From Simulation to Real Scans: Anomaly Detection in Maritime Cargo with Muon Scattering Tomography
Maritime cargo inspection requires imaging technologies capable of detecting concealed threats within dense, sealed containers, a role for which Muon Scattering Tomography (MST) is well suited: it images their interior through the density-dependent deflection of naturally occurring cosmic muons. However, MST remains constrained by the scarcity of labeled scans and by a cosmic muon flux that is both low and stochastic. Anomaly detection algorithms must therefore be trained on simulations, yet operate on measured scans acquired under different conditions, a sim-to-real gap that remains a central obstacle to operational deployment. We present the first end-to-end anomaly detection framework for maritime MST, from physically consistent simulations to validation on real container scans from the SilentBorder demonstration campaign. The task is cast as an out-of-distribution problem: the framework learns the spatial configurations of benign cargo and flags threats as deviations in the reconstruction error space, remaining agnostic to threat type and geometry. An attention U-Net, trained exclusively on benign synthetic scenes, preserves small-scale scattering signatures through its skip connections, and contraband consequently persists in the pixel-wise reconstruction error instead of being absorbed into the reconstructed background. A scoring function, the Homogeneity Index (HI), suppresses spatially uniform cosmic-ray statistical noise while amplifying coherent anomaly signatures: where pixel-level metrics collapse under a change of cargo configuration, HI retains its discriminative power. We evaluate three training strategies across two distinct cargo configurations under operational one-hour scan times, and test the best model on real muon cargo scans. The results for the studied scenarios indicate that the sim-to-real gap can be bridged.