A monolithic Fresnel bimirror for hard X‐rays and its application for coherence measurements

Experiments using a simple X‐ray interferometer to measure the degree of spatial coherence of hard X‐rays are reported. A monolithic Fresnel bimirror is used at small incidence angles to investigate synchrotron radiation in the energy interval 5–50 keV with monochromatic and white beam. The experimental set‐up was equivalent to a Young's double‐slit experiment for hard X‐rays with slit dimensions in the micrometre range. From the high‐contrast interference pattern the degree of coherence was determined.     

High‐efficiency coherence‐preserving harmonic rejection with crystal optics

This work reports a harmonic‐rejection scheme based on the combination of Si(111) monochromator and Si(220) harmonic‐rejection crystal optics. This approach is of importance to a wide range of X‐ray applications in all three major branches of modern X‐ray science (scattering, spectroscopy, imaging) based at major facilities, and especially relevant to the capabilities offered by the new diffraction‐limited storage rings. It was demonstrated both theoretically and experimentally that, when used with a synchrotron undulator source over a broad range of X‐ray energies of interest, the harmonic‐rejection crystals transmit the incident harmonic X‐rays on the order of 10^?6. Considering the flux ratio of fundamental and harmonic X‐rays in the incident beam, this scheme achieves a total flux ratio of harmonic radiation to fundamental radiation on the order of 10^?10. The spatial coherence of the undulator beam is preserved in the transmitted fundamental radiation while the harmonic radiation is suppressed, making this scheme suitable not only for current third‐generation synchrotron sources but also for the new diffraction‐limited storage rings where coherence preservation is an even higher priority. Compared with conventional harmonic‐rejection mirrors, where coherence is poorly preserved and harmonic rejection is less effective, this scheme has the added advantage of lower cost and footprint. This approach has been successfully utilized at the ultra‐small‐angle X‐ray scattering instrument at the Advanced Photon Source for scattering, imaging and coherent X‐ray photon correlation spectroscopy experiments. With minor modification, the harmonic rejection can be improved by a further five orders of magnitude, enabling even more performance capabilities.     

Versatility of a hard X‐ray Kirkpatrick–Baez focus characterized by ptychography

In the past decade Kirkpatrick–Baez (KB) mirrors have been established as powerful focusing systems in hard X‐ray microscopy applications. Here a ptychographic characterization of the KB focus in the dedicated nano‐imaging setup GINIX (Göttingen Instrument for Nano‐Imaging with X‐rays) at the P10 coherence beamline of the PETRA III synchrotron at HASLYLAB/DESY, Germany, is reported. More specifically, it is shown how aberrations in the KB beam, caused by imperfections in the height profile of the focusing mirrors, can be eliminated using a pinhole as a spatial filter near the focal plane. A combination of different pinhole sizes and illumination conditions of the KB setup makes the prepared optical setup well suited not only for high‐resolution ptychographic coherent X‐ray diffractive imaging but also for moderate‐resolution/large‐field‐of‐view propagation imaging in the divergent KB beam.     

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.     

同步辐射X光束空间相干性的物理分析

从量子力学的不确定原理出发，推导出光源相干性和光束相干性的关系，并应用所得的结论南步辐射Ｘ光束的空间相干性进行物理分析，讨论了Ｘ光相干我学实验对Ｘ光束的一些要求。     

Characterization of X‐ray gas attenuator plasmas by optical emission and tunable laser absorption spectroscopies

X‐ray gas attenuators are used in high‐energy synchrotron beamlines as high‐pass filters to reduce the incident power on downstream optical elements. The absorption of the X‐ray beam ionizes and heats up the gas, creating plasma around the beam path and hence temperature and density gradients between the center and the walls of the attenuator vessel. The objective of this work is to demonstrate experimentally the generation of plasma by the X‐ray beam and to investigate its spatial distribution by measuring some of its parameters, simultaneously with the X‐ray power absorption. The gases used in this study were argon and krypton between 13 and 530 mbar. The distribution of the 2p excited states of both gases was measured using optical emission spectroscopy, and the density of argon metastable atoms in the 1s₅ state was deduced using tunable laser absorption spectroscopy. The amount of power absorbed was measured using calorimetry and X‐ray transmission. The results showed a plasma confined around the X‐ray beam path, its size determined mainly by the spatial dimensions of the X‐ray beam and not by the absorbed power or the gas pressure. In addition, the X‐ray absorption showed a hot central region at a temperature varying between 400 and 1100 K, depending on the incident beam power and on the gas used. The results show that the plasma generated by the X‐ray beam plays an essential role in the X‐ray absorption. Therefore, plasma processes must be taken into account in the design and modeling of gas attenuators.     

