A Multi-Energy Approach for Neutron Interrogation of Uranium Items

Bulk uranium items are often measured using active neutron interrogation systems to take advantage of the high penetrability of neutrons, providing the ability to quickly and accurately measure effective uranium masses in large, dense configurations. These active techniques employ an external neutron source to induce fission in the uranium and subsequently measure emitted prompt fission or delayed neutrons. Unfortunately, the emitted neutrons from 235U and 238U are, for all practical purposes, indistinguishable; therefore, common systems such as the Active Well Coincidence Counter and other systems based on measurement of prompt or delayed fission neutrons with a fixed interrogation source require many representative calibration standards or known isotopic information to interpret the results, thus limiting the application of these techniques for safeguards purposes. This paper presents the motivation for a multi-energy neutron interrogation technique which aims to determine the enrichment of uranium-bearing items using two neutron interrogation sources in a 252Cf Shuffler. Potentially, the dual measurement approach will allow the mass values of the isotopes in the item to be independently determined while minimizing the number of representative standards needed and/or eliminating the need for prior knowledge of the isotopic information. This work examines the change in fission rates with differing interrogating neutron energy and, therefore, the potential of this new technique to independently measure the masses of 235U and 238U in the measurement item. Initial simulation and measurement results will be presented.