Counting‐loss correction for X‐ray spectroscopy using unit impulse pulse shaping

High‐precision measurement of X‐ray spectra is affected by the statistical fluctuation of the X‐ray beam under low‐counting‐rate conditions. It is also limited by counting loss resulting from the dead‐time of the system and pile‐up pulse effects, especially in a high‐counting‐rate environment. In this paper a detection system based on a FAST‐SDD detector and a new kind of unit impulse pulse‐shaping method is presented, for counting‐loss correction in X‐ray spectroscopy. The unit impulse pulse‐shaping method is evolved by inverse deviation of the pulse from a reset‐type preamplifier and a C‐R shaper. It is applied to obtain the true incoming rate of the system based on a general fast–slow channel processing model. The pulses in the fast channel are shaped to unit impulse pulse shape which possesses small width and no undershoot. The counting rate in the fast channel is corrected by evaluating the dead‐time of the fast channel before it is used to correct the counting loss in the slow channel.     

Energy‐dependent dead‐time correction in digital pulse processors applied to silicon drift detector's X‐ray spectra

Dead‐time effects in X‐ray spectra taken with a digital pulse processor and a silicon drift detector were investigated when the number of events at the low‐energy end of the spectrum was more than half of the total, at counting rates up to 56 kHz. It was found that dead‐time losses in the spectra are energy dependent and an analytical correction for this effect, which takes into account pulse pile‐up, is proposed. This and the usual models have been applied to experimental measurements, evaluating the dead‐time fraction either from the calculations or using the value given by the detector acquisition system. The energy‐dependent dead‐time model proposed fits accurately the experimental energy spectra in the range of counting rates explored in this work. A selection chart of the simplest mathematical model able to correct the pulse‐height distribution according to counting rate and energy spectrum characteristics is included.     

Si PIN X‐ray photon counter

A Si PIN detector for visible light detection, instead of a Geiger‐Müller tube, is applied to X‐ray photon counting. We counted radiation from a checking source of a Geiger‐Müller counter with a Si PIN counter and with a Geiger‐Müller counter. White X‐ray of energy up to 20 keV emitted from a pyroelectric X‐ray emitter was also counted, and the Si PIN X‐ray counter showed a similar curve of count rate versus source distance in both measurements. Pulse counting was performed by spectroscopy circuits. An audio digitizer with computer software for signal processing was also used to simplify the photon counter. A plot of count rate versus time was obtained with this setup. With simple pulse counting circuits, Si PIN X‐ray counters have advantages such as compact structure, low cost and easy application. Copyright © 2011 John Wiley & Sons, Ltd.     

Measuring partial fluorescence yield using filtered detectors

Typically, X‐ray absorption near‐edge structure measurements aim to probe the linear attenuation coefficient. These measurements are often carried out using partial fluorescence yield techniques that rely on detectors having photon energy discrimination improving the sensitivity and the signal‐to‐background ratio of the measured spectra. However, measuring the partial fluorescence yield in the soft X‐ray regime with reasonable efficiency requires solid‐state detectors, which have limitations due to the inherent dead‐time while measuring. Alternatively, many of the available detectors that are not energy dispersive do not suffer from photon count rate limitations. A filter placed in front of one of these detectors will make the energy‐dependent efficiency non‐linear, thereby changing the responsivity of the detector. It is shown that using an array of filtered X‐ray detectors is a viable method for measuring soft X‐ray partial fluorescence yield spectra without dead‐time. The feasibility of this technique is further demonstrated using α‐Fe₂O₃ as an example and it is shown that this detector technology could vastly improve the photon collection efficiency at synchrotrons and that these detectors will allow experiments to be completed with a much lower photon flux reducing X‐ray‐induced damage.     

Performance of a four‐element Si drift detector for X‐ray absorption fine‐structure spectroscopy: resolution, maximum count rate,and dead‐time correction with incorporation into the ATHENA data analysis software

The performance of a four‐element Si drift detector for energy‐dispersive fluorescence‐yield X‐ray absorption fine‐structure measurements is reported, operating at the National Institute of Standards and Technology beamline X23A2 at the National Synchrotron Light Source. The detector can acquire X‐ray absorption fine‐structure spectra with a throughput exceeding 4 × 10⁵ counts per second per detector element (>1.6 × 10⁶ total counts per second summed over all four channels). At this count rate the resolution at 6 keV is approximately 220 eV, which adequately resolves the Mn Kα and Kβ fluorescence lines. Accurate dead‐time correction is demonstrated, and it has been incorporated into the ATHENA data analysis program. To maintain counting efficiency and high signal to background, it is suggested that the incoming count rate should not exceed ～70% of the maximum throughput.     

