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

TL;DR

This paper presents initial experimental results on estimating atomic numbers in cargo radiographs using a semiempirical transparency model, improving material identification in dual energy cargo scans.

Contribution

The work introduces a novel semiempirical transparency model calibrated with experimental data for atomic number estimation in dual energy radiography.

Findings

Accurately fit the transparency model to calibration data

Successfully reconstructed atomic numbers of steel and polyethylene

Identified high-Z lead objects within organic shielding

Abstract

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. We consider measurements taken by a Rapiscan Sentry Portal scanner, which is a dual energy betatron-based system used to inspect cargo containers and large vehicles. We demonstrate the ability to accurately fit our semiempirical transparency 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.