Development of a new matrix correction scheme for pulsed neutron interrogation of fissile material.

Abstract

This work describes the first experimental and analytical investigations performed on a new active neutron-based waste assay system, the Combined Thermal Epithermal Neutron (CTEN) instrument. To further the understanding of the pulsed neutron technique as applied to waste assay, the physics of the differential dieaway measurement is discussed in terms of slowing down and diffusion theory and in historical perspective by comparison with the classic dieaway experiments. Results from Monte Carlo simulations give insight into the differential dieaway neutron physics by providing detailed neutron energy and time spectra that could not be measured directly. This allowed us to quantify in our methodology the relationship between the very broad time- or energy-related measured quantities from CTEN and the calculated parameters based on simplified theoretical models. Functional relationships between measured quantities from an active assay and calculated parameters based on slowing down and diffusion theories are used to develop matrix corrections with clear connections to physical principles. Thus this new assay technique is a combined experimental-analytical-computational methodology that substantially improves upon current capability to assay radioactive waste containers. Conclusions are presented that are applicable to active interrogation of both non-multiplying and multiplying materials regardless of the specific application.