Yasuhisa Abe

Multi-dimensional Langevin approach to fission dynamics

Abstract Multi-dimensional Langevin approach to nuclear dissipative phenomena is presented. We solve two-dimensional Langevin equation numerically without any approximations for the study of induced fission dynamics. Transient time of the symmetric fission of 213At is calculated and discussed in comparison with empirically deduced value. Fragment kinetic energy distribution is calculated and compared with experiments.

Diffusion over a saddle with a Langevin equation

The diffusion problem over a saddle is studied using a multidimensional Langevin equation. An analytical solution is derived for a quadratic potential and the probability to pass over the barrier deduced. A very simple solution is given for the one-dimensional problem and a general scheme is shown for higher dimensions.

KEWPIE: A dynamical cascade code for decaying exited compound nuclei

A new dynamical cascade code for decaying hot nuclei is proposed and specially adapted to the synthesis of super-heavy nuclei. For such a case, the interesting channel is of the tiny fraction that will decay through particles emission, thus the code avoids classical Monte-Carlo methods and proposes a new numerical scheme. The time dependence is explicitely taken into account in order to cope with the fact that fission decay rate might not be constant. The code allows to evaluate both statistical and dynamical observables. Results are successfully compared to experimental data.

ISOSPIN DEPENDENCE OF REACTIONS 48Ca+243-251Bk

The fusion process of 48Ca induced reactions is studied with the two-step model. In this model, the fusion process is devided into two stages: first, the sticking stage where projectile and target come to the touching point over the Coulomb barrier from infinite distance, and second, the formation stage where the di-nucleus formed with projectile and target evolve to form the spherical compound nucleus from the touching point. By the use of the statistical evaporation model, the residue cross sections for different neutron evaporation channels are analyzed. From the results, optimum reactions are given to synthesize Z=117 element with 48Ca induced reactions.

Inhomogeneous friction leading to current in periodic system

We study the diffusion process of a Brownian particle moving in a one-dimensional ratchet with space-dependent friction that is subjected to an external source. The equation of motion for a particle involves a multiplicative fluctuation, a nonlinear friction and an external driving force or a Gaussian white noise. The average position of the particle is simulated numerically in terms of the Langevin Monte Carlo method and discussed by means of the adiabatic approximation and the effective potential. The influence of coordinate-dependent friction on the average position and the direction of current of the particle is investigated. The results show that a net drift can be produced in the presence of both a coordinate-dependent friction and an external fluctuation, even when the ratchet potential and the temporal force are completely symmetrical.

An Integral Algorithm for Numerical Integration of One-Dimensional Additive Colored Noise Problems

We present an integral-closed algorithm for solving a Langevin equation driven by an additive colored noise. Both the mean first passage time in a bistable system and the diffusion current in a titled periodic potential are calculated and the comparison with existing algorithms is carried out. The dependence of the numerical results on the time steps is studied. Our algorithm is shown to have high accuracy and stability.

A Bohr-Mottelson model of nuclei at finite temperature

Starting from Boltzmann-Langevin equation and performing a fluid dynamical reduction, a system of coupled Langevin equations is derived for a multipolar velocity field. This system is then applied to monopole andβ-,γ-quadrupole resonances, making a link between microscopic approaches and phenomenological macroscopic ones.

Di-nuclear molecular states in oblate-oblate system

Abstract Narrow resonances in heavy-ion scattering of 28 Si + 28 Si are studied as an example of oblate-oblate di-nuclear systems. By the use of the molecular model, a stable configuration of the system is found to be an equator-equator touching one, due to the oblate shape of the 28 Si nuclei. Normal modes at the equilibrium are solved and various molecular levels are obtained, which appear to be in good correspondence with experiments.

Competition and cooperation between thermal noise and external driving force

We consider the transport of an overdamped Brownian particle moving in a rocked flashing ratchet-like potential. Together with a zero-mean driving force and a thermal white noise, the direction and behavior of the movement of the particle can be controlled. The expression of current is obtained by means of the perturbation approximation for fast flashing cases. Both the average velocity and position of the particle are simulated by use of the Langevin approach. The current reversal and the stepping motion of the particle are obtained and shown.

Theory of fusion for superheavy elements

A new model is proposed for fusion mechanisms of massive nuclear systems, where so-called fusion hindrance exists. The model describes the whole process in two steps: two-body collision processes in an approaching phase and shape evolutions of an amalgamated system into the compound nucleus formation. It is applied to 48Ca-induced reactions and is found to reproduce the experimental fusion cross sections extremely well, without any free parameter. A schematic case is solved in an analytic way, the results of which shed light on fusion mechanisms. Combined with statistical decay theory, residue cross sections for superheavy elements can be readily calculated. Examples are given.