Michael Salamon

High-resolution and high-speed CT in industry and research

The application of industrial CT covers many orders of magnitude of object sizes, ranging from freight containers (few meters) down to liquid foams (i.e. for food industry) or even parts of insects which are only several hundreds of micrometers in size. Similarly, the specifications for acquisition speed extend over some orders of magnitude, from hours to sub-second CT. We present the current technology in terms of X-ray sources and detectors, along with numerous applications from industry and materials research: e.g. industrial high-speed CT of car pistons, in situ micro-CT of milk foam decay at micrometer spatial resolution and 8 s scan time, as well as ex situ scans of tensile tested Nickel-alloys. The Fraunhofer Development Center X-ray Technology (Fürth, Germany) and the Chair of X-ray Microscopy (University Würzburg, Germany) are currently working on extending the technological limits, demonstrated, e,g. by the development of advanced X-ray detectors or a new inhouse CT system which comprises a high-brilliance liquid metal jet anode.

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.

3-D scanning of sea freight containers using MeV X-rays

The ECSIT project analyses how innovative inspection technologies can lead to an enhanced container security and how these technologies can be embedded into a holistic concept. It has the goal to analyze the possibility and feasibility for 100% scanning of all containers which are shipped to US ports and to develop a concept for integrating necessary infrastructure. A key element of the entire concept is the scanning technology itself. MeV X-ray technology using a linear accelerator as radiation source provides the feasibility to visualize the content of a container without opening it. If a 2-D radiography is ambiguous, a 3-D evaluation of the respective location could be conducted. MeV X-ray computed tomography (CT) is such a method to provide 3-D information of the content of a container. In the context of ECSIT, Fraunhofer EZRT has developed the concept of such a continuative high energy X-ray scanning stage and evaluated its application to sea freight containers. In this paper different approaches for measuring a 3-D tomographic volume data set of objects which are very heavy and thus difficult to move in arbitrary directions will be discussed. Three different geometrical principles for data acquisition were evaluated: laminography, limited angle CT, and a gantry CT. The volume data sets were reconstructed by using a standard filtered back projection and different algebraic reconstruction techniques (ART). Real 3-D volume data of large objects measured with the set-up described above are presented. As test objects a real container packed with various typical goods like furniture or consumer electronics as well as simulated threats like a bomb mock-up was used.

Application of In Situ 3D Computed Tomography during PVT Growth of 4H-SiC for the Study of Source Material Consumption under Varying Growth Conditions

2D and 3D in-situ X-ray visualization was applied to study the behavior of the SiC source material during PVT growth under various growth conditions. Experiments were carried out in two growth chambers for the growth of 3 inch and 4 inch crystals. Growth parameters were varied in the gas room in terms of axial temperature and inert gas pressure. The study addresses the stability of the SiC source material surface. It is shown that a higher inert gas pressure (e.g. 25 mbar) inhibits an unintentional upward evolution of the SiC feedstock that interferes with the crystal growth interface. The latter is related to a suppression of a pronounced recrystallization inside the SiC source. For a low inert gas pressure (e.g. 10 mbar) it is concluded that the axial temperature gradient inside the source material needs to be decreased to less than ca. 10 K/cm.

Application of 3-D X-Ray Computed Tomography for the In Situ Visualization of the SiC Crystal Growth Interface during PVT Bulk Growth

In this paper, we present for the first time an in-situ 3-D reconstruction of the SiC crystal growth interface using X-ray computed tomography (CT). We show that the shape of the growth interface can be determined with high precision at growth temperatures above 2100 °C in a conventional 3” PVT (physical vapor transport) growth system.

Growth Conditions and In Situ Computed Tomography Analysis of Facetted Bulk Growth of SiC Boules

Two 3inch SiC boules were grown in a PVT setup using source material of different packing density. During the growth, in-situ computed tomography of the growing boules showed differences in the development of the growth interface. A slightly bent growth interface was found for the smaller packing density. For the higher packing density the resulting crystal exhibits the onset of 6 pyramidal facets on its flanks. Besides that, strong anisotropic lateral growth was found on its (000-1) facet. Numerical simulations show an impact of the powder on the thermal gradient in the growth cell and therefore on the supersaturation. It is discussed that a higher supersaturation can account for the anisotropy in the growth rate of the [1-100] and the [11-20] direction.

Instrumentation for High Resolution Tomography for Characterization of Microstructures in Light-weight Alloys

Present methods for characterization of microstructures in alloys like the analysis of different material phases, dendrites, grain sizes and boundaries are mainly based on the 2D examination of grinded and polished surfaces. In order to perform a 3D analysis a large effort of preparation in multiple grinding and polishing is necessary and finally leads to a destruction of the specimen. In addition to the present used methods, highest resolution X-ray computed tomography (Subμ-CT) can offer additional value. SubμCT is not only able to create a whole 3D sample volume for investigations but is also able to create slices of a specimen at any needed position and orientation in the volume without destroying the object and area of interest. Additionally the effort for sample preparation is reduced to a minimum compared with other investigation methods. These added values for the field of material characterization motivated research and development in the field of X-ray instrumentation in the past years.

High resolution applications in nondestructive testing

Radioscopy and computed tomography in medical applications are well established and have reached a high level of development. Compared to medical applications industrial nondestructive-testing (NDT) has to deal with a large variety of different materials and material combinations, densities and density dynamics, geometries and structures which need to be inspected. This large variation necessitates the use and intelligent combination of different system components like detectors, manipulation systems and x-ray tubes to set up specialized inspection systems for different applications to fulfil the high demands of the customer.