Utilizing broadband X‐rays in a Bragg coherent X‐ray diffraction imaging experiment

A method is presented to simplify Bragg coherent X‐ray diffraction imaging studies of complex heterogeneous crystalline materials with a two‐stage screening/imaging process that utilizes polychromatic and monochromatic coherent X‐rays and is compatible with in situ sample environments. Coherent white‐beam diffraction is used to identify an individual crystal particle or grain that displays desired properties within a larger population. A three‐dimensional reciprocal‐space map suitable for diffraction imaging is then measured for the Bragg peak of interest using a monochromatic beam energy scan that requires no sample motion, thus simplifying in situ chamber design. This approach was demonstrated with Au nanoparticles and will enable, for example, individual grains in a polycrystalline material of specific orientation to be selected, then imaged in three dimensions while under load.     

Correcting for surface topography in X‐ray fluorescence imaging

Samples with non‐planar surfaces present challenges for X‐ray fluorescence imaging analysis. Here, approximations are derived to describe the modulation of fluorescence signals by surface angles and topography, and suggestions are made for reducing this effect. A correction procedure is developed that is effective for trace element analysis of samples having a uniform matrix, and requires only a fluorescence map from a single detector. This procedure is applied to fluorescence maps from an incised gypsum tablet.     

Individual GaAs nanorods imaged by coherent X‐ray diffraction

Using scanning X‐ray diffraction microscopy with a spot size of 220 × 600 nm, it was possible to inspect individual GaAs nanorods grown seed‐free through circular openings in a SiN_x mask in a periodic array with 3 µm spacing on GaAs[111]B. The focused X‐ray beam allows the determination of the strain state of individual rods and, in combination with coherent diffraction imaging, it was also possible to characterize morphological details. Rods grown either in the centre or at the edge of the array show significant differences in shape, size and strain state.     

Development of variable‐magnification X‐ray Bragg optics

A novel X‐ray Bragg optics is proposed for variable‐magnification of an X‐ray beam. This X‐ray Bragg optics is composed of two magnifiers in a crossed arrangement, and the magnification factor, M, is controlled through the azimuth angle of each magnifier. The basic properties of the X‐ray optics such as the magnification factor, image transformation matrix and intrinsic acceptance angle are described based on the dynamical theory of X‐ray diffraction. The feasibility of the variable‐magnification X‐ray Bragg optics was verified at the vertical‐wiggler beamline BL‐14B of the Photon Factory. For X‐ray Bragg magnifiers, Si(220) crystals with an asymmetric angle of 14° were used. The magnification factor was calculated to be tunable between 0.1 and 10.0 at a wavelength of 0.112 nm. At various magnification factors (M≥ 1.0), X‐ray images of a nylon mesh were observed with an air‐cooled X‐ray CCD camera. Image deformation caused by the optics could be corrected by using a 2 × 2 transformation matrix and bilinear interpolation method. Not only absorption‐contrast but also edge‐contrast due to Fresnel diffraction was observed in the magnified images.     

A new approach to synchrotron energy‐dispersive X‐ray diffraction computed tomography

A new data collection strategy for performing synchrotron energy‐dispersive X‐ray diffraction computed tomography has been devised. This method is analogous to angle‐dispersive X‐ray diffraction whose diffraction signal originates from a line formed by intersection of the incident X‐ray beam and the sample. Energy resolution is preserved by using a collimator which defines a small sampling voxel. This voxel is translated in a series of parallel straight lines covering the whole sample and the operation is repeated at different rotation angles, thus generating one diffraction pattern per translation and rotation step. The method has been tested by imaging a specially designed phantom object, devised to be a demanding validator for X‐ray diffraction imaging. The relative strengths and weaknesses of the method have been analysed with respect to the classic angle‐dispersive technique. The reconstruction accuracy of the method is good, although an absorption correction is required for lower energy diffraction because of the large path lengths involved. The spatial resolution is only limited to the width of the scanning beam owing to the novel collection strategy. The current temporal resolution is poor, with a scan taking several hours. The method is best suited to studying large objects (e.g. for engineering and materials science applications) because it does not suffer from diffraction peak broadening effects irrespective of the sample size, in contrast to the angle‐dispersive case.     

Sorting algorithms for single‐particle imaging experiments at X‐ray free‐electron lasers

Modern X‐ray free‐electron lasers (XFELs) operating at high repetition rates produce a tremendous amount of data. It is a great challenge to classify this information and reduce the initial data set to a manageable size for further analysis. Here an approach for classification of diffraction patterns measured in prototypical diffract‐and‐destroy single‐particle imaging experiments at XFELs is presented. It is proposed that the data are classified on the basis of a set of parameters that take into account the underlying diffraction physics and specific relations between the real‐space structure of a particle and its reciprocal‐space intensity distribution. The approach is demonstrated by applying principal component analysis and support vector machine algorithms to the simulated and measured X‐ray data sets.     

