Asymmetric Fission

By: S. Mahdi Mirfathi

Despite many years of experimental and theoretical effort our understanding of fission is still far from complete. The induced fission of compound nuclei has been investigated exhaustively for many decades. The model description of scission ranged from the early statistical model of Fong to the advanced dynamical description of the random-neck rupture model of Nadtochy and Adeev. When a heavy nucleus in nuclear fission divides into two fragments, daughters separate and accelerate due to their repulsive coulomb force. After many decades of intensive research in the field of fusion-fission reactions, various aspects of the problem still remain under debate and many theoretical methods have been developed to study dynamics of these reactions. Difference between particle multiplicity such as charged particles, gamma rays and neutrons in theoretical and experimental view as the main consequence of earlier investigation, show that statistical approach is not main way for definition of energy dissipation in heavy-ion reactions. There are several approaches to energy dissipation in heavy-ion fusion-fission reactions. Statistical theory as a earlier one provided good result for description of this reactions. Dynamical approach to the treatment of fission fragments characteristics is another one. In our project we find classical corrected neutron multiplicity in the base of both statistical and dynamical approaches in the frame work of asymmetric fission for two typical systems and then we compare our results with experimental data. These constraints may reflect characteristic features of fusion-fission reactions especially after scission point.

Asymmetric Statistical Model

Our strategy differs with existing literatures in two main respects. We consider a general functions for mass and charge distributions, also we consider possible asymmetry for nascent fragments. In the following our interest concentrated on two important steps, before and after scission configurations in energies near coulomb barrier because pre-scission neutron multiplicity related to fragments mass distribution whether post-scission neutron multiplicity correlate with total kinetic energy of fission fragments. In addition, we calculate excitation energy and fission energy with more reliable approaches based on asymmetric mass and charge division. By using excitation energy, sum of the coulomb potential and deformation energy of the nucleus and rotational energy of system due to it’s spin that can be obtained easily one can calculate the, total, pre-scission and post-scission neutron multiplicity.

Asymmetric Dynamical Model

There are several parameterization to incorporate investigating the dynamical evolution of fissioning nucleus Here we used the well-known”funny hills” parameterization. Until now researchers did not focused on asymmetric mass division in dynamical study of fission, now we consider the general case where both asymmetric distribution and the dynamical approaches are present. In this section we are going to proceed one more step in the line of dynamical approaches. The coupled Langevin equations used in the dynamical calculations to change our funny hills parameters. The crucial parameter in a diffusion model for fission is the nuclear friction that gives the strength of the coupling between the fission degree of freedom and the rest of the system which is considered as a heat bath. In this way one-body dissipation is usually considered to be more successful in describing fission dynamics than two-body viscosity.

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