Kinoform optics applied to X‐ray photon correlation spectroscopy

Moderate‐demagnification higher‐order silicon kinoform focusing lenses have been fabricated to facilitate small‐angle X‐ray photon correlation spectroscopy (XPCS) experiments. The geometric properties of such lenses, their focusing performance and their applicability for XPCS measurements are described. It is concluded that one‐dimensional vertical X‐ray focusing via silicon kinoform lenses significantly increases the usable coherent flux from third‐generation storage‐ring light sources for small‐angle XPCS experiments.     

A Geiger‐mode avalanche photodiode array for X‐ray photon correlation spectroscopy

X‐ray photon correlation spectroscopy (XPCS) provides an opportunity to study the dynamics of systems by measuring the temporal fluctuations in a far‐field diffraction pattern. A two‐dimensional detector system has been developed to investigate fluctuations in the frequency range of several Hz to kHz. The X‐ray detector system consists of a thin 100 µm scintillation crystal coupled to a Geiger‐mode avalanche photodiode array. In this article the elements of the system are detailed and the detector for XPCS measurements is demonstrated.     

Fast two‐dimensional detection for X‐ray photon correlation spectroscopy using the PILATUS detector

The first X‐ray photon correlation spectroscopy experiments using the fast single‐photon‐counting detector PILATUS (Paul Scherrer Institut, Switzerland) have been performed. The short readout time of this detector permits access to intensity autocorrelation functions describing dynamics in the millisecond range that are difficult to access with charge‐coupled device detectors with typical readout times of several seconds. Showing no readout noise the PILATUS detector enables measurements of samples that either display fast dynamics or possess only low scattering power with an unprecedented signal‐to‐noise ratio.     

X‐ray photon correlation spectroscopy under flow

X‐ray photon correlation spectroscopy was used to probe the diffusive dynamics of colloidal particles in a shear flow. Combining X‐ray techniques with microfluidics is an experimental strategy that reduces the risk of X‐ray‐induced beam damage and also allows time‐resolved studies of processes taking place in flow cells. The experimental results and theoretical predictions presented here show that in the low shear limit for a `transverse flow' scattering geometry (scattering wavevector q perpendicular to the direction of flow) the measured relaxation times are independent of the flow rate and determined only by the diffusive motion of the particles. This is not generally valid and, in particular, for a `longitudinal flow' ( q ∥ flow) scattering geometry the relaxation times are strongly affected by the flow‐induced motion of the particles. The results here show that the Brownian diffusion of colloidal particles can be measured in a flowing sample and that, up to flux limitations, the experimental conditions under which this is possible are easier to achieve at higher values of q.     

Combined measurement of X‐ray photon correlation spectroscopy and diffracted X‐ray tracking using pink beam X‐rays

Combined X‐ray photon correlation spectroscopy (XPCS) and diffracted X‐ray tracking (DXT) measurements of carbon‐black nanocrystals embedded in styrene–butadiene rubber were performed. From the intensity fluctuation of speckle patterns in a small‐angle scattering region (XPCS), dynamical information relating to the translational motion can be obtained, and the rotational motion is observed through the changes in the positions of DXT diffraction spots. Graphitized carbon‐black nanocrystals in unvulcanized styrene–butadiene rubber showed an apparent discrepancy between their translational and rotational motions; this result seems to support a stress‐relaxation model for the origin of super‐diffusive particle motion that is widely observed in nanocolloidal systems. Combined measurements using these two techniques will give new insights into nanoscopic dynamics, and will be useful as a microrheology technique.     

X‐ray photon correlation spectroscopy in systems without long‐range order: existence of an intermediate‐field regime

Successful X‐ray photon correlation spectroscopy studies often require that signals be optimized while minimizing power density in the sample to decrease radiation damage and, at free‐electron laser sources, thermal impact. This suggests exploration of scattering outside the Fraunhofer far‐field diffraction limit d²/λR, where d is the incident beam size, λ is the photon wavelength and R is the sample‐to‐detector distance. Here it is shown that, in an intermediate regime d²/λ > Rdξ/λ, where ξ is the structural correlation length in the material, the ensemble averages of the scattered intensity and of the structure factor are equal. Similarly, in the regime d²/λ > Rdξ(τ)/λ, where ξ(τ) is a time‐dependent dynamics length scale of interest, the ensemble‐averaged correlation functions g₁(τ) and g₂(τ) of the scattered electric field are also equal to their values in the far‐field limit. This broadens the parameter space for X‐ray photon correlation spectroscopy experiments, but detectors with smaller pixel size and variable focusing are required to more fully exploit the potential for such studies.     

