An Analysis Technique for Active Neutron Multiplicity Measurements Based on First Principles

摘要

Passive neutron multiplicity counting is commonly used to quantify the total mass of plutonium in asample, without prior knowledge of the sample geometry. However, passive neutron counting is lessapplicable to uranium measurements due to the low spontaneous fission rates of uranium. Activeneutron multiplicity measurements are therefore used to determine the ~(235)U mass in a sample.Unfortunately, there are still additional challenges to overcome for uranium measurements, such as thecoupling of the active source and the uranium sample. Techniques, such as the coupling method, havebeen developed to help reduce the dependence of calibration curves for active measurements onuranium samples; although, they still require similar geometry known standards. An advanced activeneutron multiplicity measurement method is being developed by Texas A&M University, in collaborationwith Los Alamos National Laboratory (LANL) in an attempt to overcome the calibration curverequirements. This method can be used to quantify the ~(235)U mass in a sample containing uraniumwithout using calibration curves. Furthermore, this method is based on existing detectors and nondestructiveassay (NDA) systems, such as the LANL Epithermal Neutron Multiplicity Counter (ENMC).This method uses an inexpensive boron carbide liner to shield the uranium sample from thermal andepithermal neutrons while allowing fast neutrons to reach the sample. Due to the relatively low andconstant fission and absorption energy dependent cross-sections at high neutron energies for uraniumisotopes, fast neutrons can penetrate the sample without significant attenuation. Fast neutroninterrogation therefore creates a homogeneous fission rate in the sample, allowing for first principlemethods to be used to determine the ~(235)U mass in the sample. This paper discusses the measurementmethod concept and development, including measurements and simulations performed to date, as wellas the potential limitations.