Sub‐microsecond‐resolved multi‐speckle X‐ray photon correlation spectroscopy with a pixel array detector

Small‐angle X‐ray photon correlation spectroscopy (XPCS) measurements spanning delay times from 826 ns to 52.8 s were performed using a photon‐counting pixel array detector with a dynamic range of 0–3 (2 bits). Fine resolution and a wide dynamic range of time scales was achieved by combining two modes of operation of the detector: (i) continuous mode, where data acquisition and data readout are performed in parallel with a frame acquisition time of 19.36 µs, and (ii) burst mode, where 12 frames are acquired with frame integration times of either 2.56 µs frame^?1 or 826 ns frame^?1 followed by 3.49 ms or 1.16 ms, respectively, for readout. The applicability of the detector for performing multi‐speckle XPCS was demonstrated by measuring the Brownian dynamics of 10 nm‐radius gold and 57 nm‐radius silica colloids in water at room temperature. In addition, the capability of the detector to faithfully record one‐ and two‐photon counts was examined by comparing the statistical distribution of photon counts with expected probabilities from the negative binomial distribution. It was found that in burst mode the ratio of 2 s to 1 s is markedly smaller than predicted and that this is attributable to pixel‐response dead‐time.     

Computer‐assisted area detector masking

Area detectors have become the predominant type of detector for the rapid acquisition of X‐ray diffraction, small‐angle scattering and total scattering. These detectors record the scattering for a large area, giving each shot good statistical significance to the resulting scattered intensity I(Q) pattern. However, many of these detectors have pixel level defects, which cause error in the resulting one‐dimensional patterns. In this work, new software to automatically find and mask these dead pixels and other defects is presented. This algorithm is benchmarked with both ideal simulated and experimental datasets.     

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.     

Energy resolution of the CdTe‐XPAD detector: calibration and potential for Laue diffraction measurements on protein crystals

The XPAD3S‐CdTe, a CdTe photon‐counting pixel array detector, has been used to measure the energy and the intensity of the white‐beam diffraction from a lysozyme crystal. A method was developed to calibrate the detector in terms of energy, allowing incident photon energy measurement to high resolution (approximately 140 eV), opening up new possibilities in energy‐resolved X‐ray diffraction. In order to demonstrate this, Laue diffraction experiments were performed on the bending‐magnet beamline METROLOGIE at Synchrotron SOLEIL. The X‐ray energy spectra of diffracted spots were deduced from the indexed Laue patterns collected with an imaging‐plate detector and then measured with both the XPAD3S‐CdTe and the XPAD3S‐Si, a silicon photon‐counting pixel array detector. The predicted and measured energy of selected diffraction spots are in good agreement, demonstrating the reliability of the calibration method. These results open up the way to direct unit‐cell parameter determination and the measurement of high‐quality Laue data even at low resolution. Based on the success of these measurements, potential applications in X‐ray diffraction opened up by this type of technology are discussed.     

Analysis of the halo background in femtosecond slicing experiments

The slicing facility FemtoSpeX at BESSY II offers unique opportunities to study photo‐induced dynamics on femtosecond time scales by means of X‐ray magnetic circular dichroism, resonant and non‐resonant X‐ray diffraction, and X‐ray absorption spectroscopy experiments in the soft X‐ray regime. Besides femtosecond X‐ray pulses, slicing sources inherently also produce a so‐called `halo' background with a different time structure, polarization and pointing. Here a detailed experimental characterization of the halo radiation is presented, and a method is demonstrated for its correct and unambiguous removal from femtosecond time‐resolved data using a special laser triggering scheme as well as analytical models. Examples are given for time‐resolved measurements with corresponding halo correction, and errors of the relevant physical quantities caused by either neglecting or by applying a simplified model to describe this background are estimated.     

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.     

Fast continuous energy scan with dynamic coupling of the monochromator and undulator at the DEIMOS beamline

In order to improve the efficiency of X‐ray absorption data recording, a fast scan method, the Turboscan, has been developed on the DEIMOS beamline at Synchrotron SOLEIL, consisting of a software‐synchronized continuous motion of the monochromator and undulator motors. This process suppresses the time loss when waiting for the motors to reach their target positions, as well as software dead‐time, while preserving excellent beam characteristics.     

The stopped‐drop method: a novel setup for containment‐free and time‐resolved measurements

A novel setup for containment‐free time‐resolved experiments at a free‐hanging drop is reported. Within a dead‐time of 100 ms a drop of mixed reactant solutions is formed and the time evolution of a reaction can be followed from thereon by various techniques. As an example, a small‐angle X‐ray scattering study on the formation mechanism of EDTA‐stabilized CdS both at a synchrotron and a laboratory X‐ray source is presented here. While the evolution can be followed with one drop only at a synchrotron source, a stroboscopic mode with many drops is preferable for the laboratory source.     

Performance of the micro‐PIC gaseous area detector in small‐angle X‐ray scattering experiments

The application of a two‐dimensional photon‐counting detector based on a micro‐pixel gas chamber (µ‐PIC) to high‐resolution small‐angle X‐ray scattering (SAXS), and its performance, are reported. The µ‐PIC is a micro‐pattern gaseous detector fabricated by printed circuit board technology. This article describes the performance of the µ‐PIC in SAXS experiments at SPring‐8. A dynamic range of >10⁵ was obtained for X‐ray scattering from a polystyrene sphere solution. A maximum counting rate of up to 5 MHz was observed with good linearity and without saturation. For a diffraction pattern of collagen, weak peaks were observed in the high‐angle region in one accumulation of photons.     