X‐ray analog pixel array detector for single synchrotron bunch time‐resolved imaging

Dynamic X‐ray studies can reach temporal resolutions limited by only the X‐ray pulse duration if the detector is fast enough to segregate synchrotron pulses. An analog integrating pixel array detector with in‐pixel storage and temporal resolution of around 150 ns, sufficient to isolate pulses, is presented. Analog integration minimizes count‐rate limitations and in‐pixel storage captures successive pulses. Fundamental tests of noise and linearity as well as high‐speed laser measurements are shown. The detector resolved individual bunch trains at the Cornell High Energy Synchrotron Source at levels of up to 3.7 × 10³ X‐rays per pixel per train. When applied to turn‐by‐turn X‐ray beam characterization, single‐shot intensity measurements were made with a repeatability of 0.4% and horizontal oscillations of the positron cloud were detected.     

X‐ray speckle visibility spectroscopy in the single‐photon limit

The technique of speckle visibility spectroscopy has been employed for the measurement of dynamics using coherent X‐ray scattering. It is shown that the X‐ray contrast within a single exposure can be related to the relaxation time of the intermediate scattering function, and this methodology is applied to the diffusion of 72 nm‐radius latex spheres in glycerol. Data were collected with exposure times as short as 2 ms by employing a resonant shutter. The weak scattering present for short exposures necessitated an analysis formalism based on the spatial correlation function of individual photon charge droplets on an area detector, rather than the usual methods employed for intensity correlations. It is demonstrated that this method gives good agreement between theory and experiment and thus holds promise for extending area‐detector‐based coherent scattering methods to the study of faster dynamics than previously obtainable.     

Application of a pnCCD in X‐ray diffraction: a three‐dimensional X‐ray detector

The first application of a pnCCD detector for X‐ray scattering experiments using white synchrotron radiation at BESSY II is presented. A Cd arachidate multilayer was investigated in reflection geometry within the energy range 7 keV < E < 35 keV. At fixed angle of incidence the two‐dimensional diffraction pattern containing several multilayer Bragg peaks and respective diffuse‐resonant Bragg sheets were observed. Since every pixel of the detector is able to determine the energy of every incoming photon with a resolution ΔE/E? 10^?2, a three‐dimensional dataset is finally obtained. In order to achieve this energy resolution the detector was operated in the so‐called single‐photon‐counting mode. A full dataset was evaluated taking into account all photons recorded within 10⁵ detector frames at a readout rate of 200 Hz. By representing the data in reciprocal‐space coordinates, it becomes obvious that this experiment with the pnCCD detector provides the same information as that obtained by combining a large number of monochromatic scattering experiments using conventional area detectors.     

A fast energy‐dispersive multi‐element detector for X‐ray absorption spectroscopy

In this paper results are presented from fluorescence‐yield X‐ray absorption fine‐structure spectroscopy measurements with a new seven‐cell silicon drift detector (SDD) module. The complete module, including an integrated circuit for the detector readout, was developed and realised at DESY utilizing a monolithic seven‐cell SDD. The new detector module is optimized for applications like XAFS which require an energy resolution of ～250–300 eV (FWHM Mn Kα) at high count rates. Measurements during the commissioning phase proved the excellent performance for this type of application.     

Photon statistics and speckle visibility spectroscopy with partially coherent X‐rays

A new approach is proposed for measuring structural dynamics in materials from multi‐speckle scattering patterns obtained with partially coherent X‐rays. Coherent X‐ray scattering is already widely used at high‐brightness synchrotron lightsources to measure dynamics using X‐ray photon correlation spectroscopy, but in many situations this experimental approach based on recording long series of images (i.e. movies) is either not adequate or not practical. Following the development of visible‐light speckle visibility spectroscopy, the dynamic information is obtained instead by analyzing the photon statistics and calculating the speckle contrast in single scattering patterns. This quantity, also referred to as the speckle visibility, is determined by the properties of the partially coherent beam and other experimental parameters, as well as the internal motions in the sample (dynamics). As a case study, Brownian dynamics in a low‐density colloidal suspension is measured and an excellent agreement is found between correlation functions measured by X‐ray photon correlation spectroscopy and the decay in speckle visibility with integration time obtained from the analysis presented here.     

