The influence of the neck parameter on the fission dynamics at low excitation energy is studied based on the three-dimensional Langevin approach in which the nuclear shape is described with the two-center shell model (TCSM) parametrization, and the elongation, the mass asymmetry and the fragment deformation are set to be the generalized coordinates of the Langevin equation. We first study the influence of the neck parameter on the scission configuration. We find that there is almost no obvious correlation between the neck parameter ε and the mass asymmetry η at the scission point indicating that the ε has no obvious impact on the fragment mass distribution. The elongation Z0/R0 and its correlation with the mass asymmetry η at the scission point are obviously influenced by the neck parameter ε, which has a strong effect on the total kinetic energy (TKE) distribution of fragments. The pre-neutron emission fragment mass distributions for 14 MeV n+^{233,235,238}U and ^{239}Pu are calculated and then based on these results the post-neutron emission fragment mass distributions are obtained by using the experimental data of prompt neutron emission. The calculated post-neutron emission fragment mass distributions can reproduce the experimental data well. The TKE distributions for 14 MeV n+^{235}U fission are calculated for ε=0.25,0.35,0.45, and the results show that the TKE distribution cannot be described very well for the three cases. However, the trend of the calculated TKE distribution with ε is just as that is expected from the scission configuration calculations and the results with ε=0.35 present a better agreement with the experiment data compared with the other two cases.

We have calculated the three-dimensional potential energy surface (PES) for the fission of compound nucleus 236U using the covariant density functional theory with constraints on the axial quadrupole and octupole deformations (β2, β3) as well as the nucleon number in the neck q N. By considering the additonal degree of freedom q N, coexistence of the elongated and compact fission modes is predicted for 0.9≤ β3≤1.3. Remarkably, the PES becomes very shallow across a large range of quadrupole and octupole deformations for small q N, and consequently, the scission line in (β2, β3) plane will extend to a shallow band, which leads to a fluctuation for the estimated total kinetic energies by several to ten MeV and for the fragment masses by several to about ten nucleons.