The principles of a novel technique for nondestructive and simultaneous mapping of the three-dimensional grain and the absorption microstructure of a material are explained. The technique is termed X-ray diffraction contrast tomography, underlining its similarity to conventional X-ray absorption contrast tomography with which it shares a common experimental setup. The grains are imaged using the occasionally occurring diffraction contribution to the X-ray attenuation coefficient each time a grain fulfils the diffraction condition. The three-dimensional grain shapes are reconstructed from a limited number of projections using an algebraic reconstruction technique. An algorithm based on scanning orientation space and aiming at determining the corresponding crystallographic grain orientations is proposed. The potential and limitations of a first approach, based on the acquisition of the direct beam projection images only, are discussed in this first part of the paper. An extension is presented in the second part of the paper [Johnson, King, Honnicke, Marrow & Ludwig (2008). J. Appl. Cryst. 41, 310-318], addressing the case of combined direct and diffracted beam acquisition. research papers J. Appl. Cryst. (2008). 41, 302-309 Wolfgang Ludwig et al. X-ray diffraction contrast tomography I 303

We present a method in which the contributions from the individual crystallites in a polycrystalline sample are separated and treated as essentially single crystal data sets. The process involves the simultaneous determination of the orientation matrices of the individual crystallites in the sample, the subsequent integration of the individual peaks, and filtering and summing of the subsequent integrated intensities, in order to arrive at a single-crystal like data set which may be treated normally. In order to demonstrate the method, we consider as a test case a small molecule structure, cupric acetate monohyrade. We show that it is possible to obtain a single-crystal quality structure solution and refinement, in which accurate anisotropic thermal parameters and hydrogen atom positions are obtained.

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MICROSYMPOSIA C23 nanoparticle). This is the pesky problem Peter Piper picked. In this talk I will illustrate the problem and discuss some efforts we are taking to overcome these difficulties. I will describe some developments inspired by computer science that could be brought to bear on problems like this. We are not currently able to answer the question posed in the title, but even thinking about it is proving to be a lot of fun.

After about eight years of very active development, the Computational Crystallography Toolbox (cctbx) has evolved into a large collection of reusable, open source libraries covering many aspects of crystallography. The cctbx has roots in both smallmolecule crystallography and macro-molecular crystallography and is used in both fields. The cctbx libraries are organized in a hierarchy of modules. It is possible to use subsets of the modules independently of other modules that may not be required for certain applications. We will present an overview of the cctbx architecture and the underlying software technology that has enabled the sustained growth of the libraries. Links: cctbx.sourceforge.net, phenix-online.org

The overlap of diffraction spots from different grains was investigated to understand the influence of experimental factors on the x-ray diffraction data quality and to optimize the experimental parameters for data collection on polycrystalline samples. Diffraction patterns for photoactive polycrystals were indexed and sorted with respect to grains using multigrain approaches. The indexing of diffraction spots and the identification of grains for tetrathiafulvalene-p-chloranil samples were performed using the ImageD11, GrainSpotter, GRAINDEX and Cell_now programs. In many cases, comparison of the results from these programs shows good agreement. For the individual grains from polycrystalline samples, the crystal structure was solved and refined using the SHELXTL program. After the structural refinement of the grains, the best and the average R1 values were 1.93% and 2.06%, respectively, which are on a comparable resolution level with that obtained from the x-ray single crystal measurements.