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Abstract: The angular distributions of the fission fragments of a gold target have been measured in peripheral collisions initiated by several beams: Ar, Kr, Xe, Pb at energies ranging from 27 to 60 MeV/u. The angular momentum transfers in the various reactions have been deduced from these angular distributions and compared with theoretical predictions. The possibility to obtain the angular momentum transfer from the measured angular distribution of the light charged particles is also discussed.

Topics: Total angular momentum quantum number (72%), Angular momentum of light (72%), Angular momentum coupling (71%), Angular momentum (65%), Orbital angular momentum of light (65%)

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Abstract: Recent experimental results concerning heavy systems (Pb + Au, Pb + Ag, Pb + Al, Gd + C, Gd + U, Xe + Sn, …) obtained at GANIL with the INDRA and NAUTILUS 4 π arrays will be presented. The study of reaction mechanisms has shown the dominant binary and highly dissipative character of the process. The two heavy and excited fragments produced after the first stage of the interaction can decay into various decay modes from evaporation to multifragmentation including fission. However, deviations from this simple picture have been found by analyzing angular and velocity distributions of light charged particles, and fragments. Indeed, there is a certain amount of matter in excess emitted between the two primary sources suggesting either the existence of a mid-rapidity source similar to the one observed in the relativistic regime (participants) or a strong deformation induced by the dynamics of the collision (neck instability). This last possibility has been suggested by analyzing in detail the angular distributions of the fragments. More precisely, we observe an isotropic component which is compatible with the prediction of statistical models and a second one corresponding to breakup aligned with the recoil direction of the projectile like source which should be compared with the predictions of dynamical calculations based on microscopic transport models.

39 citations

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Abstract: The excitation energy and angular momentum transferred to quasiprojectiles have been measured in the 129 Xe+ nat Sn collisions at bombarding energies between 25 and 50 MeV/nucleon. The excitation energy of quasiprojectiles has been determined from the kinetic energy of all decay products (calorimetry). It increases with the violence of the collision, approaching 10 MeV/nucleon in the most dissipative ones. The angular momentum has been deduced from the kinetic energies and angular distributions of the emitted light charged particles (p, d, t, 3 He and α ). The (apparent) spin value decreases with the violence of the collision. Larger spin values are observed at the lowest bombarding energy. Data are compared with the predictions of dynamical and statistical models. They reproduce the data in a quantitative way indicating that large spin values are transferred to quasiprojectiles during the interaction. The results show that the one-body dissipation formalism still applies at intermediate bombarding energies and low-energy dissipations. With the increase of the energy, the data seem to be better described when the two-body interaction is accounted for.

29 citations

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Abstract: Ternary fission of heavy hot composite systems with excitation energies of 1.5–2.5 MeV / amu has been studied in the reactions of 14N(53 A MeV) with 197Au and 232Th. The ternary yields have been explored as a function of the charge Z L of the lightest fragment: while Z L increases from 6 to 25, the cross sections decrease from 5 to 0.08 mb for N + Au and from 15 to 0.8 mb for N + Th. The velocity vector v → L of the lightest fragment has been investigated in the rest frame of the other two heavier fragments. Two different components are observed: (i) an isotropic one with values of v L corresponding to the Coulomb repulsion from the combined heavier fragments before separation and, (ii) an anisotropic contribution with the lightest fragment emitted with lower v L perpendicular to the scission axis of the two heavier fragments. The latter component is distinguished from the isotropic one by an enhanced fraction of mass-symmetric ternary events and by up to 50 MeV lower total kinetic fragment energies. These features are indicative of a collinear stretched scission configuration, where the lightest fragment is positioned between the two heavier ones.

11 citations

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Abstract: The detailed features of the multistep evaporation process concerning the decay of the 59 Cu compound nucleus, formed by a 32 S-beam at an energy E Lab = 8 MeV/amu, have been analyzed with respect to mean decay paths leading to a selected residual nucleus, in the frame of an out-of-plane angular correlation experiment, at 90°. For a given residual nucleus, mean multiplicities for neutrons, protons and α-particles as well as mean excitation energies and angular momenta from initial steps down to ground states have been calculated. For each step of the cascades, mean decay probabilities for neutrons, protons and α-particles as well as mean kinetic energies and transferred orbital angular momenta have been deduced. Whereas in the case of protons the anisotropy turns out to be actually independent of the compound nucleus J CN -angular momentum distribution, for α-particles it is strongly depending on J CN and can therefore give severe constraints on the higher angular momenta sustained by the compound nucleus.

1 citations

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Abstract: Complex fragments of 3 Z ⪅35 have been detected in the reverse-kinematics reactions of 93 Nb plus 9 Be, 12 C and 27 Al at bombarding energies of E / A =11.4, 14.7 and 18.0 MeV. Velocity spectra and angular distributions show the presence of projectile and target-like components along with a component isotropic (in the reaction plane). This latter component aappears as a Coulomb ring in the invariant cross section plots indicating the presence of a binary decay which is confirmed by the coincidence data. Statistical model calculations indicate that for the Nb+Be and C reactions, the isotropic component is associated with the binary decay of compound nuclei formed in complete fusion reactions. The charge distributions for these two systems are consistent with the conditional barriers predicted with the rotating finite-range model. For the Nb+Al reactions, there is an additional isotropic component besides compound nucleus decay, which may arise from fast fission.

373 citations

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Abstract: We present a semi-classical approach of the heavy-ion collision theory in the intermediate energy domain (10–100 MeV incident kinetic energy per nucleon) based on the Vlasov equation and its extension — the Landau-Vlasov equation — when the residual interaction is accounted for through a collision kernel. We use the coherent state set as an overcomplete basis for the decomposition of the nuclear phase-space distributions. We show that the uniform repartition of coherent states in phase space provides semi-classical descriptions of nuclei at equilibrium which are the correct initial conditions of the Vlasov and Landau-Vlasov dynamical equations. In the slab geometry, we compare the results of the Vlasov equation with those of the TDHF theory for the crossing of a potential barrier and the collision of two slabs. We present sample results of three-dimensional calculations of heavy-ion collisions with a Skyrme self-consistent interaction and inclusion of the Coulomb interaction; the individual collisions being described by the Uehling-Uhlenbeck kernel. These calculations illustrate the incomplete fusion process for central collisions at 27 MeV/u incident energy and the onset of an abrasion-like process for more peripheral collisions at 35 MeV/u.

188 citations

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Abstract: The nucleons transferred between two colliding nuclei transport charge, mass, linear and angular momentum, and energy. General expressions are obtained for the corresponding transport coefficients by modelling the dinuclear system as two Fermi-Dirac gases in contact. The drift and diffusion coefficients are related by a generalized Einstein relation, reflecting the fluctuation-dissipation theorem; particular simplicity arises in the extreme quantal and classical limits. Application of the nuclear proximity method leads to simple factorized formulas valid for a certain general family of binary configurations relevant to nuclear collisions: the corresponding form factors can be estimated in a parameter-free semiclassical (Thomas-Fermi) mean-field model of nuclei. A simulation method is suggested for the numerical solution of the ensuing multi-dimensional transport problem.

111 citations