Frank Nachtrab

Multi-energy X-ray imaging as a quantitative method for materials characterization.

IO N X-ray imaging is an established method for the characterization of material samples. Two-dimensional, radiographic methods as well as three-dimensional computed tomography are used very successfully in many fi elds. However both usually have a drawback: They provide only qualitative information rather than quantitatively exact physical properties. Thus a priori information and experience in evaluating the images are necessary to interpret the results. In this paper we present multi-energy X-ray imaging as a quantitative method for material characterization that is applicable for twoand three-dimensional analysis. The method we developed can be used in a classical “dual energy” approach as well as with newest generation “ spectral”, i.e. energy resolving, X-ray detectors. In X-ray imaging, three quantities of the object defi ne the attenuation of X-rays and thereby image contrast: the atomic number, the density and the thickness of the object that must be penetrated. For radiographic images, thickness and density can be united to the integral along the X-ray path through the object, resulting in the extinction, i.e. the exponent of Lambert-Beer’s law (see Equation 1 ). Computed tomography directly provides the attenuation coeffi cient depending on the atomic number and the density only. In reality usually more than one element is present, but the attenuation coeffi cient of a compound object is simply the linear combination of its constituents. Generally, the attenuation coeffi cients depend on both the energy of the X-ray photons and the type of material. If an object is imaged using different X-ray spectra or using an energy resolving detector, information on the type of material becomes available. Those so called “dual energy” techniques have been known since the mid-70s [ 1 ] and are established in medical imaging.

Laboratory X-ray microscopy with a nano-focus X-ray source

In this paper we describe the optimization of transmission X-ray targets by Monte-Carlo simulation for a laboratory X-ray microscopy setup. We identified two optimal target layer thicknesses (0.1 μm and 0.7 μm) for a high-resolution target and a high-flux target. Measurements show a decrease in focal spot size by one third or an increase in X-ray flux by a factor of three compared to those of a standard micro-focus target. Focal spot sizes down to 154 nm and 260 nm are achievable with the optimized targets. Simulation results for the X-ray flux match well to the experimental results, whereas the results for the focal spot sizes still show discrepancies attributed to the simplified simulation setup.

High-speed in-situ tomography of liquid protein foams

Abstract For the engineering of foamed food products, knowledge about the foam structure as well as about its dynamics and stability are of critical importance. Using fast tomography in the laboratory as well as ultra-fast phase-contrast synchrotron tomography accurate information about the entire pore distribution in milk protein foams is obtained almost instantaneously. This study displays the four-dimensional structural dynamics of milk-protein foam decay over time with unparalleled temporal and spatial resolution of the measurement data down to 1 s scan time (for 11 μm voxel sampling) and to 2.7 μm voxel sampling (for 2 s scan time). Pore size investigation is applied to a 15 min cine-tomography series monitoring the four-dimensional time decay of β-lactoglobulin foam, providing new insights into the dynamics of liquid protein foams.

Progress in sub-micrometer resolution computed tomography

Micro-CT with resolutions in the order of 1 μm is readily available nowadays but below 1 μm the maximum achievable resolution is not only limited by the components parameters like pixel size and focal spot size but also depends strongly on the stability of the whole CT system. We present the performance of our Sub-μm CT based on commercially available components and will show that it is possible to overcome the limitations resulting from instabilities of the system and reach resolution in the range of 500 nm. To overcome this limitation of conventional X-ray tubes we developed a nanofocus X-ray source built from a modified electron probe micro analyzer (EPMA). We present the setup of an X-ray microscope based on this source and first resolution measurements.

Energy resolving CT systems using Medipix2 and MHSP detectors

Energy resolved imaging has been possible with a newest generation of radiation detectors with photon counting and spectroscopic capabilities. This innovation gives the possibility to enhance the image quality by applying techniques using the energy information. In this work two X-ray Computed Tomography (CT) Systems were assembled with two different energy resolving detectors: Medipix2 and MicroHole & Strip Plate (MHSP). These detectors have the aforesaid characteristics and showed a good performance for X-ray imaging. The Energy Weighting Technique (EWT) and Basis Material Decomposition (BMD) techniques were applied with good results. An improvement of 31% in the CNR was achieved by applying the EWT in the MHSP data and, using Medipix2, two basis materials (Carbon based and Aluminium) were decomposed successfully with densities close to the real values.

Simple solutions for spectroscopic, photon counting X-ray imaging detectors

The central idea of our approach is to use standard imaging sensors for industrial optical imaging as X-ray sensors. We use them as a simple solution for a spectroscopic, single photon counting X-ray detector with a reduction of the pixel size by a factor of almost 10 compared to commercially available photon counting X-ray detectors. In principle, each of the sensors photodiodes can act as a direct converting X-ray sensor pixel. We compare the acquisition of images in integrating and photon-counting mode and notice a much better spatial resolution in photon-counting mode compared to integrating- mode. Using the benefits of direct detection we gather spectroscopic information of the incident photons. Gray value and energy deposition are correlated linearly. Energy resolutions down to 700 eV are by limitation to single event clusters. Furthermore detection efficiency, multiplicity, DQE(0), MTF and the radiation hardness are investigated.

Comparison of different sources for laboratory X-ray microscopy

This paper describes the setup of two different solutions for laboratory X-ray microscopy working with geometric magnification. One setup uses thin-film transmission targets with an optimized tungsten-layer thickness and the electron gun and optics of an electron probe micro analyzer to generate a very small X-ray source. The other setup is based on a scanning electron microscope and uses microstructured reflection targets. We also describe the structuring process for these targets. In both cases we show that resolutions of 100 nm can be achieved. Also the possibilities of computed tomography for 3D imaging are explored and we show first imaging examples of high-absorption as well as low-absorption specimens to demonstrate the capabilities of the setups.