Quantum calculation of the dipole excitation in fusion reactions

Evidence for the Role of Proton Shell Closure in Quasifission Reactions from X-Ray Fluorescence of Mass-Identified Fragments

The atomic numbers and the masses of fragments formed in quasifission reactions are simultaneously measured at scission in ^{48}Ti+^{238}U reactions at a laboratory energy of 286 MeV. The atomic numbers are determined from measured characteristic fluorescence x rays, whereas the masses are obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasifission fragments on a broad angular range, the important role of the proton shell closure at Z=82 is evidenced by the associated maximum production yield, a maximum predicted by time-dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the N/Z (neutron over proton ratios of the fragments) evolution in quasifission reactions.

Microscopic Phase-Space Exploration Modeling of ^{258}Fm Spontaneous Fission

We show that the total kinetic energy (TKE) of nuclei after the spontaneous fission of ^{258}Fm can be well reproduced using simple assumptions on the quantum collective phase space explored by the nucleus after passing the fission barrier. Assuming energy conservation and phase-space exploration according to the stochastic mean-field approach, a set of initial densities is generated. Each density is then evolved in time using the nuclear time-dependent density-functional theory with pairing. This approach goes beyond the mean-field theory by allowing spontaneous symmetry breaking as well as a wider dynamical phase-space exploration leading to larger fluctuations in collective space. The total kinetic energy and mass distributions are calculated. New information on the fission process: fluctuations in scission time, strong correlation between TKE and collective deformation, as well as prescission particle emission, are obtained. We conclude that fluctuations of the TKE and mass are triggered by quantum fluctuations.

Interplay between quantum shells and orientation in quasifission

The quasifission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in (40)Ca+(238)U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed (238)U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of (238)U produce quasifission fragments in the magic Z=82 region, while collisions with the side are the only ones that may result in fusion.

Suppression of charge equilibration leading to the synthesis of exotic nuclei

Charge equilibration between two colliding nuclei can take place in the early stage of heavy-ion collisions. A basic mechanism of charge equilibration is presented in terms of the extension of single-particle motion from one nucleus to the other, from which the upper energy limit of the bombarding energy is introduced for significant charge equilibration. The formula for this limit is presented, and is compared to various experimental data. It is examined also by comparison to three-dimensional time-dependent density functional calculations. The suppression of charge equilibration, which appears in collisions at the energies beyond the upper energy limit, gives rise to remarkable effects on the synthesis of exotic nuclei with extreme proton-neutron asymmetry.

Collision dynamics of two 238U atomic nuclei

Collisions of actinide nuclei form, during very short times of few 10;{-21} s, the heaviest ensembles of interacting nucleons available on Earth. Such collisions have been proposed as an alternative way to produce heavy and superheavy elements. They are also used to produce superstrong electric fields by the huge number of interacting protons to test spontaneous positron-electron (e;{+}e;{-}) pair emission predicted by the quantum electrodynamics theory. The time-dependent Hartree-Fock theory is used to study collision dynamics of two 238U atomic nuclei. In particular, the role of nuclear deformation on collision time and on reaction mechanisms such as nucleon transfer is emphasized. The highest collision times (approximately 4 x 10;{-21} s at 1200 MeV) should allow experimental signature of spontaneous e;{+}e;{-} emission in case of bare uranium ions. Surprisingly, we also observe ternary fission due to purely dynamical effects.