Gratings-based phase contrast X-ray imaging provides additional materials signatures in textured media based on the deflection of the X-ray beam. Using this technique with a hard (~160 kVp) X-ray spectrum has shown potential for improved materials discrimination in applications such as explosives detection. Typical phase contrast measurements rely on relatively broad bremsstrahlung spectra, resulting in measurement responses averaged across wide energy ranges. Here, we present results for gratings-based phase contrast measurements using a spectroscopic imaging detector. This allows for direct observation of phase-contrast material cross sections as a function of energy, without the need for a mono-energetic X-ray source. Further, the measurements provide a direct understanding of spectral variations and a technical basis for application of hard X-ray gratings-based phase contrast measurements in the presence of attenuating materials.
Gratings-based phase contrast x-ray imaging offers enhanced material information in an x-ray imaging measurement, a key consideration for improving performance in explosives detection. Application of phase contrast imaging to explosives detection requires addressing several key technical issues: identifying a patterning element (grating) that offers an appropriate tradeoff between sensitivity and robust operation at high energies, developing techniques that allow for quantitative interpretation of new signatures under a broad range of attenuation conditions, and designing a system that allows for rapid measurement while providing sufficient signal-to-noise. We present results illustrating the value of phase contrast x-ray signatures for explosives detection, and demonstrate the ability to obtain quantitative metrics in the presence of intervening materials. Finally, we demonstrate preliminary results from a gratings-based phase contrast system in a scanning configuration.
Gratings-based x-ray imaging can provide additional materials signatures, including refraction which is proportional to variations in electron density, and scatter which is sensitive to sub-resolution texture. Phase contrast measurements have been conducted using a variety of approaches, including Talbot-Lau interferometry, coded aperture systems, and single absorption grid systems. Because of the simultaneous requirements for fine spatial patterns to detect small angular changes, and the thickness of material required to modulate a penetrating beam, many phase contrast measurements are conducted at relatively low energy, below 100 kV. Many applications in security screening require higher energies in order to penetrate larger objects.
Here, we use a single absorption grid with direct imaging of the projected pattern to perform phase contrast measurements. A second grid is used for a beam hardening correction. We present measurements of pattern visibility as a function of energy up to 450 kV, demonstrating that the necessary beam patterning can be extended to higher energies. We also present measurements of a textured and homogeneous material as a function of energy, demonstrating that a texture signature is still present as energy is increased, and that the beam-hardening correction correctly accounts for and removes spectral effects on pattern visibility. To the best of our knowledge, this represents the highest energy demonstration of this technique to date, and enables new application areas.
Pacific Northwest National Laboratory (PNNL) is performing a computational assessment of the impact of several
important gamma-ray detector material properties (e.g. energy resolution and intrinsic detection efficiency) on the
scenario-specific spectroscopic performance of these materials. The research approach combines 3D radiation transport
calculations, detector response modeling, and spectroscopic analysis of simulated energy deposition spectra to map the
functional dependence of detection performance on the underlying material properties. This assessment is intended to
help guide formulation of performance goals for new detector materials within the context of materials discovery
programs, with an emphasis on applications in the threat reduction, nonproliferation, and safeguards/ verification user
communities. The research results will also provide guidance to the gamma-ray sensor design community in estimating
relative spectroscopic performance merits of candidate materials for novel or notional detectors.