Time‐resolved X‐ray PIV technique for diagnosing opaque biofluid flow with insufficient X‐ray fluxes

X‐ray imaging is used to visualize the biofluid flow phenomena in a nondestructive manner. A technique currently used for quantitative visualization is X‐ray particle image velocimetry (PIV). Although this technique provides a high spatial resolution (less than 10 µm), significant hemodynamic parameters are difficult to obtain under actual physiological conditions because of the limited temporal resolution of the technique, which in turn is due to the relatively long exposure time (～10 ms) involved in X‐ray imaging. This study combines an image intensifier with a high‐speed camera to reduce exposure time, thereby improving temporal resolution. The image intensifier amplifies light flux by emitting secondary electrons in the micro‐channel plate. The increased incident light flux greatly reduces the exposure time (below 200 µs). The proposed X‐ray PIV system was applied to high‐speed blood flows in a tube, and the velocity field information was successfully obtained. The time‐resolved X‐ray PIV system can be employed to investigate blood flows at beamlines with insufficient X‐ray fluxes under specific physiological conditions. This method facilitates understanding of the basic hemodynamic characteristics and pathological mechanism of cardiovascular diseases.     

In situ X‐ray diffraction study on the growth kinetics of NiO nanoparticles

The growth kinetics of NiO nanoparticles have been studied by in situ X‐ray diffraction using two detection systems (conventional and imaging plate). NiO nanoparticles were formed by thermal decomposition after heating of an amorphous compound formed by the coprecipitation method. It was found that the detection method using an imaging plate is more efficient than the conventional detection mode for observing changes in the crystallite growth of nanocrystalline materials. Studies have been carried out to investigate the effects of the heating rates on the particles growth. The results suggest that the growth process of the particles is accelerated when the samples are treated at low heating rates. The evolution of particles size and the diffusion coefficient obtained from X‐ray powder diffraction patterns are discussed in terms of the thermal conditions for the two types of detection.     

Propagation of an X‐ray beam modified by a photonic crystal

A method of calculating the transmission of hard X‐ray radiation through a perfect and well oriented photonic crystal and the propagation of the X‐ray beam modified by a photonic crystal in free space is developed. The method is based on the approximate solution of the paraxial equation at short distances, from which the recurrent formula for X‐ray propagation at longer distances is derived. A computer program for numerical simulation of images of photonic crystals at distances just beyond the crystal up to several millimetres was created. Calculations were performed for Ni inverted photonic crystals with the [111] axis of the face‐centred‐cubic structure for distances up to 0.4 mm with a step size of 4 µm. Since the transverse periods of the X‐ray wave modulation are of several hundred nanometres, the intensity distribution of such a wave is changed significantly over the distance of several micrometres. This effect is investigated for the first time.     

Simultaneous X‐ray fluorescence and scanning X‐ray diffraction microscopy at the Australian Synchrotron XFM beamline

Owing to its extreme sensitivity, quantitative mapping of elemental distributions via X‐ray fluorescence microscopy (XFM) has become a key microanalytical technique. The recent realisation of scanning X‐ray diffraction microscopy (SXDM) meanwhile provides an avenue for quantitative super‐resolved ultra‐structural visualization. The similarity of their experimental geometries indicates excellent prospects for simultaneous acquisition. Here, in both step‐ and fly‐scanning modes, robust, simultaneous XFM‐SXDM is demonstrated.     

Investigation of defect luminescence from multicrystalline Si wafer solar cells using X‐ray fluorescence and luminescence imaging

Multicrystalline silicon wafer solar cells reveal performance‐ reducing defects by luminescence. X‐ray fluorescence spectra are used to investigate the elemental constituents from regions of solar cells yielding reverse‐bias or sub‐bandgap luminescence from defects. It is found that a higher concentration of metals is present in regions yielding reverse‐bias electroluminescence than in regions yielding sub‐bandgap electroluminescence. This suggests, dislocations do not create strong breakdown currents in the absence of impurity precipitates.

     

Confocal full‐field X‐ray microscope for novel three‐dimensional X‐ray imaging

A confocal full‐field X‐ray microscope has been developed for use as a novel three‐dimensional X‐ray imaging method. The system consists of an X‐ray illuminating `sheet‐beam' whose beam shape is micrified only in one dimension, and an X‐ray full‐field microscope whose optical axis is normal to the illuminating sheet beam. An arbitral cross‐sectional region of the object is irradiated by the sheet‐beam, and secondary X‐ray emission such as fluorescent X‐rays from this region is imaged simultaneously using the full‐field microscope. This system enables a virtual sliced image of a specimen to be obtained as a two‐dimensional magnified image, and three‐dimensional observation is available only by a linear translation of the object along the optical axis of the full‐field microscope. A feasibility test has been carried out at beamline 37XU of SPring‐8. Observation of the three‐dimensional distribution of metallic inclusions in an artificial diamond was performed.     

