D. E. Sayers

Diffraction enhanced x-ray imaging

Diffraction enhanced imaging is a new x-ray radiographic imaging modality using monochromatic x-rays from a synchrotron which produces images of thick absorbing objects that are almost completely free of scatter. They show dramatically improved contrast over standard imaging applied to the same phantom. The contrast is based not only on attenuation but also the refraction and diffraction properties of the sample. This imaging method may improve image quality for medical applications, industrial radiography for non-destructive testing and x-ray computed tomography.

Criteria for automatic x-ray absorption fine structure background removal

Criteria for terminating smoothing to remove a cubic spline background from the x‐ray absorption coefficient are stated in terms of three parameters obtained from the k3 weighted Fourier transform of the resulting x‐ray absorption fine structure data. The three parameters are HR, the average value of the transform magnitude between 0 and 0.25 A, HM, the maximum value in the transform magnitude between 1 and 5 A, and HN, the average value of the transform magnitude between 9 and 10 A. The termination criteria are HR−HN ⩾0.05HM; or if HN ≳0.1HM, then HR ⩾0.1HM. The incorporation of the criteria into a computer program to facilitate automatic background removal is discussed. Examples of application of the technique to copper, β‐PtO2, and ferritin samples are presented.

DIFFRACTION ENHANCED X-RAY IMAGING OF ARTICULAR CARTILAGE

OBJECTIVE To introduce a novel X-ray technology, diffraction-enhanced X-ray imaging (DEI), in its early stages of development, for the imaging of articular cartilage. DESIGN Disarticulated and/or intact human knee and talocrural joints displaying both undegenerated and degenerated articular cartilage were imaged with DEI. A series of three silicon crystals were used to produce a highly collimated monochromatic X-ray beam to achieve scatter-rejection at the microradian level. The third crystal (analyser) was set at different angles resulting in images displaying different characteristics. Once the diffraction enhanced (DE) images were obtained, they were compared to gross and histological examination. RESULTS Articular cartilage in both disarticulated and intact joints could be visualized through DEI. For each specimen, DE images were reflective of their gross and histological appearance. For each different angle of the analyser crystal, there was a slight difference in appearance in the specimen image, with certain characteristics changing in their contrast intensity as the analyser angle changed. CONCLUSIONS DEI is capable of imaging articular cartilage in disarticulated, as well as in intact joints. Gross cartilage defects, even at early stages of development, can be visualized due to a combination of high spatial resolution and detection of X-ray refraction, extinction and absorption patterns. Furthermore, DE images displaying contrast heterogeneities indicative of cartilage degeneration correspond to the degeneration detected by gross and histological examination.

Implementation of diffraction-enhanced imaging experiments: at the NSLS and APS

Di!raction-enhanced imaging is a recently developed X-ray imaging technique that has demonstrated enhanced contrast for dense, highly absorbing materials of interest in materials science and medicine. The implementation of this technique in experiments at the National Synchrotron Light Source and at the Advanced Photon Source is described in detail. ( 2000 Elsevier Science B.V. All rights reserved.

XAFS study of adsorbed and mineral forms of phosphate.

The solubility of inorganic phosphorus in soils is regulated by surface-adsorbed phosphate or phosphate minerals. The objective of this study was to determine whether different phosphate species of relevance to soils showed distinguishing XAFS spectral features. Phosphorus KXANES spectra for Fe-phosphates were characterized by a unique pre-edge feature near -3 eV (relative energy) that increased in intensity with increasing mineral crystallinity and was very weak for phosphate adsorbed on goethite. Spectra of Ca-phosphates and a soil sample exhibited a distinct shoulder on the high-energy side of the absorption edge. Spectra of Al-phosphates were characterized by a weak pre-edge feature at -1 eV.

Stability of C54 titanium germanosilicide on a silicon‐germanium alloy substrate

The stability of C54 Ti(Si1−yGey)2 films in contact with Si1−xGex substrates was investigated. The C54 Ti(Si1−yGey)2 films were formed from the Ti‐Si1−xGex solid phase metallization reaction. It was determined that initially C54 Ti(Si1−yGey)2 forms with a Ge index y approximately the same as the Ge index x of the Si1−xGex substrate (i.e., y≊x). After the formation of the C54 titanium germanosilicide, Si and Ge from the Si1−xGex substrate continue to diffuse into the C54 layer, presumably via lattice and grain boundary diffusion. Some of the Si diffusing into the C54 lattice replaces Ge on the C54 lattice and the Ge index of the C54 Ti(Si1−yGey)2 decreases (i.e., y<x). We propose that this process is driven by a reduction in C54 crystal energy which accompanies the replacement of Ge with Si on the C54 lattice. The excess Ge diffuses to the C54 grain boundaries where it combines with Si1−xGex from the substrate and precipitates as Si1−zGez which is Ge‐rich relative to the substrate (z≳x). This segregation a...

