Discriminating Materials Using a Multi-particle Approach in an Active Interrogation Environment

Abstract

Spectroscopic transmission radiography using MeV-class radiation is a powerful method for identifying the elemental composition of dense objects. This method has been extensively used in nuclear nonproliferation, safeguards, and nuclear security applications that employ active interrogation (AI). Of particular interest is the identification of special nuclear material (SNM), which relies on a characteristic signature such as energy-dependent transmission or the emission of delayed radiation unique to the fission process that occurs in SNM. Neither fast neutron nor photon probes are universally applicable, as each suffers from poor penetrability at one extreme of the atomic number spectrum. While dual-energy photon radiography is adequate for measurement of materials from low to moderate atomic numbers, its discrimination performance diminishes at high atomic numbers, leading to poor contrast between elements such as lead and uranium. Further, transmission radiography is not sufficient to confirm or exclude the presence of SNM. If two primary measurement modes offered by AI (spectroscopic transmission radiography and the detection of fission signatures) commonly used independently are integrated, more information about the interrogated material may be revealed. This dissertation combines these sources of information to characterize the elemental composition with improved accuracy and to indicate the presence of fissile material. Using a combination of neutrons and gamma rays produced by nuclear reactions induced by deuterons incident on boron nitride target, material identification was performed. The neutron time-of-flight technique was employed to measure the transmission over a broad range of neutron energies and was combined with spectroscopic photon transmission to provide a more accurate measurement of elemental composition. It is shown that the buildup and decay of delayed-neutron emission can be used not only to detect the presence of fissionable material but also to further distinguish among various uranium isotopes and infer the uranium enrichment level. This work demonstrates the first use of a single multi-particle, multi-energy source and a single type of detector to simultaneously perform neutron and photon spectroscopic radiography.