Current Density and Angular Distribution of Neutrons Emitted During Scission

A time-dependent approach to the angular distribution of the scission neutrons with respect to the fission axis based on a recently developed dynamical scission model is presented. It implies the numerical solution of the bi-dimensional time-dependent Schrödinger equation with time-dependent potential for the motion of a neutron inside a nucleus that undergoes fission. The time evolution is calculated from the configuration with a minimum neck radius (αi), the start of the scission process, to the configuration of two just-separated fragments (αf), the end of the scission process. The resulting neutron wave packets are then propagated further in time but with the configuration of the fissioning system frozen at αf. The numerical solutions at a given time t are used to calculate the current density , a key quantity in the angular distribution evaluation. We investigate the nucleus 236U at two mass asymmetries defined by the light fragment mass . The number of neutrons that leave a sphere of radius (a test is done with ) around the fissioning nucleus in a solid angle dΩ and in a time interval dt, dνsc/dΩ, is calculated. The integration in time of this quantity from 0 to ∞ gives the angular distribution. In practice we could only reach . At this time however the majority of the scission neutrons left the sphere. The scission neutron emission is found to take place mainly along the fission axis with a small preference for the light fragment similar with what is experimentally observed for all prompt neutrons. A ratio νL/νH close to the experimental value (1.41) is obtained. We concluded that the distinction between scission and evaporated neutrons, based on their angular distribution, is more challenging than expected.