Manfred Schuster

High-resolution differential phase contrast imaging using a magnifying projection geometry with a microfocus x-ray source

Differential x-ray phase contrast imaging using a grating interferometer was combined with a magnifying cone beam geometry using a conventional microfocus x-ray tube. This brings the advantages of a magnifying cone beam setup, namely, a high spatial resolution in the micron range and the possibility of using an efficient, low resolution detector, into differential phase contrast imaging. The authors present methodical investigations which show how the primary measurement signal depends on the magnification factor. As an illustration of the potential of this quantitative imaging technique, a high-resolution x-ray phase contrast tomography of an insect is presented.

Determination of the chemical composition of distorted InGaN/GaN heterostructures from x-ray diffraction data

An evaluation algorithm for the determination of the chemical composition of strained hexagonal epitaxial films is presented. This algorithm is able to separate the influence of strain and composition on the lattice parameters measured by x-ray diffraction. The measurement of symmetric and asymmetric reflections delivers the strained lattice parameters a and c of hexagonal epitaxial films. These lattice parameters are used to calculate the relaxed lattice parameters employing the theory of elasticity. From the relaxed parameters, the chemical composition of the epitaxial film can be determined by Vegards rule. The algorithm has been applied to InGaN/GaN/Al2O3(00.1) heterostructures.

Inverse geometry for grating-based x-ray phase-contrast imaging

Phase-contrast imaging using conventional polychromatic x-ray sources and grating interferometers has been developed and demonstrated for x-ray energies up to 60 keV. Here, we conduct an analysis of possible grating configurations for this technique and present further geometrical arrangements not considered so far. An inverse interferometer geometry is investigated that offers significant advantages for grating fabrication and for the application of the method in computed tomography (CT) scanners. We derive and measure the interferometer’s angular sensitivity for both the inverse and the conventional configuration as a function of the sample position. Thereby, we show that both arrangements are equally sensitive and that the highest sensitivity is obtained, when the investigated object is close to the interferometer’s phase grating. We also discuss the question whether the sample should be placed in front of or behind the phase grating. For CT applications, we propose an inverse geometry with the sample ...

The fractional Talbot effect in differential x-ray phase-contrast imaging for extended and polychromatic x-ray sources

The influence of different physical parameters, such as the source size and the energy spectrum, on the functional capability of a grating interferometer applied for phase‐contrast imaging is discussed using numerical simulations based on Fresnel diffraction theory. The presented simulation results explain why the interferometer could be well combined with polychromatic laboratory x‐ray sources in recent experiments. Furthermore, it is shown that the distance between the two gratings of the interferometer is not in general limited by the width of the photon energy spectrum. This implies that interferometers that give a further improved image quality for phase measurements can be designed, because the primary measurement signal for phase measurements can be increased by enlargement of this distance. Finally, the mathematical background and practical instructions for the quantitative evaluation of measurement data acquired with a polychromatic x‐ray source are given.

Signal-to-noise ratio in x ray dark-field imaging using a grating interferometer

In this work, we report an analytical and experimental investigation of the signal-to-noise ratio for a recently developed method called x ray dark-field imaging. Our approach is based on the propagation of signal and noise through the reconstruction algorithm. We find that the statistical nature of the dark-field images can be understood by a Rician distribution. The analysis shows that, for high flux, the noise in the dark-field images is proportional to the noise in the raw data. In the limit of low flux and, thus, low signal-to-noise ratio, the dark-field signal exhibits a breakdown of the signal transmission, which can be described by an asymptotic behavior of the underlying noise distribution. In this limit, the dark-field signal is no longer connected to the coherence degradation, but rather to the attenuation in the sample. The model is verified in exemplary test measurements using a compact laboratory setup with a polychromatic source and a photon counting detector.

Inspection of refractive x-ray lenses using high-resolution differential phase contrast imaging with a microfocus x-ray source

A refractive x-ray lens was characterized using a magnifying cone beam setup for differential phase contrast imaging in combination with a microfocus x-ray tube. Thereby, the differential and the total phase shift of x rays transmitted through the lens were determined. Lens aberrations have been characterized based on these refractive properties.

Photodiodes as detectors with high dynamical range for X-ray reflectivity measurements

Abstract Silicon pin photodiodes from different manufacturers were tested with synchrotron radiation and found to be excellently suited as detectors for the X-ray range between 5 and 25 keV. They have a high dynamical range (more than 6 decades) and a high degree of linearity. They can be used in the current mode down to 100 photons/s. This corresponds to a photocurrent of 50 fA. The performance of the signal processing (current-voltage amplifier, voltage-frequency converter, frequency counter) is of great importance. The Keithley current-voltage amplifier 18000-20, whose range can be switched electronically, is superior in performance to the Keithley amplifier 427, when the intensity range exceeds 4 decades.

Fluorescence x-ray standing wave study on (AlAs)(GaAs) superlattices

X-ray standing waves (XSW) were used to investigate the structure of molecular beam epitaxy (MBE) grown (AlAs)3(GaAs)7 short-period superlattices (SPSL). The modulation of the Al K, As L, and Ga L x-ray fluorescence induced by XSW was measured at the zero-order superlattice (SL) satellite (AlAs)(GaAs)(004,0) and the GaAs(004) substrate Bragg reflection. From the shape of the fluorescence yield modulations and the diffraction pattern, a model of the interfaces is derived by comparing the experimental data with dynamical calculations of the x-ray wave field distribution and reflectivity. A straightforward analysis of the fluorescence measurements at the SL satellite shows that in AlAs layers a high crystalline order is established, whereas in GaAs layers a fraction of the Ga and As atoms is not on ideal lattice sites, but is displaced towards the substrate. The data can be explained by a model in which, at each AlAs/GaAs interface of the GaAs layers, two Ga atom planes are displaced by 0.035 nm and 0.008 nm and one As atom plane by 0.023 nm. The displacements within the GaAs layers exhibit a mirror symmetry with respect to the centre of each layer.

X‐ray standing wave measurements on III–V compound heterostructures

New applications of the x‐ray standing wave method are described, in which not only Bragg reflections from the substrate, but also from a heteroepitaxial layer and from a superlattice satellite are utilized. The investigated samples are an InP/InGaAsP structure on InP(001) and a short‐period AlAs/GaAs superlattice on GaAs(001). For efficient detection of the photoelectron yield without vacuum requirements, the sample was placed inside a gas cell, and the induced photocurrent was monitored. In addition to the electron yield, which probes only a thin surface layer, the depth‐integrating x‐ray fluorescence was analyzed. The results give new insights into the structure of heteroepitaxial systems and their buried interfaces, as well as information about electron escape depths and spatial coherence of x‐ray wavefields.

Siemens SMART CCD detector applied to protein crystallography with synchrotron and rotating anode x-ray sources

The performance of the Siemens CCD detectors with 1K and 2K chips is evaluated using radiation from bending magnet beamlines at SSRL and NSLS and from a rotating anode source with three type of optics. Structure solution quality data is obtainable in short times for macromolecular and small molecule crystallography. General experiments, such as time-resolved powder diffraction and overlayer diffraction are also possible.