Capabilities and Limitations of Dual Energy X-ray Scanners for Cargo Content Atomic Number Discrimination

To combat the risk of nuclear smuggling, radiography systems are deployed at ports to scan cargo containers for concealed illicit materials. Dual energy radiography systems enable a rough elemental analysis of cargo containers due to the Z-dependence of gamma attenuation, allowing for improved material detection. This work studies the theoretical capabilities for atomic number discrimination using dual energy systems by considering three case studies: dual energy {6,4} MeV bremsstrahlung beams, dual energy {10,6} MeV bremsstrahlung beams, and dual energy {10,4} MeV bremsstrahlung beams. Results of this analysis show that two different materials can sometimes produce identical measurements due to competition between photoelectric absorption and pair production, leading to a fundamental ambiguity when differentiating between materials of vastly different atomic numbers. This non-uniqueness property is present at low area densities and for high-Z materials. Furthermore, this analysis suggests that atomic number discrimination becomes significantly more challenging as atomic number increases. The dual energy {10,4} MeV bremsstrahlung case study showed significantly improved atomic number discrimination capabilities compared to the dual energy {10,6} MeV and {6,4} MeV case studies due to the increased prevalence of pair production in the high energy beam when compared to the low energy beam.