Epithermal Neutron Resonance Analysis Using A Compact DT Generator

Nuclear material identification and isotopic composition analysis are important capabilities for safeguards and the verification of nuclear arms control and nonproliferation treaties. Epithermal neutron resonance imaging has been proposed for spent fuel assay measurements, and the development of a mobile neutron imaging system could provide such a tool for on-site applications. This technique is highly sensitive to isotopes of interest for safeguards applications (e.g. 235,238 U, 238,239,240 Pu) and yields unique transmission spectra for a target containing actinides and some mid- and high-Z elements. Paired with a position-sensitive neutron detector, such a system could provide spatial information of the target isotopic composition. Recent progress in the development of high intensity neutron sources could make mobile epithermal neutron imaging designs increasingly feasible to serve as a tool for nuclear inspectors. In this work, Monte Carlo (MC) simulations were performed to further optimize a previous design for an epithermal neutron resonance-based analysis apparatus using a D-T portable neutron generator. Additionally, experimental work is being performed to test the feasibility of this method. The MC simulations of moderation time distributions for different moderator compositions and thicknesses were measured to provide a multi-group correction for neutron time-of-flight energy reconstruction. A simplified neutron resonance analysis was developed to characterize simulated neutron transmission spectra based on energy reconstruction for short (~2 m) time-of-flight distances and minimum detection thresholds for various target objects were calculated. Initial experimental validation tests of the technique were conducted using a D-T neutron generator source and enriched Li-6 glass scintillator. GEANT4 simulations demonstrated the advantages of the D-T neutron generator relative to a D-D based source. The feasibility of using a mobile neutron generator based on the 7 Li(p,n)Be 7 near-threshold reaction was also studied in GEANT4 simulations and initial results demonstrated the potential for orders of magnitude increase in epithermal neutron flux with a tradeoff for larger system size. Results of MC simulations and experimental measurements will be presented.