X‐ray irradiation induces local rearrangement of silica particles in swollen rubber

X‐ray photon correlation spectroscopy (XPCS) of swollen rubber containing spherical silica nanoparticles is reported. It is shown that irradiation by intense X‐rays leads to the breakdown of cross‐links, thereby inducing the local rearrangement of silica nanoparticles. This rearrangement process depends on the cross‐link density and is characterized by a compressed exponential relaxation with aging behaviour, which resembles a common feature of complex fluids observed with XPCS.     

Soft X‐ray absorption spectroscopy and resonant inelastic X‐ray scattering spectroscopy below 100 eV: probing first‐row transition‐metal M‐edges in chemical complexes

X‐ray absorption and scattering spectroscopies involving the 3d transition‐metal K‐ and L‐edges have a long history in studying inorganic and bioinorganic molecules. However, there have been very few studies using the M‐edges, which are below 100 eV. Synchrotron‐based X‐ray sources can have higher energy resolution at M‐edges. M‐edge X‐ray absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS) could therefore provide complementary information to K‐ and L‐edge spectroscopies. In this study, M_2,3‐edge XAS on several Co, Ni and Cu complexes are measured and their spectral information, such as chemical shifts and covalency effects, are analyzed and discussed. In addition, M_2,3‐edge RIXS on NiO, NiF₂ and two other covalent complexes have been performed and different d–d transition patterns have been observed. Although still preliminary, this work on 3d metal complexes demonstrates the potential to use M‐edge XAS and RIXS on more complicated 3d metal complexes in the future. The potential for using high‐sensitivity and high‐resolution superconducting tunnel junction X‐ray detectors below 100 eV is also illustrated and discussed.     

Coherent X‐ray scattering beamline at port 9C of Pohang Light Source II

The coherent X‐ray scattering beamline at the 9C port of the upgraded Pohang Light Source (PLS‐II) at Pohang Accelerator Laboratory in Korea is introduced. This beamline provides X‐rays of 5–20 keV, and targets coherent X‐ray experiments such as coherent diffraction imaging and X‐ray photon correlation spectroscopy. The main parameters of the beamline are summarized, and some preliminary experimental results are described.     

Picosecond time‐resolved laser pump/X‐ray probe experiments using a gated single‐photon‐counting area detector

The recent developments in X‐ray detectors have opened new possibilities in the area of time‐resolved pump/probe X‐ray experiments; this article presents the novel use of a PILATUS detector to achieve X‐ray pulse duration limited time‐resolution at the Advanced Photon Source (APS), USA. The capability of the gated PILATUS detector to selectively detect the signal from a given X‐ray pulse in 24 bunch mode at the APS storage ring is demonstrated. A test experiment performed on polycrystalline organic thin films of α‐perylene illustrates the possibility of reaching an X‐ray pulse duration limited time‐resolution of 60 ps using the gated PILATUS detector. This is the first demonstration of X‐ray pulse duration limited data recorded using an area detector without the use of a mechanical chopper array at the beamline.     

A multi‐MHz single‐shot data acquisition scheme with high dynamic range: pump–probe X‐ray experiments at synchrotrons

The technical implementation of a multi‐MHz data acquisition scheme for laser–X‐ray pump–probe experiments with pulse limited temporal resolution (100 ps) is presented. Such techniques are very attractive to benefit from the high‐repetition rates of X‐ray pulses delivered from advanced synchrotron radiation sources. Exploiting a synchronized 3.9 MHz laser excitation source, experiments in 60‐bunch mode (7.8 MHz) at beamline P01 of the PETRA III storage ring are performed. Hereby molecular systems in liquid solutions are excited by the pulsed laser source and the total X‐ray fluorescence yield (TFY) from the sample is recorded using silicon avalanche photodiode detectors (APDs). The subsequent digitizer card samples the APD signal traces in 0.5 ns steps with 12‐bit resolution. These traces are then processed to deliver an integrated value for each recorded single X‐ray pulse intensity and sorted into bins according to whether the laser excited the sample or not. For each subgroup the recorded single‐shot values are averaged over ～10⁷ pulses to deliver a mean TFY value with its standard error for each data point, e.g. at a given X‐ray probe energy. The sensitivity reaches down to the shot‐noise limit, and signal‐to‐noise ratios approaching 1000 are achievable in only a few seconds collection time per data point. The dynamic range covers 100 photons pulse^?1 and is only technically limited by the utilized APD.     

