Elemental x-ray imaging using Zernike phase contrast

We develop an element-specific x-ray microscopy method by using Zernike phase contrast imaging near absorption edges, where a real part of refractive index changes abruptly. In this method two phase contrast images are subtracted to obtain the target element: one is at the absorption edge of the target element and the other is near the absorption edge. The x-ray exposure required by this method is expected to be significantly lower than that of conventional absorption-based x-ray elemental imaging methods. Numerical calculations confirm the advantages of this highly efficient imaging method.     

Quantitative Evaluation Methods of In-Line X-Ray Phase Contrast Techniques

By revealing the relationship between edge visibility and imaging parameters in in-line phase contrast imaging （PCI）, we propose a method to quantitatively measure the contribution of absorption and phase shift from acquired images. We also prove that edge visibility will grow with the increasing source-object distance and object-detector distance. The result is validated by relative phase factor and by experiments conducted on a microfocus x-ray source. This method provides a new approach to evaluate in-line PCI images and is helpful for deciding imaging parameters.     

近场X射线位相衬度成像及位相恢复算法

理论分析了X射线衍射位相成像和近场位相恢复算法。定义了一个最佳成像距离，给出了基于最佳成像距离和特征空间频率的衍射场光强分布新的表达形式。基于模拟位相物体给出了数值模拟结果。由此得出了探测距离和多色辐照对相衬图像和位相恢复结果的影响。本文给出的结果将对同轴x射线相衬成像实验具有一定的指导作用。     

Nanoscale imaging of buried structures with elemental specificity using resonant x-ray diffraction microscopy

We report the first demonstration of resonant x-ray diffraction microscopy for element specific imaging of buried structures with a pixel resolution of approximately 15 nm by exploiting the abrupt change in the scattering cross section near electronic resonances. We performed nondestructive and quantitative imaging of buried Bi structures inside a Si crystal by directly phasing coherent x-ray diffraction patterns acquired below and above the Bi M5 edge. We anticipate that resonant x-ray diffraction microscopy will be applied to element and chemical state specific imaging of a broad range of systems including magnetic materials, semiconductors, organic materials, biominerals, and biological specimens.     

In-situ study of precipitates in Al–Zn–Mg–Cu alloys using anomalous small-angle x-ray scattering

In the present work,the precipitate compositions and precipitate amounts of these elements(including the size distribution,volume fraction,and inter-precipitate distance) on the Cu-containing 7000 series aluminum alloys(7150 and 7085 Al alloys),are investigated by anomalous small-angle x-ray scattering(ASAXS) at various energies.The scattering intensity of 7150 alloy with T6 aging treatment decreases as the incident x-ray energy approaches the Zn absorption edge from the lower energy side,while scattering intensity does not show a noticeable energy dependence near the Cu absorption edge.Similar results are observed in the 7085 alloy in an aging process(120℃) by employing in-situ ASAXS measurements,indicating that the precipitate compositions should include Zn element and should not be strongly related to Cu element at the early stage after 10 min.In the aging process,the precipitate particles with an initial average size of ~ 8 ?A increase with aging time at an energy of 9.60 ke V,while the increase with a slower rate is observed at an energy of 9.65 ke V as near the Zn absorption edge.     

X ray absorption spectroscopy investigation of phase transition in Ge,GaAs and GaP

Abstract

The phase transitions of GaAs, GaP and Ge under pressure have been investigated by x-ray absorption spectroscopy (XAS). At the onset of the transition the Debye-Waller factor increases and the x-ray absorption near edge structure (XANES) is progressively modified. A mixing of the low and high pressure phase can be determined by XAS as well as amorphization of the sample on pressure release.     

Mixed transfer function and transport of intensity approach for phase retrieval in the Fresnel region

We present a method for phase retrieval in propagation-based x-ray imaging, based on the contrast transfer and transport of intensity equation approaches. We show that the contrast transfer model does not coincide with the transport of intensity in the limit of small propagation distances, and we derive a new model that alleviates this problem. Using this model, we devise an algorithm to retrieve the phase from slowly varying samples that is valid beyond the limit of small distances. We show its utility by imaging in three dimensions a biological sample that causes both strong absorption and phase shift.     

