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To combat the risk of nuclear smuggling, radiography systems are deployed atports to scan cargo containers for concealed illicit materials. Dual energyradiography systems enable a rough elemental analysis of cargo containers dueto the $Z$-dependence of photon attenuation, allowing for improved materialdetection. This work studies the capabilities for atomic number discriminationusing dual energy MeV systems by considering dual energy $\{6, 4\}$ MeV, $\{10,6\}$ MeV, and $\{10, 4\}$ MeV bremsstrahlung beams. Results of this analysisshow that two different pure materials can sometimes produce identicaltransparency measurements, leading to a fundamental ambiguity whendifferentiating between materials of different atomic numbers. Previousliterature has observed this property, but the extent of the limitation ispoorly understood and the cause of the degeneracy is generally inadequatelyexplained. This non-uniqueness property stems from competition betweenphotoelectric absorption and pair production and is present even in systemswith perfect resolution and zero statistical noise. These findings arevalidated through Monte Carlo transparency simulations. Results of this studyshow that currently deployed commercial radiographic systems are fundamentallyincapable of distinguishing between high-$Z$ nuclear materials andmiscellaneous mid-$Z$ cargo contents.

Fundamental limitations of dual energy X-ray scanners for cargo content atomic number discrimination