Development of picosecond time‐resolved X‐ray absorption spectroscopy by high‐repetition‐rate laser pump/X‐ray probe at Beijing Synchrotron Radiation Facility

A new setup and commissioning of transient X‐ray absorption spectroscopy are described, based on the high‐repetition‐rate laser pump/X‐ray probe method, at the 1W2B wiggler beamline at the Beijing Synchrotron Radiation Facility. A high‐repetition‐rate and high‐power laser is incorporated into the setup with in‐house‐built avalanche photodiodes as detectors. A simple acquisition scheme was applied to obtain laser‐on and laser‐off signals simultaneously. The capability of picosecond transient X‐ray absorption spectroscopy measurement was demonstrated for a photo‐induced spin‐crossover iron complex in 6 mM solution with 155 kHz repetition rate.     

High‐resolution spectroscopy on an X‐ray absorption beamline

A bent‐crystal spectrometer based on the Rowland circle geometry has been installed and tested on the BM30b/FAME beamline at the European Synchrotron Radiation Facility to improve its performances. The energy resolution of the spectrometer allows different kinds of measurements to be performed, including X‐ray absorption spectroscopy, resonant inelastic X‐ray scattering and X‐ray Raman scattering experiments. The simplicity of the experimental device makes it easily implemented on a classical X‐ray absorption beamline. This improvement in the fluorescence detection is of particular importance when the probed element is embedded in a complex and/or heavy matrix, for example in environmental sciences.     

Small‐angle solution scattering using the mixed‐mode pixel array detector

Solution small‐angle X‐ray scattering (SAXS) measurements were obtained using a 128 × 128 pixel X‐ray mixed‐mode pixel array detector (MMPAD) with an 860 µs readout time. The MMPAD offers advantages for SAXS experiments: a pixel full‐well of >2 × 10⁷ 10 keV X‐rays, a maximum flux rate of 10⁸ X‐rays pixel^?1 s^?1, and a sub‐pixel point‐spread function. Data from the MMPAD were quantitatively compared with data from a charge‐coupled device (CCD) fiber‐optically coupled to a phosphor screen. MMPAD solution SAXS data from lysozyme solutions were of equal or better quality than data captured by the CCD. The read‐noise (normalized by pixel area) of the MMPAD was less than that of the CCD by an average factor of 3.0. Short sample‐to‐detector distances were required owing to the small MMPAD area (19.2 mm × 19.2 mm), and were revealed to be advantageous with respect to detector read‐noise. As predicted by the Shannon sampling theory and confirmed by the acquisition of lysozyme solution SAXS curves, the MMPAD at short distances is capable of sufficiently sampling a solution SAXS curve for protein shape analysis. The readout speed of the MMPAD was demonstrated by continuously monitoring lysozyme sample evolution as radiation damage accumulated. These experiments prove that a small suitably configured MMPAD is appropriate for time‐resolved solution scattering measurements.     

Pixelated transmission‐mode diamond X‐ray detector

Fabrication and testing of a prototype transmission‐mode pixelated diamond X‐ray detector (pitch size 60–100 µm), designed to simultaneously measure the flux, position and morphology of an X‐ray beam in real time, are described. The pixel density is achieved by lithographically patterning vertical stripes on the front and horizontal stripes on the back of an electronic‐grade chemical vapor deposition single‐crystal diamond. The bias is rotated through the back horizontal stripes and the current is read out on the front vertical stripes at a rate of ～1 kHz, which leads to an image sampling rate of ～30 Hz. This novel signal readout scheme was tested at beamline X28C at the National Synchrotron Light Source (white beam, 5–15 keV) and at beamline G3 at the Cornell High Energy Synchrotron Source (monochromatic beam, 11.3 keV) with incident beam flux ranges from 1.8 × 10^?2 to 90 W mm^?2. Test results show that the novel detector provides precise beam position (positional noise within 1%) and morphology information (error within 2%), with an additional software‐controlled single channel mode providing accurate flux measurement (fluctuation within 1%).