Multiple-energy X-ray Research Articles

Three-Dimensional Chemical Analysis with Synchrotron Tomography at Multiple X-ray Energies: Brominated Aromatic Flame Retardant and Antimony Oxide in Polystyrene

Synchrotron X-ray tomography is used to image, at 3.34 μm resolution, a mixture containing high-impact polystyrene (HIPS) and a two-component flame retardant, a brominated phthalimide dimer (Saytex BT-93), and a synergist, antimony oxide (Sb 2 O 3 ). Complete tomography data sets were acquired at seven X-ray energies in the range of 12-40 keV, closely spanning Br and Sb Is electron binding energies at 13.474 and 30.491 keV, respectively. Data acquisition required 8 h. Quantification is done for a representative subvolume of 0.582 mm 3 containing 250 3 cubic volume elements (voxels) which is fitted to a model for X-ray voxel linear attenuation coefficient, yielding the three-dimensional concentration distribution of the flame retardant and the synergist. With successive thresholding, a particle size distribution algorithm yields regions of low flame retardant concentration-a potential safety concern-and regions of excessively high flame retardant concentration-a waste of an expensive product.

X-ray spectrum generation for a multiphase flow meter

This paper describes the design of a geometry for the generation of X-rays to measure the relative amounts of oil, water, and gas in a flow. The measurement principle is based on the attenuation of X-rays in the mixture. The X-rays are generated by fluorescence in slanted foils placed in front of an end-window X-ray tube. EGS4 Monte Carlo simulations have been used to determine an optimum geometry. We conclude that with an X-ray tube power in the order of only /spl sim/ 2.5 W, successful multiple energy X-ray absorption analysis should be possible.

Multiple-energy X-ray radiography and digital subtraction for a particle-composition sensor

Composite mineral particles in a monolayer were X-ray radiographed with monochromatic beams at different incident energies. The digital radiographic images were subtracted according to the dual-energy (DE) principle, which is extended to multiple-energy (ME) for the purpose of this work. The subtraction allows the selective derivation of two images thicknesses, one for each mineralogical phase component. The integration of the thickness gives the volumetric content in the phase. Preliminary experimental results were compared to the grade obtained by micro-CT, showing good agreement. The possible developments related to the fast DE evaluation of particle grades are discussed.

Multiple-energy x-ray holography: Polarization effect

We present the theory for multiple-energy x-ray holography (MEXH), using a multipole expansion for the scattered field. We find that light polarization plays a crucial role in the reconstruction of the image, and we suggest how to use it in order to eliminate aberration effects. The method we propose is alternative to the scattered-wave-included Fourier transform method, but has the advantage that no theoretical calculations are required to redefine the hologram.

Development of apparatus for multiple energy X-ray holography at SPring-8

We developed an apparatus for multiple energy X-ray holography (MEXH) at third generation synchrotron facility, SPring-8. The combination system of a cylindrical lithium fluoride crystal and an avalanche photo diode in our apparatus enabled high countrate detection of the fluorescence from samples. From the measurements repeated 10 times, reproducible holographic undulations were obtained from the strontium titanate single crystal, although 10% intensity change of the incident beam occurred. This result revealed that the holographic data provided by our MEXH apparatus was highly stable to the variation of the incident beam intensity.

Local-structure analysis around dopant atoms using multiple energy x-ray holography

We used multiple energy x-ray holography (MEXH) to image the local atomic environment of Zn atoms doped in a GaAs wafer using synchrotron radiation and a multielement solid-state detector. The obtained atomic images revealed that the Zn atoms occupied substitutional site. By the comparison of the reconstructed images 1.41 \AA{} above and below the emitter atom, the suppression of twin images due to MEXH was confirmed experimentally for certain atoms.

Direct 3D imaging of Al70.4Pd21Mn8.6 quasicrystal local atomic structure by X-ray holography

Inverse x-ray fluorescence holography was used to explore the local atomic order of a nearly perfect quasicrystal with composition Al70. 4Pd21Mn8.6. We have demonstrated the possibility of direct 3D imaging of the atomic decoration in a quasicrystal. We have obtained the average 3D environment of selected coordination shells around the Mn atoms. These results open the way to obtaining further and more complete information about the various coordination shells in complex materials by measuring multiple energy x-ray holograms at different sites.

