Fission of fermium isotopes using Langevin calculations

Fragment mass distributions in the fission of 246,264Fm at low excitation energies are studied using the three-dimensional Langevin equations with potential energy landscape of the two-centershell model. After overcoming the 1st saddle point, the large-amplitude shape motion along a specific shape degrees of freedom is found in the 2nd minimum of the potential energy landscape due to the nature of the friction tensor in the Langevin equation. The 2nd-saddle points are found at two positions which links to the compact-symmetric scission and elongated-asymmetric scission, respectively, and they are located at both ends of the shape motion in the 2nd minimum. Height of the mass-symmetric 2nd saddle point relative to the asymmetric saddle point decreases toward heavier Fm isotopes. It was shown that not only the structure of the potential energy landscape but also the wide-range directive uctuation of nuclear shape have a crucial role to understand the mechanism of the sharp transition of mass asymmetry.