Fission-Fragment Neutron Multiplicities: Correlations with Mass Number and Total Kinetic Energy, and the Dominance of the Superlong Fission Mode at Elevated Neutron Energies

This study investigates the average neutron multiplicity of fission fragments in ²³⁵U neutron-induced fission, with a focus on its correlations with fragment mass number (A) and total kinetic energy (TKE). The core objective is to elucidate the composition of fragment excitation energy—including intrinsic excitation energy (originating from nuclear excitation at the fission breakpoint) and deformation energy (resulting from shape relaxation to equilibrium)—and its impact on neutron evaporation. Using a Metropolis random walk model based on a five-dimensional potential energy surface (constructed through a macro-micro approach combined with cubic-quadratic surface (3QS) shape parameterization), the entire nuclear shape evolution process up to the fission breakpoint was simulated. Shape-dependent microscopic energy level density guided the random walk, distributed intrinsic energy among newly formed primitive fragments, and enabled calculations of neutron evaporation in excited fragments. Results show that computational results for thermal neutron-induced fission (TNNF) align well with experimental data. As incident neutron energy increases (e.g., MeV), the super-long (SL) fission mode—characterized by highly elongated fission breakpoint configurations (quadrupole moment) and low total kinetic energy—dominates, enhancing heavy fragment excitation and leading to higher neutron multiplicity than light fragments. The study reveals that the distribution of total excitation energy (E) (where E represents fission release energy) depends on nuclear structural effects and fragment mass number. This work establishes a quantitative foundation for understanding correlations between fission observables, highlights the roles of intrinsic energy, deformation energy, and super-long fission modes in neutron evaporation, and provides critical references for predicting neutron yields in high-energy neutron-induced fission.