Comparison of dual-energy, Z-SCAN, and Z-SPEC material separation techniques for high-energy x-ray cargo inspection

Abstract

The most widely used technology for non-intrusive active inspection of cargo containers and trucks is x-ray radiography at high energies (4-9 MeV). Techniques such as dual-energy imaging, spectroscopy (Z-SPEC), and statistical waveform analysis (Z-SCAN) can be used to extract the effective atomic number Z of the cargo material from the x-ray transmission data because the mass attenuation coefficient depends on energy as well as Z. Knowledge of Z in turn leads to improved detection capability of contraband and threats, including special nuclear materials (SNM) and shielding. Dual-energy imaging is the method currently used in commercially available cargo scanners. Z-SPEC and Z-SCAN have both been demonstrated in prototype systems. The relative merits of these three methods have, however, not been evaluated in the past. Here, we employ an idealized model to compare these techniques in principle. In the process, we derive a slight modification of the Z-SPEC method which turns out to be usable at higher transmission, and which appears to improve on the Z-SCAN method. We find that dual-energy imaging is especially effective with high x-ray dose rate, and thus is preferred at high cargo transmission. The modified Z-SPEC technique, however, performs better than dual-energy at low transmission, and has the highest material separation capability overall when an intensity-modulated x-ray source is used. Z-SPEC and Z-SCAN perform even better with a high duty factor source and/or fast detectors. We show results for a number of different x-ray source dose rates, x-ray source energies, and x-ray pulse rates, with and without the use of an intensity-modulated x-ray source, and discuss some implications.