Spectral X-ray detectors provide a direct observation of the energy spectrum of a transmitted X-ray beam, both for typical and phase-contrast gratings-based systems. The sensitivity of a gratings-based X-ray system to small-angle deflections from a given particle size is dependent on X-ray energy. Therefore, spectral (energy-sensitive) detectors could be sensitive to particle size, even with a broad spectrum from a commercial X-ray generator. Furthermore, these detectors allow direct observation of how the X-ray spectrum is changing as the beam is passing through an object and gratings, and how this affects grating visibilities used to determine the presence of small-angle deflections. This is a particular issue for higher-energy systems where artifacts from beam hardening are common. We present results exploring the particle-size dependent signatures that are available from a spectral gratings-based phase-contrast X-ray imaging system, and the feasibility of observing them with lab-based, broad-spectrum X-ray generators.
Gratings-based phase contrast imaging provides additional materials signatures that can enhance detection for security screening. However, screening of larger objects requires more penetrating energies, which presents a challenge to gratings-based systems: more penetrating x-rays are more difficult to pattern, while the cross sections for small angle scattering are reduced. We present a study of phase contrast x-ray imaging at endpoint energies above 160 kV, discussing pattern visibility, spectral corrections, and detection of material microstructure. We then discuss considerations for potential implementation.
X-ray screening can be used to aid in the detection of a variety of threats, including items such as explosives and narcotics. Conventional x-ray imaging provides limited means of distinguishing between different materials. Gratings-based phase contrast x-ray imaging provides images reflecting three distinct materials signatures: absorption, similar to conventional x-ray imaging; phase contrast, proportional to electron density variations; and dark field, which can be related to material texture. These additional signatures may enhance the ability to distinguish materials in x-ray images, leading to detection improvements. Here we discuss considerations for system design and improved detection: approach to measurements at relevant energies for security screening, signature studies of explosives and opioids as well as common benign materials, and development of a prototype screening system.
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 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 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.
Direct examination of natural and engineered environments has revealed that the majority of microorganisms in these systems live in structured communities termed biofilms. To gain a better understanding for how biofilms function and interact with their local environment, fundamental capabilities for enhanced visualization, compositional analysis, and functional characterization of biofilms are needed. For pore-scale and community-scale analysis (100’s of nm to 10’s of microns), a variety of surface tools are available. However, understanding biofilm structure in complex three-dimensional (3-D) environments is considerably more difficult. X-ray microtomography can reveal a biofilm’s internal structure, but obtaining sufficient contrast to image low atomic number (Z) biological material against a higher-Z substrate makes detecting biofilms difficult. Here we present results imaging Shewanella oneidensis biofilms on a Hollow-fiber Membrane Biofilm Reactor (HfMBR), using the x-ray microtomography system at sector 2-BM of the Advanced Photon Source (APS), at energies ranging from 12.9-15.4 keV and pixel sizes of 0.7 and 1.3 μm/pixel. We examine the use of osmium (Os) as a contrast agent to enhance biofilm visibility and demonstrate that staining improves imaging of hydrated biofilms. We also present results using a Talbot interferometer to provide phase and scatter contrast information in addition to absorption. Talbot interferometry allows imaging of unstained hydrated biofilms with phase contrast, while absorption contrast primarily highlights edges and scatter contrast provides little information. However, the gratings used here limit the spatial resolution to no finer than 2 μm, which hinders the ability to detect small features. Future studies at higher resolution or higher Talbot order for greater sensitivity to density variations may improve imaging.
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.
We report the results of Differential Aperture X-ray Microscopy (DAXM) measurements near Te precipitates in CdZnTe
grown via low-pressure Bridgman. White-beam Laue patterns were acquired with 3-D spatial resolution (with 0.25 μm
resolution in the scanning directions and 1 μm resolution in depth) at depths of up to 35 μm deep normal to the surface.
We find very little crystal strain (< 10-3) or rotation (<0.05 degrees) near Te precipitates. We also examine local
deformations in the vicinity of a microhardness indent, and find that although significant rotations exist, the spatial
extent is limited to a few tens of microns. Furthermore, observed crystal strains are limited to 5 x 10-3 or less in regions
near the microhardness indent.
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