A Study of prompt neutron emission in thermal-neutron-induced fission of 235U

Abstract

An original experiment was performed to measure the angular correlation of fission neutrons from thermal-neutron-induced fission of 235 u, with respect to the light fission fragment direction, as a function of fragment mass division and neutron energy. A Monte Carlo model, with a realistic description of the fission fragment de-excitation process, was developed to simulate the observed neutron-fragment angular correlation data. The model was capable of investigating various possible forms of neutron emission which were classified into emission before, during and after full fragment acceleration, and correspondingly named scission acceleration and prompt neutron emission. Simulated neutron-fragment angular correlations displaying similar distributions with respect to the light fragment direction for different forms of neutron emission are shown to exhibit differing distributions when examined as a function of fragment mass division or neutron energy, thus illustrating the sensitivity of the experiment to the forms of neutron emission occurring in fission. A primary conclusion of the investigation was that neutron emission solely from fully accelerated fragments, whether isotropically or anisotropically emitted in the fragment centre of mass system, was unable to adequately describe the observed neutron-fragment angular correlations. Simulation of the fission process with some neutron emission before or during fragment acceleration exhibited a closer correspondence with observed phenomena. Within the scope of this work the form of neutron emission that produced the closest overall correspondence with experimental data was a simulation in which 20% of the emitted neutrons were isotropically emitted scission neutrons with a Maxwellian energy distribution of temperature 1.0 MeV. The remaining neutrons were emitted from fully accelerated fragments, being isotropic in the fragment centre of mass frame, except for the n-th(n > 1) neutrons from the light fragment, which were emitted anisotropically. The form of the anisotropy is described by (l+b 2 cos ), where  =- 0 lies along the fission axis, and b has any value between 0.02 and 0.1 and represents the influence of fission fragment angular momentum on the fragment de-excitation process.