Theoretical interpretation of spin-polarized X-ray absorption spectra of Mn in MnP

On the basis of the multiple scattering approach in combination with the spin-polarized self-consistent potential calculation, an interpretation is proposed for the spin-polarized x-ray absorption spectra near the Mn K edge in MnP. The effect of the core vacancy potential on the spectra is analyzed and found to be insignificant. The method used for the calculations allowed the separation of the effect of the dipole transition matrix element on the spectra and the effect of the density of unoccupied electron states. It is shown that the transition matrix element causes an intensity redistribution only near the absorption jump, while the difference in the densities of states is most pronounced in the energy region 35–55 eV away from the main edge and leads to a shift in energy for the spectra corresponding to the two spin directions. The effect of the spin-dependent broadening caused by the dependence of the mean free path (as a function of energy) on the photoelectron spin is studied. It is shown that this factor considerably affects only the intensities of the peaks in the energy region lying within less than 12 eV from the main edge.     

Current status of the TwinMic beamline at Elettra: a soft X‐ray transmission and emission microscopy station

The current status of the TwinMic beamline at Elettra synchrotron light source, that hosts the European twin X‐ray microscopy station, is reported. The X‐ray source, provided by a short hybrid undulator with source size and divergence intermediate between bending magnets and conventional undulators, is energy‐tailored using a collimated plane‐grating monochromator. The TwinMic spectromicroscopy experimental station combines scanning and full‐field imaging in a single instrument, with contrast modes such as absorption, differential phase, interference and darkfield. The implementation of coherent diffractive imaging modalities and ptychography is ongoing. Typically, scanning transmission X‐ray microscopy images are simultaneously collected in transmission and differential phase contrast and can be complemented by chemical and elemental analysis using across‐absorption‐edge imaging, X‐ray absorption near‐edge structure or low‐energy X‐ray fluorescence. The lateral resolutions depend on the particular imaging and contrast mode chosen. The TwinMic range of applications covers diverse research fields such as biology, biochemistry, medicine, pharmacology, environment, geochemistry, food, agriculture and materials science. They will be illustrated in the paper with representative results.     

Amplification of the phase contrast signal at very high x-ray energies

X-ray phase contrast imaging is increasingly being used in several fields, both at synchrotron facilities and with laboratory sources, due to its increased sensitivity compared to conventional x-ray methods. One important problem is the development of methods to make it suitable for use at very high x-ray energies, needed in many applications. We show how the edge illumination concept, which stands at the basis of the coded-aperture method, allows achieving hyperintense phase signals at energies close to 100 keV, by showing images of both weak phase objects and highly absorbing fossils with a high iron content.     

Single-shot grating-based x-ray differential phase contrast imaging with a modified analyzer grating

X-ray grating interferometer has attracted widely attention in the past years due to its capability in achieving x-ray phase contrast imaging with low brilliance source. However, the widely used phase stepping information extraction method reduces system stability and prolongs data acquisition time by several times compared with conventional x-ray absorptionbased imaging. The mechanical stepping can be avoided by using a staggered grating, but at the cost of low vertical spatial resolution. In this paper, employing a modified staggered grating and the angular signal radiography, we proposed a single-shot grating-based x-ray differential phase contrast imaging with decent vertical spatial resolution. The theoretical framework was deduced and proved by numerical experiments. Absorption, phase, and scattering computed tomography can be performed without phase stepping. Therefore, we believe this fast and highly stable imaging method with decent resolution would be widely applied in x-ray grating-based phase contrast imaging.     

Investigation of noise properties in grating-based x-ray phase tomography with reverse projection method

The relationship between noise variance and spatial resolution in grating-based x-ray phase computed tomography(PCT) imaging is investigated with reverse projection extraction method, and the noise variances of the reconstructed absorption coefficient and refractive index decrement are compared. For the differential phase contrast method, the noise variance in the differential projection images follows the same inverse-square law with spatial resolution as in conventional absorption-based x-ray imaging projections. However, both theoretical analysis and simulations demonstrate that in PCT the noise variance of the reconstructed refractive index decrement scales with spatial resolution follows an inverse linear relationship at fixed slice thickness, while the noise variance of the reconstructed absorption coefficient conforms with the inverse cubic law. The results indicate that, for the same noise variance level, PCT imaging may enable higher spatial resolution than conventional absorption computed tomography(ACT), while ACT benefits more from degraded spatial resolution. This could be a useful guidance in imaging the inner structure of the sample in higher spatial resolution.     