Diamond X‐ray beam‐position monitoring using signal readout at the synchrotron radiofrequency

Single‐crystal diamond is a material with great potential for the fabrication of X‐ray photon beam‐position monitors with submicrometre spatial resolution. Low X‐ray absorption combined with radiation hardness and excellent thermal‐mechanical properties make possible beam‐transmissive diamond devices for monitoring synchrotron and free‐electron laser X‐ray beams. Tests were made using a white bending‐magnet synchrotron X‐ray beam at DESY to investigate the performance of a position‐sensitive diamond device using radiofrequency readout electronics. The device uniformity and position response were measured in a 25 µm collimated X‐ray beam with an I‐Tech Libera `Brilliance' system. This readout system was designed for position measurement and feedback control of the electron beam in the synchrotron storage ring, but, as shown here, it can also be used for accurate position readout of a quadrant‐electrode single‐crystal diamond sensor. The centre‐of‐gravity position of the F4 X‐ray beam at the DORIS III synchrotron was measured with the diamond signal output digitally sampled at a rate of 130 Msample s^?1 by the Brilliance system. Narrow‐band filtering and digital averaging of the position signals resulted in a measured position noise below 50 nm (r.m.s.) for a 10 Hz bandwidth.     

X‐ray position‐sensitive duo‐lateral diamond detectors at SOLEIL

The performance of a diamond X‐ray beam position monitor is reported. This detector consists of an ionization solid‐state chamber based on a thin single‐crystal chemical‐vapour‐deposition diamond with position‐sensitive resistive electrodes in a duo‐lateral configuration. The detector's linearity, homogeneity and responsivity were studied on beamlines at Synchrotron SOLEIL with various beam sizes, intensities and energies. These measurements demonstrate the large and homogeneous (absorption variation of less than 0.7% over 500 µm × 500 µm) active area of the detector, with linear responses independent of the X‐ray beam spatial distribution. Due to the excellent charge collection efficiency (approaching 100%) and intensity sensitivity (0.05%), the detector allows monitoring of the incident beam flux precisely. In addition, the in‐beam position resolution was compared with a theoretical analysis providing an estimation of the detector's beam position resolution capability depending on the experimental conditions (X‐ray flux, energy and readout acquisition time).     

The coherent radiation fraction of low‐emittance synchrotrons

In this work the coherence properties of the synchrotron radiation beam from an X‐ray undulator in a fourth‐generation storage ring are analyzed. A slightly focused X‐ray beam is simulated using a wavefront propagation through a non‐redundant array of slits and the mutual coherence function is directly obtained and compared with the Gaussian–Schell approximation. The numerical wave propagation and the approximate analytical approaches are shown to agree qualitatively, and it is also shown that, when the coherent fraction is selected by a finite aperture before the focusing element, even achromatic focusing systems like total reflection mirrors become slightly chromatic. This effect is also well accounted for in the Gaussian–Schell model. The wavefront propagation simulation through the non‐redundant array was repeated with an imperfect mirror demonstrating that, although the wavefront is distorted, its coherent length is practically unchanged.     

Rapid in situ X‐ray position stabilization via extremum seeking feedback

X‐ray beam stability is crucial for acquiring high‐quality data at synchrotron beamline facilities. When the X‐ray beam and defining apertures are of similar dimensions, small misalignments driven by position instabilities give rise to large intensity fluctuations. This problem is solved using extremum seeking feedback control (ESFC) for in situ vertical beam position stabilization. In this setup, the intensity spatial gradient required for ESFC is determined by phase comparison of intensity oscillations downstream from the sample with pre‐existing vertical beam oscillations. This approach compensates for vertical position drift from all sources with position recovery times <6 s and intensity stability through a 5 µm aperture measured at 1.5% FWHM over a period of 8 hours.     