Electronic structure of noncrystalline transition metal silicate and aluminate alloys

A localized molecular orbital description ~LMO! for the electronic states of transition metal ~TM! noncrystalline silicate and aluminate alloys establishes that the lowest conduction band states are derived from d states of TM atoms. The relative energies of these states are in agreement with the LMO approach, and have been measured by x-ray absorption spectroscopy for ZrO2 ‐ SiO2 alloys, and deduced from an interpretation of capacitance‐voltage and current‐voltage data for capacitors with Al2O3 ‐T a 2O5 alloy dielectrics. The LMO model yields a scaling relationship for band offset energies providing a guideline for selection of gate dielectrics for advanced Si devices.

Diffraction enhanced imaging contrast mechanisms in breast cancer specimens

We have investigated the contrast mechanisms of the refraction angle, and the apparent absorption images obtained from the diffraction enhanced imaging technique (DEI) and have correlated them with the absorption contrast of conventional radiography. The contrast of both the DEI refraction angle image and the radiograph have the same dependence on density differences of the tissues in the visualization of cancer; in radiography these differences directly relate to the contrast while in the DEI refraction angle image it is the density difference and thickness gradient that gives the refraction angle. We show that the density difference of fibrils in breast cancer as measured by absorption images correlate well with the density difference derived from refraction angle images of DEI. In addition we find that the DEI apparent absorption image and the image obtained with the DEI system at the top of the reflectivity curve have much greater contrast than that of the normal radiograph (x8 to 33-fold higher). This is due to the rejection of small angle scattering (extinction) from the fibrils enhancing the contrast.

X-Ray absorption edge and extended x-ray absorption fine structure studies of PtTiO2 catalysts

X-Ray absorption edge and extended x-ray absorption fine structure (EXAFS) spectroscopy was used to study PtTiO2 prepared by two different techniques. Edge studies show that the observable electronic structure of very small particles of reduced PtTiO2 are independent of preparation technique. After H2 reduction at 473 °K, PtTiO2 has about 10% fewer unfilled (vacant) d states (0.03 fewer d state vacancies) per Pt atom than PtSiO2, prepared and reduced the same way, and has about 15% more unfilled d states (0.04 more d state vacancies) per Pt atom than bulk Pt. Reduction of PtTiO2 in H2 at 698 °K results in less than a 4% reduction in the number of unfilled d states per Pt atom (less than 0.015 hole reduction per Pt atom) as compared to the 473 °K reduction, indicating a very small amount of electron transfer to the Pt induced by the high-temperature H2 reduction of PtTiO2. It was concluded that the reported effect of high-temperature reduction of PtTiO2 on CO and H2 chemisorption is not due to a substantial change in the extent of electron transfer from the support but is due to more subtle and specific electronic changes.

Silicide formation and stability of Ti SiGe and Co SiGe

Abstract The formation and stability of the products of Ti and Co reacting with Si1 − x Gex substrates were investigated. For the Ti SiGe system, when a C54 Ti(Si1 − yGey)2 layer forms, the Ge index y is initially the same as the Ge index of the Si1−xGex substrate (i.e. y = x). Thereafter Si1 − xGex from the substrate continues to diffuse into the C54 layer via lattice and grain-boundary diffusion. Some of the Si which diffuses into the C54 lattice replaces Ge in the lattice, and the C54 Ti(Si1 − yGey)2 becomes silicon enriched (i.e. y Co SiGe system, it was determined that a silicon-enriched Co(Si1 − yGey) layer was formed at ~ 400 °C. As the annealing temperature was increased, the reacted layer became even more Si enriched. For both materials systems, Ge-enriched Si1 − zGe(z > x) islands were observed. It was found that for Co Si 1 − xe x the reacted layer consisted of CoSi2 and Si1 − zGez, after high-temperature annealing (≈700 °C). We propose that these processes are driven by a reduction in the crystal energy of the C54 Ti(Si1 − yGey)2 phase in the Ti SiGe system and the Co(Si1 − yGey) phase in the Co SiGe system which accompanies the replacement of Ge with Si.