Anton Kachatkou

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.

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 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.

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.

Dynamic range enhancement algorithms for CMOS sensors with non-destructive readout

Image sensors capable of a non-destructive readout (NDR) allow reading several frames during the integration time without affecting a photo charge being collected in the pixelpsilas well. We show that two published algorithms for processing the sequence of NDR frames into a high dynamic range (HDR) image suffer from ldquostepped gradientrdquo artifacts using data taken with an ldquooff-the-shelfrdquo CMOS image sensor. We present a novel algorithm that is capable of alleviating this image artifact that is due to suboptimal saturated pixel treatment. Our procedure works for both black and white and color images. In addition, we present joint demosaicing and denoising for color HDR image reconstruction and its performance is compared with conventional demosaicing algorithms. The overall performance of the combined methods provides a DR increase of up to 44 dB compared to raw data. We demonstrate our algorithms using images taken with a black-and-white and color NDR capable camera system.

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.

A compact and portable X-ray beam position monitor using Medipix3

The present work reports on the design and implementation of a novel portable X-ray beam diagnostics (XBPM) device. The device is transparent to the X-ray beam and provides real-time measurements of beam position, intensity, and size. The measurement principle is based on a pinhole camera which records scattered radiation from a Kapton foil which is placed in the beam path. The use of hybrid detectors (Medipix3) that feature a virtually noiseless readout system with capability of single photon detection and energy resolving power enables the diagnostics with a better resolution and higher sensitivity compared to the use of traditional indirect X-ray detection schemes. We describe the detailed system design, which consists of a vacuum compatible focal plane sensor array, a sensor conditioning and readout board and a heterogeneous data processing unit, which also acts as a network server that handles network communications with clients. The readout protocol for the Medipix3 sensor is implemented using field programmable gate array (FPGA) logic resulting in a versatile and scalable system that is capable of performing advanced functions such as data compression techniques and feature extraction. For the system performance measurements, we equipped the instrument with a single Medipix3 die, bump bonded to a Si sensor, rather than four for which it was designed. Without data compression, it is capable of acquiring magnified images and profiles of synchrotron X-ray beams at a transfer rate through Ethernet of 27 frames/s for one Medipix3 die.

Performance of the Lancelot Beam Position Monitor at the Diamond Light Source

The Lancelot beam position and profile monitor records the scattered radiation off a thin, low-density foil, which passes through a pinhole perpendicular to the path of the beam and is detected by a Medipix3RX sensor. This arrangement does not expose the detector to the direct beam at synchrotrons and results in a negligible drop in flux downstream of the module. It allows for magnified images of the beam to be acquired in real time with high signal-to-noise ratios, enabling measurements of tiny displacements in the position of the centroid of approximately 1 μm. It also provides a means for independently measuring the photon energy of the incident monoenergetic photon beam. A constant frame rate of up to 245 Hz is achieved. The results of measurements with two Lancelot detectors installed in different environments at the Diamond Light Source are presented and their performance is discussed.