A sub‐micrometer resolution hard X‐ray microprobe system of BL8C at Pohang Light Source

A microprobe system has been installed on the nanoprobe/XAFS beamline (BL8C) at PLS‐II, South Korea. Owing to the reproducible switch of the gap of the in‐vacuum undulator (IVU), the intense and brilliant hard X‐ray beam of an IVU can be used in X‐ray fluorescence (XRF) and X‐ray absorption fine‐structure (XAFS) experiments. For high‐spatial‐resolution microprobe experiments a Kirkpatrick–Baez mirror system has been used to focus the millimeter‐sized X‐ray beam to a micrometer‐sized beam. The performance of this system was examined by a combination of micro‐XRF imaging and micro‐XAFS of a beetle wing. These results indicate that the microprobe system of the BL8C can be used to obtain the distributions of trace elements and chemical and structural information of complex materials.     

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 beam‐position feedback system with easy‐to‐use beam‐position monitor

X‐ray beam‐position stability is indispensable in cutting‐edge experiments using synchrotron radiation. Here, for the first time, a beam‐position feedback system is presented that utilizes an easy‐to‐use X‐ray beam‐position monitor incorporating a diamond‐fluorescence screen. The acceptable range of the monitor is above 500 µm and the feedback system maintains the beam position within 3 µm. In addition to being inexpensive, the system has two key advantages: it works without a scale factor for position calibration, and it has no dependence on X‐ray energy, X‐ray intensity, beam size or beam shape.     

Coherence and wavefront characterization of Si‐111 monochromators using double‐grating interferometry

A study of the coherence and wavefront properties of a pseudo‐channel‐cut monochromator in comparison with a double‐crystal monochromator is presented. Using a double‐grating interferometer designed for the hard X‐ray regime, the complex coherence factor was measured and the wavefront distortions at the sample position were analyzed. A transverse coherence length was found in the vertical direction that was a factor of two larger for the channel‐cut monochromator owing to its higher mechanical stability. The wavefront distortions after different optical elements in the beam, such as monochromators and mirrors, were also quantified. This work is particularly relevant for coherent diffraction imaging experiments with synchrotron sources.     

Application of kinoform lens for X‐ray reflectivity analysis

In this paper the first practical application of kinoform lenses for the X‐ray reflectivity characterization of thin layered materials is demonstrated. The focused X‐ray beam generated from a kinoform lens, a line of nominal size ～50 µm × 2 µm, provides a unique possibility to measure the X‐ray reflectivities of thin layered materials in sample scanning mode. Moreover, the small footprint of the X‐ray beam, generated on the sample surface at grazing incidence angles, enables one to measure the absolute X‐ray reflectivities. This approach has been tested by analyzing a few thin multilayer structures. The advantages achieved over the conventional X‐ray reflectivity technique are discussed and demonstrated by measurements.     

X‐Tream quality assurance in synchrotron X‐ray microbeam radiation therapy

Microbeam radiation therapy (MRT) is a novel irradiation technique for brain tumours treatment currently under development at the European Synchrotron Radiation Facility in Grenoble, France. The technique is based on the spatial fractionation of a highly brilliant synchrotron X‐ray beam into an array of microbeams using a multi‐slit collimator (MSC). After promising pre‐clinical results, veterinary trials have recently commenced requiring the need for dedicated quality assurance (QA) procedures. The quality of MRT treatment demands reproducible and precise spatial fractionation of the incoming synchrotron beam. The intensity profile of the microbeams must also be quickly and quantitatively characterized prior to each treatment for comparison with that used for input to the dose‐planning calculations. The Centre for Medical Radiation Physics (University of Wollongong, Australia) has developed an X‐ray treatment monitoring system (X‐Tream) which incorporates a high‐spatial‐resolution silicon strip detector (SSD) specifically designed for MRT. In‐air measurements of the horizontal profile of the intrinsic microbeam X‐ray field in order to determine the relative intensity of each microbeam are presented, and the alignment of the MSC is also assessed. The results show that the SSD is able to resolve individual microbeams which therefore provides invaluable QA of the horizontal field size and microbeam number and shape. They also demonstrate that the SSD used in the X‐Tream system is very sensitive to any small misalignment of the MSC. In order to allow as rapid QA as possible, a fast alignment procedure of the SSD based on X‐ray imaging with a low‐intensity low‐energy beam has been developed and is presented in this publication.     

A sagittally focusing double‐multilayer monochromator for ultrafast X‐ray imaging applications

The development of a sagittally focusing double‐multilayer monochromator is reported, which produces a spatially extended wide‐bandpass X‐ray beam from an intense synchrotron bending‐magnet source at the Advanced Photon Source, for ultrafast X‐ray radiography and tomography applications. This monochromator consists of two W/B₄C multilayers with a 25 Å period coated on Si single‐crystal substrates. The second multilayer is mounted on a sagittally focusing bender, which can dynamically change the bending radius of the multilayer in order to condense and focus the beam to various points along the beamline. With this new apparatus, it becomes possible to adjust the X‐ray beam size to best match the area detector size and the object size to facilitate more efficient data collection using ultrafast X‐ray radiography and tomography.     

