A computation-based analysis of photon-induced fission

A single burst of radiation created by 100-ns pulsed power can be used to induce photofission in fissionable material. In NRL experiments, the 8-MV Mercury bremsstrahlung spectrum irradiates a depleted-uranium plate several meters from the diode, and the resulting delayed-fission neutrons are measured. In this work, the LSP particle-in-cell code is used to model electron flow in the vacuum feed and diode, and the ITS Monte-Carlo codes are used to extract the x-ray spectrum from the diode and transport it to the DU plate. The delayed-fission-neutron yield is determined from the known dependence of the fission cross-section on photon energy. In this calculational chain, the only free parameter is the fraction of ions flowing in the diode, which affects the orbits of electrons impacting the bremsstrahlung converter. The ion fraction therefore determines the radial and angular distributions of x-rays emitted by the converter, which in turn, determine the DU neutron yield, the radial x-ray distribution emitted by the anode, and the angular distribution of x-ray dose. These three experimental measurements are compared to the modeling to determine the ion fraction that gives the best agreement between the measurements and computations: about 0.4% for the experiments discussed here.