactions40Ar+141Pr,20Ne+165Ho and12C+175Lu leading to the iridium like nucleus. As a result prescission lifetimes were obtained and compared with the experimental values. The comparison between the calculated and experimental lifetimes indicates that the one-body dissipation picture is much more relevant in describing the fusion-fission dynamics than the two-body one.

Analyzing powers and isotope ratios have been measured for ejectiles with Z\ensuremath{\le}7 emitted at forward angles in the 200-MeV p\ensuremath{\rightarrow}${+}^{\mathrm{nat}}$Ag reaction. The observed analyzing powers are consistent with zero, and thus do not provide evidence for a significant contribution from cluster knockout, or similar direct formation mechanisms. Fragment kinetic-energy spectra above the Coulomb peak are compared with a coalescence calculation. The isotopic composition of the elemental kinetic-energy spectra is found to favor N/Z\ensuremath{\ge}1 nuclei for fragment energies near the exit-channel Coulomb energy, whereas species with N/Z\ensuremath{\le}1 are more abundant in the high-energy spectral tails. This behavior is consistent with the predictions of an accreting source calculation.

Two-proton correlation functions, measured at small relative momenta in the $^{3}\mathrm{He}$+Ag reaction at 200 MeV, increase monotonically with the kinetic energy of the detected particles. Very strong correlations are observed for intermediate energy protons. These correlations scale with the weaker correlation functions measured for heavier projectiles, and can be reproduced by the Boltzmann-Uehling-Uhlenbeck equation. Even stronger correlations measured for beam velocity protons, however, may be remnants of the initial state wave functions of the $^{3}\mathrm{He}$ projectiles.

Two-proton correlation functions have been measured at {theta}{sub lab}{approx}25{degree} for the forward kinematics'' reactions {sup 14}N+{sup 27}Al, {sup 14}N+{sup 197}Au at {ital E}/{ital A}=75 MeV, for the inverse kinematics'' reaction {sup 129}Xe+{sup 27}Al at {ital E}/{ital A}=31 MeV, and for the nearly symmetric reaction {sup 129}Xe+{sup 122}Sn at {ital E}/{ital A}=31 MeV. For the reactions at 75 MeV per nucleon, the correlation functions exhibit pronounced maxima at relative proton momenta, {ital q}{approx}20 MeV/{ital c}, and minima at {ital q}{approx}0 MeV/{ital c}. These correlations indicate emission from fast, nonequilibrium processes. They are analyzed in terms of standard Gaussian source parametrizations and compared to microscopic simulations performed with the Boltzmann-Uehling-Uhlenbeck equation. For the reactions at 31 MeV per nucleon, the two-proton correlation functions do not exhibit maxima at {ital q}{approx}20 MeV/{ital c}, but only minima at {ital q}{approx}0 MeV/{ital c}. These correlations indicate emission on a slower time scale. They can be reproduced by calculations based on the Weisskopf formula for evaporative emission from fully equilibrated compound nuclei. For all reactions, the measured longitudinal and transverse correlation functions are very similar, in agreement with theoretical predictions.

Two-proton correlation functions measured for the {sup 14}N+{sup 27}Al reaction at {ital E}/{ital A}=75 MeV are compared to correlation functions predicted for collision geometries obtained from numerical solutions of the Boltzmann-Uehling-Uhlenbeck (BUU) equation. The calculations are in rather good agreement with the experimental correlation function, indicating that the BUU equation gives a reasonable description of the space-time evolution of the reaction.

The emission of complex fragments (3⩽ Z ≲12) from reactions of 3 He with nat Ag has been studied at bombarding energies of E = 480, 900, 1800, 2700 and 3600 MeV. Between 900 and 1800 MeV slopes of the fragment kinetic energy spectra at backward angles undergo a change in character, becoming much flatter; in addition, the Coulomb peak is found to broaden significantly and shift to lower energies. Power-law fits to the fragment charge distributions result in decreasing values of the exponent τ up to a bombarding energy of 1800 MeV; at this value and beyond, a constant value of τ =2.1±0.1 is observed. Elemental cross sections at the highest energy are found to increase by approximately two orders of magnitude relative to data near 100 MeV. The data suggest a change in reaction mechanism between incident energies of 900 and 1800 MeV.

The excitation-energy distribution of projectilelike fragments formed in the {ital E}/{ital A}=8.5 MeV {sup 74}Ge+{sup 165}Ho reaction has been obtained by applying a statistical evaporation calculation to the difference between primary fragment mass yields derived from kinematic coincidence measurements and directly measured yields. A gradual transition from approximate equipartition of excitation energy for small energy losses to a division that favors the targetlike fragment for highly damped events is observed, in agreement with previous measurements. For a given energy loss, heat partition is found to correlate with the direction of net nucleon transfer. The variances of the excitation-energy distributions are shown to increase monotonically as a function of energy loss. The average data are compared with predictions of the nucleon exchange transport model. The results suggest that energy-loss mechanisms in addition to nucleon exchange may be important in the early stages of the collision.

Multi-breakup processes for the {sup 124}Sn+{sup 64}Ni system at 35 MeV/nucleon have been studied with the forward part of the CHIMERA detector. An extensive comparison between experimental data corresponding to almost complete ternary events and constrained molecular dynamics (CoMD-II) calculations suggests different characteristic times in the selected processes. This is in agreement with previous studies of the same reaction already published concerning the prompt intermediate-mass-fragment emission. Stimulated by CoMD-II calculations, we investigate the existence of more complex dynamical multi-breakup processes occurring on the same time scale. A detailed study of the rotational dynamics leading to slower dynamical fission processes is also presented.