Energy-dispersive, x-ray reflectivity density measurements of porous SiO2 xerogels

X-ray reflectivity has been used to measure nondestructively the density of thin, porous, silica xerogels used for interlayer dielectric applications. The critical angle, defined through total external reflection, was measured for multiple x-ray energies to correct for sample misalignment error in the determination of the density for the films. This density was used to extrapolate the percentage porosity, assuming a bulk SiO2 density standard. The results were compared to those obtained by Rutherford backscattering and ellipsometry techniques.

Holographies and EXAFS in quantum electrodynamics

We calculate the cross section of the photoionization process consistently in the framework of quantum electrodynamics, including the initial-state photon interaction and the final-state photoelectron interaction with neighboring atoms. The obtained photoionization cross section explains multiple energy x-ray holography (MEXH), photoelectron holography, and extended x-ray absorption fine structure (EXAFS). In EXAFS, the contribution of the initial-state photon interaction is found to oscillate with energy and can disturb the EXAFS signal. It becomes non-negligible at higher energies. At 1 keV above the absorption edge, it can be in comparable order of the EXAFS signal. The hologram functions of MEXH and x-ray fluorescence holography are consistent with those in classical electrodynamics.

Elemental quantification using multiple-energy x-ray absorptiometry

A novel implementation of multiple-energy x-ray absorptiometry (MEXA) for elemental quantification has been developed. Species are resolved on the basis of their differential attenuation spectra across a wide energy range, ideally including absorption edges. By measuring the incident and exiting x-ray spectra and using known values of mass attenuation coefficients over selected energy bands, the density line integral of the species along the x-ray path can be calculated from all the selected energy channels simultaneously by non-linear least squares methods. Effects of `escape' peak phenomena are modelled and corrections for them are included in the MEXA software. The applications of MEXA are illustrated by single measurements on aluminium and zirconium foils, quantitation of aqueous KI diffusing into a porous solid, simultaneous measurement of acidic diffusant and porous solid with which it reacts and which it dissolves and microtomographic reconstructions of liquid and solid specimens containing caesium and/or iodine.

Multiple-Energy X-ray Holography with Atomic Resolution

Multiple-energy x-ray holography (MEXH) is a particular implementation of an x-ray holographic method capable of directly imaging atomic structures in the bulk, at surfaces and interfaces of solids. It utilizes an interference field generated by incident and scattered x-rays within the sample, then determines its strength at specific atomic sites from integral fluorescence yields. Scanning large portions of reciprocal space, a hologram is recorded, which can be converted to a real space image by Fourier transformation. Image aberrations that adversely affect results in single-energy techniques are successfully avoided in this novel multi-energy version.

Multiple-energy X-ray holography with atomic resolution

Abstract Recent approaches to achieving atomic resolution with holographic methods and directly image atomic structures in the bulk at surfaces and interfaces of solids have become increasingly successful. Multiple-energy X-ray holography (MEXH), a particular implementation of such a method, is reviewed and its first applications to bulk crystals and nano-clusters suspended in a crystalline matrix is presented. MEXH utilizes X-rays to generate an interference field within the sample, then determines its strength at specific atomic sites from fluorescence emissions. Scanning reciprocal space, a hologram is recorded from which local atomic structures can be reconstructed by Fourier transformation. Image aberrations known to distort single-energy holograms are effectively suppressed in this novel multi-energy technique.

Quantum electrodynamics of the internal source x-ray holographies: Bremsstrahlung, fluorescence, and multiple-energy x-ray holography

Quantum electrodynamics (qed) is used to derive the differential cross sections measured in the three new experimental internal source ensemble x-ray holographies: bremsstrahlung (BXH), fluorescence (XFH), and multiple-energy (MEXH) x-ray holography. The polarization dependence of the BXH cross section is also obtained. For BXH, we study analytically and numerically the possible effects of the virtual photons and electrons which enter qed calculations in summing over the intermediate states. For the low photon and electron energies used in the current experiments, we show that the virtual intermediate states produce only very small effects. This is because the uncertainty principle limits the distance that the virtual particles can propagate to be much shorter than the separation between the regions of high electron density in the adjacent atoms. We also find that using the asymptotic form of the scattering wave function causes about a 5-10% error for near forward scattering.

