  Fragment mass distributions from the fission of U and Pu isotopes at low
excitation energies are studied using a dynamical model based on the
fluctuation-dissipation theorem formulated as Langevin equations. The present
calculations reproduced the overall trend of the asymmetric mass distribution
without parameter adjustment for the first time using the Langevin approach.
The Langevin trajectories show a complicated time evolution on the potential
surface, which causes the time delay of fission, showing that dynamical
treatment is vital. It was found that the shell effect of the potential energy
landscape has a dominant role in determining the mass distribution, although it
is rather insensitive to the strength of dissipation. Nevertheless, it is
essential to include the effect of dissipation, since it has a crucial role in
giving "fluctuation" to Langevin trajectories as well as for explaining the
multiplicities of pre-scission neutrons as the excitation energy increases.
Therefore, the present approach can serve as a basis for more refined analysis.
