Bulk uranium items are often measured using active neutron interrogation systems to take advantage of the relatively high penetrability of neutrons, providing the ability to quickly and accurately measure uranium masses in large, dense configurations. 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 [uranium-235] and 238U [uranium-238] are, for all practical purposes, indistinguishable; therefore, commonly used systems such as the Active Well Coincidence Counter, the 252Cf [californium-252] Shuffler, and other systems based on measurement of prompt or delayed fission neutrons require many representative calibration standards and/or well-known isotopic information to interpret the results (i.e., extract an isotopic mass from the effective fissionable mass), thus limiting these techniques for safeguards applications. The primary objective of this research was to develop and demonstrate a dual-energy neutron interrogation technique using a 252Cf Shuffler measurement chamber for determination of uranium enrichment, thus eliminating the need for a (traditionally separate) gamma isotopic measurement.
This new technique exploits the change in fission rates as a function of interrogating neutron energy to independently determine the 235U and 238U content in the measurement item. Dual neutron interrogation energies were achieved by adding a deuterium- tritium (D-T) neutron generator into the measurement chamber of the Oak Ridge National Laboratory 252Cf Shuffler. Results from traditional 252Cf measurements and the new D-T measurements were then used to develop a relationship between uranium enrichment and the ratio of the two delayed neutron count rates. Parameter studies were performed to optimize the measurements for each source using a combination of modeling/simulation and experimental measurements. This dissertation presents the detailed development of this novel dual-energy neutron interrogation technique. The results are promising and with engineering refinements could be deployed for routine assay of certain types of materials