Chatterjee [ital et] [ital al]. [Phys. Rev. C 44, R2249 (1991)] report that the sharing of dissipated energy in the [sup 58]Ni + [sup 208]Pb reaction at [ital E]/[ital A]=6.5 MeV is not influenced appreciably by the direction and magnitude of net charge (and mass) transfer. We point out that neither this conclusion nor one arguing for a correlation between mass flow and excitation energy division is supported by the data and analysis presented in their communication.

Inclusive and coincidence measurements have been performed to study symmetric fragmentation of $^{44}\mathrm{Ti}$ binary decay from the $^{32}\mathrm{C}$ reaction at 280 MeV incident energy. Element distributions after binary decay were measured. Angular distributions and fragment correlations are presented. Total c.m. kinetic energy for the symmetric products is extracted from our data and from Monte-Carlo model calculations including Q-value fluctuations. This result was compared to liquid drop model calculations and standard fission systematics. Comparison between the experimental value of the total kinetic energy and the rotating liquid-drop model predictions locates the angular momentum window for symmetric splitting of $^{44}\mathrm{Ti}$ between 33\ensuremath{\Elzxh} and 38\ensuremath{\Elzxh}. It also showed that 50% of the corresponding rotational energy contributes to the total kinetic energy values. The dominant reaction mechanism was found to be symmetric splitting followed by evaporation.

Inclusive charged particle spectra were measured from nuclear reactions induced by 156 MeV 6Li on 40Ca. At forward angles the spectra exhibit broad break-up distributions centered around the energy corresponding to the beam velocity. The double differential cross sections together with previous results for a 208Pb target were analyzed in the framework of the DWBA approach to projectile break-up taking into account elastic and inelastic reactions of the break-up fragments. The high energy tails of the background due to preequilibrium emission of complex charged particles were estimated on the basis of the coalescence model.

Inclusive and coincidence measurements have been performed to study symmetric products from the reactions 74--186 MeV /sup 12/C+ /sup 40/Ca, 141 MeV /sup 9/Be+ /sup 40/Ca, and 153 MeV /sup 6/Li+ /sup 40/Ca. The binary decay of the composite system has been verified. Energy spectra, angular distributions, and fragment correlations are presented. The total kinetic energies for the symmetric products from these very light composite systems are compared to liquid drop model calculations and fission systematics.

Nuclide distributions have been measured for damped collision products formed in the reaction of E/A=8.5 MeV $^{58}\mathrm{Ni}$ and $^{64}\mathrm{Ni}$ ions with $^{238}\mathrm{U}$. The data demonstrate that in these very asymmetric systems the evolution of the nucleon-exchange process as a function of energy loss depends strongly on the N/Z value of the projectile and the corresponding gradient in the potential-energy surface. Comparison of the data with transport model calculations shows qualitative agreement with the N and Z centroids, variances, and correlation coefficients. However, absolute discrepancies exist which suggest the need for improvement in the model.

Mass and charge distributions have been measured for damped projectile-like fragments in the reaction /sup 74/Ge+/sup 165/Ho at 8.5 MeV per nucleon bombarding energy. Coincidences were measured between angle-correlated reaction partners in order to derive the primary mass distribution and the excitation energy distribution for projectile-like fragments. The evolution of the primary N and Z distributions as a function of energy loss is found to deviate significantly from predictions of the nucleon exchange transport model. The fraction of excitation energy residing in the projectile-like fragment is shown to depend both upon energy loss and the direction of the nucleon flow.

Experimental fission excitation functions for compound nuclei $^{52}\mathrm{Fe}$, $^{49}\mathrm{Cr}$, $^{46}\mathrm{V}$, and $^{44}\mathrm{Ti}$ formed in heavy-ion reactions are analyzed in the Hauser-Feshbach/Bohr-Wheeler formalism using fission barriers based on the rotating liquid drop model of Cohen et al. and on the rotating finite range model of Sierk. We conclude that the rotating finite range approach gives better reproduction of experimental fission yields, consistent with results found for heavier systems.

Projectile-like fragments have been studied with high A and Z resolution for the damped reactions /sup 238/U+/sup 40/Ca, /sup 48/Ca, /sup 58/Ni, and /sup 64/Ni at 8.5 MeV/nucleon. These dinuclear systems exhibit widely different driving forces. Qualitative agreement between experiment and dynamical nucleon-exchange transport theory is obtained for the first and second moments of the fragment atomic (Z) and neutron (N) number distributions. However, systematic differences exist between the experimental and theoretical average values of Z and N suggesting the need for more realistic driving forces.

In past experiments, mass asymmetric projectile-target combinations 124Sn+64Ni and 112Sn+58Ni were investigated at ELab(112,124Sn)=35 A.MeV beam energy by using the 4π multi-detector CHIMERA. From a quantitative comparison of cross sections associated to Statistical and Dynamical Fission of the Projectile-Like Fragments, it resulted that Dynamical Fission process is about two times more probable in the neutron rich 124Sn+64Ni system than in the 112Sn +58 Ni neutron poor one. In contrast, no sizable difference was found for Statistical Fission mechanism. The observed difference in the strength of the Dynamical effects could arise from the difference in entrance channel Isospin (N/Z) content. In order to disentangle Isospin effects from effects due to the different masses of the two systems, a new experiment 124Xe+64Zn at 35 A.MeV beam energy has been recently carried out.