On‐line optical and X‐ray spectroscopies with crystallography: an integrated approach for determining metalloprotein structures in functionally well defined states

X‐ray‐induced redox changes can lead to incorrect assignments of the functional states of metals in metalloprotein crystals. The need for on‐line monitoring of the status of metal ions (and other chromophores) during protein crystallography experiments is of growing importance with the use of intense synchrotron X‐ray beams. Significant efforts are therefore being made worldwide to combine different spectroscopies in parallel with X‐ray crystallographic data collection. Here the implementation and utilization of optical and X‐ray absorption spectroscopies on the modern macromolecular crystallography (MX) beamline 10, at the SRS, Daresbury Laboratory, is described. This beamline is equipped with a dedicated monolithic energy‐dispersive X‐ray fluorescence detector, allowing X‐ray absorption spectroscopy (XAS) measurements to be made in situ on the same crystal used to record the diffraction data. In addition, an optical microspectrophotometer has been incorporated on the beamline, thus facilitating combined MX, XAS and optical spectroscopic measurements. By uniting these techniques it is also possible to monitor the status of optically active and optically silent metal centres present in a crystal at the same time. This unique capability has been applied to observe the results of crystallographic data collection on crystals of nitrite reductase from Alcaligenes xylosoxidans, which contains both type‐1 and type‐2 Cu centres. It is found that the type‐1 Cu centre photoreduces quickly, resulting in the loss of the 595 nm peak in the optical spectrum, while the type‐2 Cu centre remains in the oxidized state over a much longer time period, for which independent confirmation is provided by XAS data as this centre has an optical spectrum which is barely detectable using microspectrophotometry. This example clearly demonstrates the importance of using two on‐line methods, spectroscopy and XAS, for identifying well defined redox states of metalloproteins during crystallographic data collection.     

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.     

More power to X‐rays: New developments in X‐ray spectroscopy

Recent developments in X‐ray spectroscopy in the last decade are reviewed. A specific emphasis is placed on displaying the strong natural connection between X‐ray spectroscopy and materials science. Brief explanations of several X‐ray spectroscopic methods are given. X‐ray spectroscopic instruments such as table‐top X‐ray sources are discussed in detail, whereas those employing synchrotron and other sources are briefly addressed. The spectroscopic methods and results from materials investigations are reviewed according to their positions in a 3D parameter space of time, length, and energy. New experimental measurements on atoms, molecules, nanomaterials, and bulk materials that include insulators, semiconductors, metals and magnetic materials using both static and time‐resolved methods are reviewed.     

BioCARS: a synchrotron resource for time‐resolved X‐ray science

BioCARS, a NIH‐supported national user facility for macromolecular time‐resolved X‐ray crystallography at the Advanced Photon Source (APS), has recently completed commissioning of an upgraded undulator‐based beamline optimized for single‐shot laser‐pump X‐ray‐probe measurements with time resolution as short as 100 ps. The source consists of two in‐line undulators with periods of 23 and 27 mm that together provide high‐flux pink‐beam capability at 12 keV as well as first‐harmonic coverage from 6.8 to 19 keV. A high‐heat‐load chopper reduces the average power load on downstream components, thereby preserving the surface figure of a Kirkpatrick–Baez mirror system capable of focusing the X‐ray beam to a spot size of 90 µm horizontal by 20 µm vertical. A high‐speed chopper isolates single X‐ray pulses at 1 kHz in both hybrid and 24‐bunch modes of the APS storage ring. In hybrid mode each isolated X‐ray pulse delivers up to ～4 × 10¹⁰ photons to the sample, thereby achieving a time‐averaged flux approaching that of fourth‐generation X‐FEL sources. A new high‐power picosecond laser system delivers pulses tunable over the wavelength range 450–2000 nm. These pulses are synchronized to the storage‐ring RF clock with long‐term stability better than 10 ps RMS. Monochromatic experimental capability with Biosafety Level 3 certification has been retained.     

A high‐energy‐resolution resonant inelastic X‐ray scattering spectrometer at ID20 of the European Synchrotron Radiation Facility

An end‐station for resonant inelastic X‐ray scattering and (resonant) X‐ray emission spectroscopy at beamline ID20 of ESRF – The European Synchrotron is presented. The spectrometer hosts five crystal analysers in Rowland geometry for large solid angle collection and is mounted on a rotatable arm for scattering in both the horizontal and vertical planes. The spectrometer is optimized for high‐energy‐resolution applications, including partial fluorescence yield or high‐energy‐resolution fluorescence detected X‐ray absorption spectroscopy and the study of elementary electronic excitations in solids. In addition, it can be used for non‐resonant inelastic X‐ray scattering measurements of valence electron excitations.