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

Peter Lalor, Massachusetts Institute of Technology

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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.

Abstract Author(s): Peter Lalor, Areg Danagoulian