Roelof van Silfhout

XAS and XES Techniques Shed Light on the Dark Side of Ziegler–Natta Catalysts: Active-Site Generation†

The local structure and the electronic properties of the active Ti sites in heterogeneous Ziegler–Natta catalysts, generated in situ by interaction of the precatalyst with different Al‐alkyl activators, were investigated by combining X‐ray absorption and valence‐to‐core X‐ray emission spectroscopy (XAS and vtc‐XES), coupled with UV/Vis, FTIR, and DFT theoretical calculations. Irrespective of the activator used, the active system was found to be a highly dispersed TiCl3‐like phase in which the Ti sites are surrounded, not only by bridged chlorine ligands (with the same bond length of bulk TiCl3), but also by terminal chlorine ligands, at a much shorter distance. These results set Ziegler–Natta catalysts in the category of complex nanomaterials. Despite the observation that the investigated catalysts polymerize ethylene, cutting‐edge XAS and XES techniques do not yet offer unequivocal proof for the presence of any alkyl chain attached to the Ti sites, as a consequence of the small fraction of the active sites.

High-resolution transparent x-ray beam location and imaging

We present a high-resolution in situ imaging and localization method of energetic particle beams. Recording of the scattered radiation from a thin featureless foil, placed in the path of the beam, and taken with a pinhole or coded aperture camera arrangement magnifies beam movements at the sensor. At the same time, a magnified image of the beam is available with an exceptional signal-to-noise ratio. We show measurement results of the level of precision that can be achieved and compare them to theoretical limits based on the signal-to-noise levels.

A transparent two-dimensional in situ beam-position and profile monitor for synchrotron X-ray beamlines

A compact, inexpensive and easy-to-construct two-dimensional in situ beam-position and profile monitor for synchrotron X-ray beamlines is presented. The device is based on the collection of spatially resolved scattered radiation from a polyimide foil. The X-ray beam passes through a foil placed in the path of the beam, which absorbs no more than 3% of the beam at 12 keV. The scattered radiation is collected at an angle of 90 degrees through a collimator located below the foil onto a CCD sensor. The device was tested on bending-magnet beamline BM26 at the ESRF synchrotron radiation source and has a positional sensitivity better than 10 microm with a large working range of 25 mm x 25 mm. Although the device is optimized for use in the range 10-12 keV, it can easily be modified for use with higher-energy beams by using a suitably chosen scattering foil.

Real-time photon beam localization methods using high-resolution imagers and parallel processing using a reconfigurable system

We report on a high-precision, real-time photon beam localization and characterization instrument. The device uses a 2-D (image) sensor coupled to a hardware image processing system. The system uses two different algorithms that both run in parallel on field programmable gate array logic using data from a single active pixel array sensor. The first algorithm mimics the ubiquitous quadrant photodiode design that features high precision but has limited range and requires calibration cycles. The second algorithm calculates the location of the center of gravity. We compare the merits of both methods by measuring the displacement of a light beam. It is shown that the center of gravity (or centroid) method offers the advantage of large dynamic range with excellent linearity. By the method of operating on identical image data captured from these experiments, we aim to investigate both algorithms and compare their performance.

In situ synchrotron x-ray photon beam characterization

We have investigated two in situ methods of measuring x-ray beam parameters such as integrated intensity, position, and intensity distribution. These virtually transparent methods both rely on the collection of scattered radiation from a thin amorphous foil. The scattered radiation is collected by an active pixel sensor placed below the foil, well out of the direction of the beam path. These methods measure a cross-sectional image of the beam as opposed to a profile or beam centroid position provided by existing in situ detection methods. We present the results of measurements taken at a third generation synchrotron radiation source and provide analytical methods of deriving beam profile, position, and absolute intensity.

In situ X-ray beam imaging using an off-axis magnifying coded aperture camera system

This paper presents an imaging model and a reconstruction algorithm for obtaining X-ray beam cross-sectional images from the data recorded by an X-ray beam monitor based on a coded aperture camera that collects radiation scattered from a thin foil placed in the X-ray beam at an oblique angle.

Overview of the tunable beamlines for protein crystallography at the EMBL Hamburg Outstation; an analysis of current and future usage and developments.

The EMBL Hamburg Outstation currently operates two tunable protein crystallography beamlines suitable for single and multiple anomalous diffraction (SAD/MAD) experiments. The first beamline, designated X31, is located on a bending magnet of the DORIS III storage ring whereas the second beamline, BW7A, is positioned at a multipole wiggler at the same storage ring. X31 is equipped with an energy stabilization device to ensure constant wavelength during longer data-collection periods. The in-house built crystallographic end-station is now equipped with a Mar345 imaging-plate scanner as a detector. The wiggler beamline BW7A features a novel sagitally focusing monochromator. The end-station used here has also been developed and built in-house. The beamline is currently operated with a Mar 165 CCD detector. In this paper the hardware and software developments of the last years will be summarized and the outlook for substantial upgrades will be given. The future plans include the design and construction of a third tunable beamline, designated X12, for protein crystallography. The development of automated beamlines for protein crystallography is of particular importance with respect to structural genomics initiatives. The analysis of the projects of the last years shows the wide range of anomalous scatterer used on the tunable beamlines thus demonstrating the need of a wide range of accessible energies and fast and reliable energy changes.

On the resolution and linearity of lensless in situ X-ray beam diagnostics using pixelated sensors

We present a theoretical model that describes the resolution and linearity of a novel transparent X-ray beam imaging and position measurement method. Using a pinhole or coded aperture camera with pixelated area sensors to image a small fraction of radiation scattered by a thin foil placed at oblique angles with respect to the beam, a very precise measurement of the beam position is made. We show that the resolution of the method is determined by incident beam intensity, beam size, camera parameters, sensor pixel size and noise. The model is verified experimentally showing a sub-micrometer resolution over a large linear range.

Position and flux stabilization of X-ray beams produced by double-crystal monochromators for EXAFS scans at the titanium K-edge

The simultaneous and active feedback stabilization of X-ray beam position and monochromatic beam flux during EXAFS scans at the titanium K-edge as produced by a double-crystal monochromator beamline is reported. The feedback is generated using two independent feedback loops using separate beam flux and position measurements. The flux is stabilized using a fast extremum-searching algorithm that is insensitive to changes in the synchrotron ring current and energy-dependent monochromator output. Corrections of beam height are made using an innovative transmissive beam position monitor instrument. The efficacy of the feedback stabilization method is demonstrated by comparing the measurements of EXAFS spectra on inhomogeneous diluted Ti-containing samples with and without feedback applied.

Real-time in-situ X-ray beam diagnostics

We report on a novel diagnostics instrument for in-situ imaging and measurements of X-ray beam parameters in real-time. The instrument is based on the robust and simple idea of a pinhole camera that collects the weakly scattered radiation from a thin sheet of a low-Z material placed in the X-ray beam at an acute angle. We demonstrate how by recording the scattered radiation with an appropriate detector, high-resolution beam characterisation can be performed in real-time. We present a mathematical model that describes the imaging process and beam position measurements. The theoretical evaluation of the resolution limit of our device and its dependence on various parameters are also discussed. Reported experimental results demonstrate the instruments capabilities in beam tracking and imaging applications.