Macroscopic effects on the fission process

Yoshihiro Aritomo (Kindai University, Higashi-Osaka, Osaka, Japan)

In the fission process, mass asymmetric fission has been considered to be a microscopic property of the internal structure of nuclei. However, computational analysis of dynamical models has revealed that the choice of mass asymmetric or symmetric fission in the induced fission plays a major role in the droplet properties (macroscopic effects) that appear in the fission process. Based on the results of this analysis, an interpretation of the dynamical description of induced fission is presented.

After the discovery of fission in 1938, the statistical model of Bohr and Wheeler was the first to explain the phenomenon of the fission process in a theoretical model. The following year, Kramers introduced the thermal fluctuation due to Brownian motion, and soon after, probably due to the difficulty of analytical calculations, he extended the Langevin equation to the Fokker-Planck equation, and since then, he moved toward the description of the fission process by the Fokker-Planck equation.

After the application of the one-dimensional Langevin equation to the fission process in 1986 [1], in 1990, calculations were made with two-dimensional degrees of freedom in the collective coordinate, tensor quantities were introduced for the transport coefficient, and the strength of the random force on the random force term was expressed as a tensor quantity [2]. In the dynamical model, the introduction of random force is essential to describe induced fission. The physical concept of a nucleus, such as uranium, being given energy from the outside, which causes violent thermal fluctuations and allows the system to overcome the fission barrier, is very consistent with these basic concepts of Brownian motion.

It is now known that the most significant factor among those governing the dynamics from the spherical region to the saddle point is due to random forces, and furthermore, that the “directionality of these random forces” plays an important role. We will discuss the fission process precisely.

[1] Y. Abe, C. Gregorie, and H. Delagrange, J. Phys. Colloq. 47, C-4-329 (1986)
[2] T. Wada, Y. Abe, N. Carjan, Phys. Rev. Lett. 70, 3538 (1993); N. Carjan, T. Wada, Y. Abe, AIP Conf. Proc. No. 250 （AIP, NY, 1992）; G.-R. Tillack, Phys. Lett. B 278, 403 (1992)