X‐ray dynamical diffraction Fraunhofer holography

An X‐ray dynamical diffraction Fraunhofer holographic scheme is proposed. Theoretically it is shown that the reconstruction of the object image by visible light is possible. The spatial and temporal coherence requirements of the incident X‐ray beam are considered. As an example, the hologram recording as well as the reconstruction by visible light of an absolutely absorbing wire are discussed.     

Silicon nitride transmission X‐ray mirrors

Transmission X‐ray mirrors have been fabricated from 300–400 nm‐thick low‐stress silicon nitride windows of size 0.6 mm × 85 mm. The windows act as a high‐pass energy filter at grazing incidence in an X‐ray beam for the beam transmitted through the window. The energy cut‐off can be selected by adjusting the incidence angle of the transmission mirror, because the energy cut‐off is a function of the angle of the window with respect to the beam. With the transmission mirror at the target angle of 0.22°, a 0.3 mm × 0.3 mm X‐ray beam was allowed to pass through the mirror with a cut‐off energy of 10 keV at the Cornell High Energy Synchrotron Source. The energy cut‐off can be adjusted from 8 to 12 keV at an angle of 0.26° to 0.18°, respectively. The observed mirror transmittance was above 80% for a 300 nm‐thick film.     

Pressure study of monoclinic ReO2 up to 1.2 GPa using X‐ray absorption spectroscopy and X‐ray diffraction

The crystal and local atomic structure of monoclinic ReO₂ (α‐ReO₂) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X‐ray absorption spectroscopy and high‐resolution synchrotron X‐ray powder diffraction and a home‐built B₄C anvil pressure cell developed for this purpose. Extended X‐ray absorption fine‐structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re—Re distances and a distorted ReO₆ octahedron in the α‐ReO₂ structure. X‐ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the α‐phase, demonstrating a modulation of the Re—Re distances. The relatively small portion of the diffraction pattern accessed in the pressure‐dependent measurements does not allow for a detailed study of the crystal structure of α‐ReO₂ under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re—Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that α‐ReO₂ is a possible inner pressure gauge for future experiments up to 1.2 GPa.     

Retrieval of the atomic displacements in the crystal from the coherent X‐ray diffraction pattern

The retrieval of spatially resolved atomic displacements is investigated via the phases of the direct(real)‐space image reconstructed from the strained crystal's coherent X‐ray diffraction pattern. It is demonstrated that limiting the spatial variation of the first‐ and second‐order spatial displacement derivatives improves convergence of the iterative phase‐retrieval algorithm for displacements reconstructions to the true solution. This approach is exploited to retrieve the displacement in a periodic array of silicon lines isolated by silicon dioxide filled trenches.     

White‐beam X‐ray radioscopy and tomography with simultaneous diffraction at the EDDI beamline

A set‐up for simultaneous imaging and diffraction that yields radiograms with up to 200 frames per second and 5.6 µm effective pixel size is presented. Tomograms and diffractograms are acquired together in 10 s. Two examples illustrate the attractiveness of combining these methods at the EDDI beamline for in situ studies.     

Development of new apertures for coherent X‐ray experiments

When one performs a coherent small‐angle X‐ray scattering experiment, the incident beam must be spatially filtered by slits on a length scale smaller than the transverse coherence length of the source which is typically around 10 µm. The Fraunhofer diffraction pattern of the slit is one of the important sources of background in these experiments. New slits which minimize this parasitic background have been designed and tested. The slit configuration apodizes the beam by the use of partially transmitting inclined slit jaws. A model is presented which predicts that the high wavevector tails of the diffraction pattern fall as the inverse fourth power of the wavevector instead of the inverse second power that is observed for standard slits. Using cleaved GaAs single‐crystal edges, Fraunhofer diffraction patterns from 3 and 5.5 keV X‐rays were measured, in agreement with the theoretical model proposed. A novel phase‐peak diffraction pattern associated with phase variations of the transmitted electric field was also observed. The model proposed adequately accounts for this phenomenon.     

X‐ray spectromicroscopy with the scanning transmission X‐ray microscope at BESSY II

Using the scanning transmission X‐ray microscope at BESSY II, colloidal structures from a Chernozem soil have been studied with a spatial resolution around 60 nm and a spectral resolution of 1700 at the K‐absorption edge of carbon. Elemental mapping has been used to determine the distribution of organic matter within the colloidal structures. Spectra have been extracted from image stacks to obtain information about the chemical state. For the analysis of the latter, principal component analysis and cluster analysis have been applied. It was possible, for example, to discriminate clay particles against organic components.