Kouhei Washiyama

One-body energy dissipation in fusion reactions from mean-field theory

Information on dissipation in the entrance channel of heavy-ion collisions is extracted by the macroscopic reduction procedure of time-dependent Hartree-Fock theory. The method gives access to a fully microscopic description of the friction coefficient associated with the transfer of energy from the relative motion toward intrinsic degrees of freedom. The reduced friction coefficient exhibits a universal behavior, i.e., almost independent of systems investigated, whose order of magnitude is comparable with the calculations based on linear response theory. Similarly to nucleus-nucleus potential, especially close to the Coulomb barrier, there are sizable dynamical effects on the magnitude and form factor of the friction coefficient.

Fluctuation and dissipation dynamics in fusion reactions from a stochastic mean-field approach

By projecting the stochastic mean-field dynamics on a suitable collective path during the entrance channel of heavy-ion collisions, expressions for transport coefficients associated with relative distance are extracted. These transport coefficients, which have forms similar to those familiar from the nucleon exchange model, are evaluated by carrying out time-dependent Hartree-Fock simulations. The calculations provide an accurate description of the magnitude and form factor of transport coefficients associated with one-body dissipation and fluctuation mechanisms.

Mass dispersion in transfer reactions with a stochastic mean-field theory

Nucleon transfer in symmetric heavy-ion reactions at energies below the Coulomb barrier is investigated in the framework of a microscopic stochastic mean-field theory. Although mean field alone is known to significantly underpredict the dispersion of the fragment mass distribution, a considerable enhancement of the dispersion is obtained in the stochastic mean-field theory. The variance of the fragment mass distribution deduced from the stochastic theory scales with the number of exchanged nucleons. Therefore, the new approach provides the first fully microscopic theory consistent with the phenomenological analysis of the experimental data.

Pairing dynamics in particle transport

We analyze the effect of pairing on particle transport in time-dependent theories based on the Hartree-FockBogoliubov (HFB) or BCS approximations. The equations of motion for the HFB density matrices are unique and the theory respects the usual conservation laws defined by commutators of the conserved quantity with the Hamiltonian. In contrast, the theories based on the BCS approximation are more problematic. In the usual formulation of time-dependent Hartree-Fock (TDHF) + BCS, the equation of continuity is violated and one sees unphysical oscillations in particle densities. This can be ameliorated by freezing the occupation numbers during theevolutioninTDHF + BCS,butthereareotherproblemswiththeBCSapproximationthatmakeitdoubtfulfor reaction dynamics. We also compare different numerical implementations of the time-dependent HFB equations. The equations of motion for the U and V Bogoliubov transformations are not unique, but it appears that the usual formulation is also the most efficient. Finally, we compare the time-dependent HFB solutions with numerically

Nucleon exchange mechanism in heavy-ion collisions at near-barrier energies

Nucleon drift and diffusion mechanisms in central collisions of asymmetric heavy ions at near-barrier energies are investigated in the framework of a stochastic mean-field approach. Expressions for diffusion and drift coefficients for nucleon transfer deduced from the stochastic mean-field approach in the semiclassical approximation have similar forms familiar from the phenomenological nucleon exchange model. The variance of fragment mass distribution agrees with the empirical formula {sigma}{sub AA}{sup 2}(t)=N{sub exc}(t). The comparison with the time-dependent Hartree-Fock calculations shows that below barrier energies, the drift coefficient in the semiclassical approximation underestimates the mean number of nucleon transfer obtained in the quantal framework. Motion of the window in the dinuclear system has a significant effect on the nucleon transfer in asymmetric collisions.

ENERGY DISSIPATION IN FUSION REACTIONS FROM DYNAMICAL MEAN-FIELD THEORY

Nucleus-nucleus interaction potentials and friction coefficients associated with one-body energy dissipation in the entrance channel of fusion reactions are extracted from the microscopic time-dependent Hartree-Fock theory. They show center-of-mass energy dependence close to the Coulomb barrier energy. This dependence indicates dynamical reorganization of internal degrees of freedom. We give a simple estimate of excitation energy from microscopic nucleon exchange.

Multipole modes of excitation in triaxially deformed superfluid nuclei

The five-dimensional quadrupole collective model based on energy density functionals (EDF) has often been employed to treat long-range correlations associated with shape fluctuations in nuclei. Our goal is to derive the collective inertial functions in the collective Hamiltonian by the local quasiparticle random phase approximation (QRPA) that correctly takes into account time-odd mean-field effects. Currently, practical framework to perform the QRPA calculation with the modern EDFs on the

Large amplitude motion with a stochastic mean-field approach

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Nucleus-nucleus potential, energy dissipation and mass dispersion infusion and transfer reactions

deformation space is not available. Toward this goal, we develop an efficient numerical method to perform the QRPA calculation on the

Extraction of nucleus‐nucleus potential and energy dissipation from dynamical mean‐field theory

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