Vicinity Of Absorption Edge Research Articles

Synchrotron K-edge subtraction imaging and its application to metallic foams

On the basis of sudden change of linear absorption coefficient (LACs) in the vicinity of absorption edge, the synchrotron K-edge subtraction technique is developed to label the three-dimensional (3D) distribution of element. In this work, tomographic scans above and below the specific absorption edge of Zn are carried out and the subtracted image is used to identify and quantify the 3D distribution of Zn element inside the cell wall of Al-Zn-Mg foam. A non-uniform spatial distribution of Zn element is found in the cell wall of as-cast foam. After long solution tratment, the concentration of Zn element tended to be homogeneous. It is confirmed that the local agglomerated Zn-bearing particles exerta negative influence on the brittle cracking of the cell wall. The K-edge subtraction technique provides a powerful tool for the quantitative microstructure characterization of three-dimensional tissues.

Speciation of deeply buriedTiOxnanolayers with grazing-incidence x-ray fluorescence combined with a near-edge x-ray absorption fine-structure investigation

Nondestructive methods based on electron emission may encounter serious difficulties when probing the chemical state of deeply buried nanolayers due to restricted information depth. The purpose of the present work is to evaluate to which extent photon emission can overcome these restrictions. Grazing-incidence x-ray fluorescence combined with a near-edge x-ray absorption fine-structure investigation (GIXRF-NEXAFS) offers access to depth-resolving analysis of buried nanolayers with respect to both the chemical speciation and the layer composition. By varying the angle of incidence, the penetration depth can be tuned from a few to several hundreds of nanometers. The information depth of the emitted fluorescence radiation is in the same general range as the soft x-ray regime. Initial measurements were performed on nominally 30 nm thick titanium nanolayers oxidized to different extents and buried below 5 nm carbon layers. These layered structures were produced by means of ion beam sputtering deposition. The plane grating monochromator beamline for undulator radiation in the laboratory of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY II provides tunable radiation of both well-known flux and high spectral purity for GIXRF-NEXAFS studies. The current results confirm that GIXRF-NEXAFS has the potential to substantially contribute to the speciation of deeply buried nanolayers. The analysis of measurements at a constant incident angle demonstrated that it is not possible to find an angle of incidence for the NEXAFS region to ensure a stable penetration depth. However, appropriate angular corrections can ensure a constant mean penetration depth, in particular, in the vicinity of absorption edges.

Detailed Tabulation of Atomic Form Factors, Photoelectric Absorption and Scattering Cross Section, and Mass Attenuation Coefficients in the Vicinity of Absorption Edges in the Soft X-Ray (Z=30–36, Z=60–89, E=0.1 keV–10 keV), Addressing Convergence Issues of Earlier Work

Reliable knowledge of the complex x-ray form factor [Re(f ) and f″] and the photoelectric attenuation coefficient (σPE) is required for crystallography, medical diagnosis, radiation safety, and XAFS studies. Discrepancies between currently used theoretical approaches of 200% exist for numerous elements from 1 to 3 keV x-ray energies. The key discrepancies are due to the smoothing of edge structure, the use of nonrelativistic wave functions, and the lack of appropriate convergence of wave functions. This paper addresses these key discrepancies and derives new theoretical results of substantially higher accuracy in near-edge soft x-ray regions. The high-energy limitations of the current approach are also illustrated. The energy range covered is 0.1 to 10 keV. The associated figures and tabulation demonstrate the current comparison with alternate theory and with available experimental data. In general, experimental data are not sufficiently accurate to establish the errors and inadequacies of theory at this level. However, the best experimental data and the observed experimental structure as a function of energy are strong indicators of the validity of the current approach. New developments in experimental measurement hold great promise in making critical comparisons with theory in the near future.

Measurements of photon-atom elastic scattering cross-sections in the photon energy range 1 keV to 4 MeV

We review the current status of measurements of photon-atom elastic scattering cross-sections for the restricted photon energy range 1 keV up to 4 MeV. Among the key experimental factors which influence the accuracy and precision of a particular type of measurement are the choice of source, detector and scattering geometry. We have examined the interests which motivate the making of measurements, looking at issues at the atomic, molecular and solid state level. Discrepancies have been noted to exist between experiments and between experiments and theory and explanations for these have been sought. It is apparent that over the last fifteen years or so there has been a substantial increase in the number of reported studies of elastic scattering cross-sections for photons of energies <100 keV impinging on high purity targets. These measurements have largely focused on scattering amplitudes in the vicinity of absorption edges. For the same range of photon energies, molecular correlations in amorphous media represent another active area of investigation. At higher photon energies, measurements have more generally involved small angles of scattering, the main interest being Rayleigh scattering from high purity targets. Account of Delbrück scattering amplitudes has been found necessary for a satisfactory explanation of experimental values of elastic scattering cross-sections. In the case of lead, at photon energies of about 1.3 MeV and large angles of scattering, the real Delbrück amplitude is about 10% of the Rayleigh amplitude.

Polarized X-ray Absorption. Quantitative Determination by Fluorescence Measurements

For non-cubic crystals, polarized X-ray absorption due to anisotropic anomalous scattering (AAS) occurring in the vicinity of absorption edges can be determined, both qualitatively and quantitatively, from polarized fluorescence measurements. For crystals exhibiting `large' faces, a method is presented that allows the simultaneous determination of all absorption-tensor elements from a single-crystal rotation experiment. Energy variation included, the method may be used for single-crystal polarized absorption and fluorescence spectroscopy. Depending on the crystal system and the face investigated, the experimental set-up can require a tilt of the polarization direction(s) against the plane of measurement as well as a precession of the crystal face normal about the rotation axis Ψ. For all systems except triclinic, the experimental conditions are discussed and fluorescence intensity functions IF(Ψ) are developed and illustrated by model calculations.

Measurement of total and photoelectric cross sections in the vicinity of absorption edges of heavier atoms

Measurement of total and photoelectric cross sections in the vicinity of absorption edges of heavier atoms

RECENT DEVELOPMENTS IN THE EXPERIMENTAL STUDIES OF XANES

X-Ray absorption spectroscopy in the vicinity of absorption edges contain information about the electronic structure and about the geometrical arrangement around the absorbing atom. This paper presents recent experimental studies in relation to the type of interpretation which is given.