Multiple energy x-ray holography: Incident-radiation polarization effects

Multiple energy x-ray holography (MEXH) measures both phase and amplitude information for x rays scattered from an incident reference beam, from which three-dimensional atomic images can be directly reconstructed. The angular distribution of the x-ray scattering is highly dependent on the polarization direction via the Thomson scattering cross section. We consider here the effect of incident x-ray polarization on images of Fe atoms reconstructed from theoretical and experimental MEXH data for $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(001)$ (hematite). We also illustrate such polarization effects theoretically in the enhancement of specific atomic structural information of ideal Fe trimers, and a Ge \ensuremath{\delta}-layer buried in Si(001), where the use of different polarization modes and experimental geometries is found to strongly influence atomic images.

Evaluation of a Scattered Radiation Field in a Cluster Relevant for Multiple-Energy X-Ray Holography

We analyze theoretically multiple-energy X-ray holography (MEXH), a new tool for material structure determination, already tested experimentally, which utilizes synchrotron radiation to generate an electromagnetic scattering field at a specific target atom inside a material sample. In MEXH the target atom is considered as a detector, which indicates the electric field strength at its core via the measurement of the total intensity of fluorescence that the atom emits in all directions. The modulations of the intensity, upon varying the direction and the energy of the incident synchrotron radiation, are registered as a hologram which offers then the image of the environment surrounding the target atom. The purpose of this work is to determine theoretically the relevant physical quantity of the process, i.e. the electromagnetic field, at the target atom position. For this purpose we use methods of quantum electrodynamics (QED). We obtain formulas which are interpreted in terms of a holographic description of the process which can then have a direct experimental application through the MEXH method.

Evaluation of a Scattered Radiation Field in a Cluster Relevant for Multiple-Energy X-Ray Holography

Evaluation of a Scattered Radiation Field in a Cluster Relevant for Multiple-Energy X-Ray Holography

X-ray fluorescence holography and multiple-energy x-ray holography: A critical comparison of atomic images

We compare x-ray fluorescence holography (XFH) and multiple-energy x-ray holography (MEXH), two techniques that have recently been used to obtain experimental three-dimensional atomic images. For single-energy holograms, these methods are equivalent by virtue of the optical reciprocity theorem. However, XFH can only record holographic information at the characteristic fluorescence energies of the emitting species, while MEXH can record holographic information at any energy above the fluorescent edge of the emitter, thus enabling the suppression of real-twin overlaps and other aberrations and artifacts in atomic images. {copyright} {ital 1997} {ital The American Physical Society}

Multiple-energy x-ray holography: Atomic images of hematite (Fe2O3).

Multiple-energy x-ray holography (MEXH) is employed to image the local atomic environment of Fe atoms in hematite. MEXH utilizes synchrotron radiation to generate an interference field within the sample, and then determines the strength of this near field at specific atomic sites from the integrated fluorescence yield. Scanning an extended volume of reciprocal space, the local atomic structure in Fe{sub 2}O{sub 3} is reconstructed by Fourier transformation from the hologram. Image aberrations known to distort single-energy holograms are effectively suppressed by summing data for several incident energies. {copyright} {ital 1996 The American Physical Society.}

An area detector data acquisition system for protein crystallography using multiple-energy anomalous dispersion techniques

We have developed an automated data acquisition system, incorporating a two-dimensional multiwire proportional counter mounted on a 5-circle diffractometer, to facilitate crystallographic data collection from protein crystals at multiple X-ray energies ∗. This system is permanently installed on a tunable X-ray beamline at the Stanford Synchrotron Radiation Laboratory and is now fully operational. It is being used to investigate the utility of multiple-energy anomalous dispersion (MEAD) phasing techniques for determining macromolecular structures. These techniques provide an alternative to the multiple isomorphous replacement method for the solution of crystallographic phases, and have a number of advantages. This paper describes the data acquisition system in some detail and also outlines a number of the important considerations for the collection of good quality data at multiple X-ray energies from a synchrotron radiation source. We will discuss the methodology for multi-energy phasing and preliminary results in a later publication.