X‐ray spectra by means of Monte Carlo simulations for imaging applications

X‐ray tubes have a broad range of applications worldwide, including several techniques for atomic physics, like X‐ray fluorescence, as well as for medical imaging, like computed tomography. The performances of X‐ray imaging detectors have shown to be significantly sensitive to the incident beam spectrum. Therefore, an accurate knowledge of the X‐ray beam becomes necessary for the emission source characterization and the whole imaging process comprehension. Direct measurements and suitable Monte Carlo simulations may be used to establish the X‐ray spectra. Dedicated Monte Carlo simulation routines, based on the PENELOPE code, have been developed to determine the Bremsstrahlung X‐ray spectra generated by conventional X‐ray tubes. The simulated spectra have been validated by comparison with the corresponding experimental data showing an overall good agreement. The incorporation of a suitably designed virtual grid allowed to assess the angular distribution of Bremsstrahlung yield, showing a remarkable anisotropy. In addition, a dedicated program has been developed for virtual imaging, which enables to perform suitable X‐ray absorption contrast images. Also, the developed program includes a user‐friendly graphic interface to allow the upload of required input parameters, which include setup arrangement, beam characteristics, sample properties and image simulation parameters (spatial resolution, tracks per run, etc.). The software includes dedicated subroutines which handle the physical process from X‐ray generation up to detector signal acquisition. The aim of the developed program is to perform virtual imaging by means of absorption contrast and using conventional X‐ray sources, which may be a useful tool for the study the X‐ray imaging techniques in several research fields as well as for educational purposes. The performed comparisons with experimental data have shown good agreement. The obtained results for X‐ray imaging may constitute useful information for the comprehension and improvement of X‐ray image quality, like absorption contrast optimization, detail visualization, definition and detectability. Copyright © 2010 John Wiley & Sons, Ltd.     

Multimodal hard X‐ray imaging of a mammography phantom at a compact synchrotron light source

The Compact Light Source is a miniature synchrotron producing X‐rays at the interaction point of a counter‐propagating laser pulse and electron bunch through the process of inverse Compton scattering. The small transverse size of the luminous region yields a highly coherent beam with an angular divergence of a few milliradians. The intrinsic monochromaticity and coherence of the produced X‐rays can be exploited in high‐sensitivity differential phase‐contrast imaging with a grating‐based interferometer. Here, the first multimodal X‐ray imaging experiments at the Compact Light Source at a clinically compatible X‐ray energy of 21 keV are reported. Dose‐compatible measurements of a mammography phantom clearly demonstrate an increase in contrast attainable through differential phase and dark‐field imaging over conventional attenuation‐based projections.     

Dose,exposure time and resolution in serial X‐ray crystallography

The resolution of X‐ray diffraction microscopy is limited by the maximum dose that can be delivered prior to sample damage. In the proposed serial crystallography method, the damage problem is addressed by distributing the total dose over many identical hydrated macromolecules running continuously in a single‐file train across a continuous X‐ray beam, and resolution is then limited only by the available molecular and X‐ray fluxes and molecular alignment. Orientation of the diffracting molecules is achieved by laser alignment. The incident X‐ray fluence (energy/area) is evaluated that is required to obtain a given resolution from (i) an analytical model, giving the count rate at the maximum scattering angle for a model protein, (ii) explicit simulation of diffraction patterns for a GroEL–GroES protein complex, and (iii) the spatial frequency cut‐off of the transfer function following iterative solution of the phase problem, and reconstruction of an electron density map in the projection approximation. These calculations include counting shot noise and multiple starts of the phasing algorithm. The results indicate counting time and the number of proteins needed within the beam at any instant for a given resolution and X‐ray flux. An inverse fourth‐power dependence of exposure time on resolution is confirmed, with important implications for all coherent X‐ray imaging. It is found that multiple single‐file protein beams will be needed for sub‐nanometer resolution on current third‐generation synchrotrons, but not on fourth‐generation designs, where reconstruction of secondary protein structure at a resolution of 7 Å should be possible with relatively short exposures.