X-ray imaging offers a number of unique properties that are favorable for NDE applications, including large penetration depth, elemental specificity, and relatively low radiation damage. While direct-projection type x-ray systems with a few um resolution have been widely deployed, recent advances in x-ray optics and imaging methodology have lead to lens-based x-ray microscopes with better than 60-nm resolution, and with integrated 3D imaging and material analysis capabilities. Used independently or in combination with established techniques based on visible light and electron microscopy, these new high-resolution x-ray systems introduces many attractive new capabilities for studying structures at micrometer to tens-of-nm scale.
The spatial resolution is a key optical parameter characterizing the performance of an imaging microscope. Zone plate based x-ray microscopy offers the highest spatial resolution over the whole electromagnet wave spectrum. Sub-20 nm resolution have been demonstrated with soft x-rays and sub-60 nm resolution have been obtained with multikeV x-rays using a laboratory source. There are two simple pathways to achieve sub-10 nm resolution x-ray imaging: (1) improving the fabrication technology to produce zone plates with an outermost zone width less than 10 nm and (2) using a higher diffraction order (such as the third diffraction order) of a currently available zone plate.
Near-edge x-ray absorption resonances provide information on molecular orbital structure; these resonances can be exploited in x-ray spectromicroscopy to give sub-50-nanometer resolution images with chemical state sensitivity. At the same time, radiation damage sets a limit to the resolution that can be obtained in absorption mode. Phase contrast imaging may provide another means of chemical state imaging with lower radiation dose. We describe here the use of experimentally measured near-edge absorption data to estimate near-edge phase resonances. This is accomplished by splicing the near-edge data into reference data and carrying out a numerical integration of the Kramers-Kronig relation.
It is now possible for large volumes of synchrotron- radiation-generated micro-tomography data to be produced at gigabyte-per-minute rates, especially when using currently available CCD cameras at a high-brightness source, such as the Advanced Photon Source (APS). Recent improvements in the speed of our detectors and stages, combined with increased photon flux supplied by a newly installed double multilayer monochromator, allow us to achieve these data rates on a bending magnet beamline. Previously, most x-ray microtomography experiments have produced data at comparatively lower rates, and often the data were analyzed after the experiment. The time needed to generate complete data sets meant putting off analysis to the completion of a run, thus preventing the user from evaluating the usefulness of a data set and consequently impairing decision making during data acquisition as to how to proceed. Thus, the ability to provide to a tomography user a fully reconstructed data set in few minutes is one of the major problems to be solved when dealing with high-throughput x- ray tomography. This is due to the complexity of the data analysis that involves data preprocessing, sinogram generation, 3D reconstruction, and rendering. At the APS, we have developed systems and techniques to address this issue. We present a method that uses a cluster-based, parallel- computing system based on the Message Passing Interface (MPI) standard. Among the advantages of this approach are the portability, ease-of-use, and low cost of the system. The combination of high-speed, online analysis with high- throughput acquisition allows us to acquire and reconstruct a 512x512x512-voxel sample with a few microns resolution in less than ten minutes.