  The mass asymmetry in the fission of U-236 at low excitation energy is
clarified by the analysis of the trajectories obtained by solving the Langevin
equations for the shape degrees of freedom. It is demonstrated that the
position of the peaks in the mass distribution of fission fragments is
determined mainly by the saddle point configuration originating from the shell
correction energy. The width of the peaks, on the other hand, results from the
shape fluctuations close to the scission point caused by the random force in
the Langevin equation. We have found out that the fluctuations between
elongated and compact shapes are essential for the fission process. According
to our results the fission does not occur with continuous stretching in the
prolate direction, similarly to that observed in starch syrup, but is
accompanied by the fluctuations between elongated and compact shapes. This
picture presents a new viewpoint of fission dynamics and the splitting
mechanism.
