Atomic number estimation of dual energy cargo radiographs: initial experimental results using a semiempirical transparency model

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 photon attenuation, allowing for improved material detection. This work presents our initial experimental findings using a novel approach to predict the atomic number of dual energy images of a loaded cargo container. Our past work introduces a semiempirical transparency model, which is able to correct for bulk scattering effects, source energy uncertainty, and detector response uncertainty through a simple calibration procedure. The semiempirical model is more accurate than a fully analytic model, and shows improved extrapolation accuracy compared to existing empirical methods. This work considers measurements taken by a Rapiscan Sentry® Portal scanner, which is a dual energy betatronbased system used to inspect cargo containers and large vehicles. We demonstrate the ability to accurately f it our model to a set of calibration measurements. We then use the calibrated model to reconstruct the atomic number of an unknown material by minimizing the chi-squared error between the measured pixel values and the model predictions. We apply this methodology to two experimental scans of a loaded cargo container. First, we incorporate an image segmentation routine to group clusters of pixels into larger, roughly homogeneous objects. By considering groups of pixels, the subsequent atomic number reconstruction step produces a lower noise result. We demonstrate the ability to accurately reconstruct the atomic number of blocks of steel and high density polyethylene. Furthermore, we are able to identify the presence of two high-Z lead test objects, even when embedded within lower-Z organic shielding. These results demonstrate the significant potential of this methodology to yield improved performance characteristics over existing methods when applied to commercial dual energy systems.