A new method of detecting interferogram in differential phase-contrast imaging system based on special structured x-ray scintillator screen

An x-ray scintillator screen with a special structure,functioning as detector and analyser grating,was proposed for collecting the interferogram of differential phase contrast imaging without absorption grating and difficulty of fabrication by a state of the art technique.On the basis of phase grating diffraction,a detecting model of the scintillator screen was built for analysing the phase and absorption information of objects.According to the analysis,a new method of phase retrievals based on two-images and the optimal structure of screen were presented.     

The local and electronic structures of Al90FexCe10−x alloys (x=3, 5, 7): XANES analysis

The x-ray absorption near-edge structure (XANES) of Al₉₀Fe_xCe_10?x (x=3, 5, 7) is measured above the Fe K edge and analyzed theoretically by the multiple scattering method. By comparing the experimental data with the theoretical findings, the most adequate model for describing the neighbor environment of Fe atoms is chosen. Since the dipole transition matrix element is a smoothly varying function of energy near the Fe K edge, the x-ray absorption fine structure in this energy range characterizes the density of free p states of Fe in the conduction band of Al₉₀Fe_xCe_10?x alloys.     

Nanoscale imaging with resonant coherent X rays: extension of multiple-wavelength anomalous diffraction to nonperiodic structures

The methodology of multiple-wavelength anomalous diffraction, widely used for macromolecular structure determination, is extended to the imaging of nonperiodic nanostructures. We demonstrate the solution of the phase problem by a combination of two resonantly recorded coherent scattering patterns at the carbon K edge (285 eV). Our approach merges iterative phase retrieval and x-ray holography approaches, yielding unique and rapid reconstructions. The element, chemical, and magnetic state specificity of our method further renders it widely applicable to a broad range of nanostructures, providing a spatial resolution that is limited, in principle, by wavelength only.     

X-ray phase-sensitive microscope imaging with a grating interferometer: Theory and simulation

A general theoretical framework is presented to explain the formation of the phase signal in an x-ray microscope integrated with a grating interferometer, which simultaneously enables the high spatial resolution imaging and the improved image contrast. By using this theory, several key parameters of phase contrast imaging can be predicted, for instance, the fringe visibility and period, and the conversion condition from the differential phase imaging (DPI) to the phase difference imaging (PDI). Additionally, numerical simulations are performed with certain x-ray optical components and imaging geometry. Comparison with the available experimental measurement [Appl. Phys. Lett. 113 063105 (2018)] demonstrates the accuracy of this developed quantitative analysis method of x-ray phase-sensitive microscope imaging.     

中子相衬层析成像方法

介绍了中子相衬和中子层析的工作原理，提出了中子相衬层析成像方法.这种方法兼备了相衬和层析的各自优点，不采用干涉术，能高分辨率地重构待测样品的三维空间位相分布，能测试和分辨吸收系数非常相近样品的空间结构. 关键词： 相衬 层析 中子相衬 中子层析     

X-ray spatial harmonic imaging of phase objects

Refractive index gradients in materials or at material interfaces lead to x-ray diffraction. Interference of this radiation with adjacent x-ray waves causes phase contrast that can be used for imaging purposes if an x-ray source with sufficient spatial coherence is used. The imaging modality presented here uses hard x radiation diffracted at interfaces, but requires only little spatial coherence. We report experiments showing, first, that image contrast is not diminished by motional blurring, and second, that contrast can be increased by orders of magnitude relative to in-line x-ray phase-contrast imaging. These properties substantially broaden the applicability of phase-sensitive imaging to moving samples and very weak density gradients.     

Multienergy anomalous diffuse scattering

A new approach to local structure determination is presented. A three-dimensional region of the reciprocal space of a SrTiO(3) single crystal was mapped by measuring x-ray diffuse scattering patterns at different sample orientations in order to reconstruct the local atomic structure. The phase problem was solved by means of anomalous scattering from strontium atoms at photon energies near their K absorption edge. Real-space reconstruction provides the average short-range order atomic arrangement in the vicinity of anomalous scatterers up to a distance of several unit cells.