July 2008 Archives

It is well-known that Vincent van Gogh (1853-1890) often reused canvases and painted over his older works. Specialists estimate that about one third of his early paintings conceal other compositions under them. Recently, an international team led by Professor K. Janssens (University of Antwerp, Belgium) and Dr J. Dik (Delft University of Technology, The Netherlands) successfully applied synchrotron radiation induced X-ray fluorescence spectroscopy to the painting entitled Patch of Grass (painted by Van Gogh in Paris in 1887 and owned by the Kroller-Muller Museum). The research group recorded X-ray fluorescence intensity maps of several tens of square cm and, in particular, the distribution of Hg and Sb, which corresponds to red and light tones, respectively. In this way, it could analyze an approximate color reconstruction of the flesh tones. Accordingly, a portrait of a woman was discovered behind the painting. The measurement was done at DESY in Hamburg, Germany. For more information, visit the Website, http://www.vangogh.ua.ac.be/, and see the paper, "Visualization of a Lost Painting by Vincent van Gogh Using Synchrotron Radiation Based X-ray Fluorescence Elemental Mapping", J. Dik et al., Anal. Chem., ASAP Article, 10.1021/ac800965g (2008).

Aerogel is a form of nanofoam, an engineered material designed for its high strength-to-weight ratio for application wherever lightness and strength are needed. Now, the internal structure is within the scope of X-ray analysis. Lawrence Livermore and Lawrence Berkeley scientists have successfully applied the coherent X-ray diffraction technique to Ta₂O₅ nanofoam, the density of which is 1.2 % to the bulk, and have reconstructed 3D images to determine its strength and potential new applications. Combining the obtained structural information with detailed simulations, the research team showed that the blob-and-beam network structure explains why the materials are weaker than expected. For more information, see the paper, "Three-Dimensional Coherent X-Ray Diffraction Imaging of a Ceramic Nanofoam: Determination of Structural Deformation Mechanisms", A. Barty et al., Phys. Rev. Lett., 101, 055501 (2008).

Scanning diffraction microscopy, or ptychography, was first developed for the scanning transmission electron microscope (STEM). In the same way, by using an X-ray nano beam, one can use a STXM. The X-ray beam is focused onto the sample via a lens, and the transmission is measured. The image is obtained by plotting the transmission as a function of the sample position, as it is rastered across the beam. The analysis is straightforward, but its resolution is limited by the beam size. On the other hand, coherent diffractive imaging (CDI) now reaches resolutions below 10 nm, but the reconstruction procedures are not always easy due to the influences of data quality, sample conditions etc. A Swiss research group led by Drs. C. David and F. Pfeiffer (Paul Scherrer Institut) recently demonstrated a ptychographic imaging method that bridges the gap between STXM and CDI by measuring complete diffraction patterns at each point of a STXM scan. The group employed an advanced large-area pixel detector, Pilatus, to obtain the diffraction pattern efficiently. These diffraction data were then treated with an image reconstruction algorithm developed by the team. Several tens of thousands of diffraction images were processed to obtain one super-resolution X-ray image. The algorithm not only reconstructs the sample but also the exact shape of the light probe resulting from the X-ray beam. The 6.8 keV X-ray beam was focused using a zone plate, and the beam size was 300 nm. The spatial resolution achieved was about five times higher. For more information, see the paper, "High-Resolution Scanning X-ray Diffraction Microscopy", P. Thibault et al., Science, 321, 379 - 382 (2008).

The corrosion of steel-based mechanical components is said to be responsible for the loss of about 3% of annual global GDP. Cracks can appear in stainless steel components when stress or strain is combined with a corrosive environment that attacks sensitive grain boundaries. In nuclear power plants, certain grain boundaries can become sensitive during heat treatments or during fast neutron irradiation. It is important to observe how these cracks grow in detail, because they have been identified as the primary cause of several critical system failures. At the European Synchrotron Radiation Facility (ESRF), Grenoble, France, Dr. A. King and his colleagues recently revealed how growing cracks interact with the 3D crystal structure of stainless steel. The sample was a wire of 0.4 mm in diameter, and 40 keV X-rays were employed. By using diffraction contrast tomography, the research group could observe the shapes, positions, and orientations of 362 different grains with some 1600 grain boundaries without destroying the sample. They put the wire into a corrosive liquid, K2S4O6, and applied a load to cause microcracks to grow between the grains. As the cracks grew, 3D tomographic scans (of 30 minutes each) were made at intervals of between several minutes and two hours to follow the progress of the cracks. It was found that the cracks grew along the boundaries between the grains. The technique has enabled visualization of the cracks as they grow and of certain special boundaries that resist cracking. Information on this method is given in the following papers; "X-ray diffraction contrast tomography: a novel technique for three-dimensional grain mapping of polycrystals. I. Direct beam case", W. Ludwig et al., J. Appl. Crystallogr. 41, 302 (2008) and "II. The combined case", G. Johnson et al., J. Appl. Crystallogr. 41, 310 (2008). For more information on the present research, see the paper, "Observations of Intergranular Stress Corrosion Cracking in a Grain-Mapped Polycrystal", A. King et al., Science, 321, 382 - 385 (2008).

When X-rays satisfy Bragg's law for a perfect crystal, a significant transparency to X-ray beams is observed. This is the so-called Bormann effect, and is caused because the X-ray electric field approaches zero amplitude at the crystal planes, corresponding to almost no scattering by atoms. Recently, Dr. S. P. Collins (Diamond Light Source, United Kingdom) and his colleagues attempted several very interesting experiments - X-ray spectroscopy under the Bormann transmission condition. The main idea is that the electric quadrupole absorption transitions could be effectively enhanced under conditions of absorption suppression. The measured sample is gadolinium gallium garnet (Gd₃Ga₅0₁₂) cut parallel to the (100) planes, and some new spectral features were observed in the L_I (8,376 eV), L_II (7,930 eV) and L_III (7,243 eV) edges for gadolinium, at different temperatures. They are basically additional peaks on the low energy side, and correspond to an electric quadrupole transition from 2s, 2p_1/2 and 2p_2/3 to the narrow, half-filled 4f states, respectively. For more information, see the paper, "Quadrupole transitions revealed by Borrmann spectroscopy", R. F. Pettifer et al., Nature, 454, 196-199 (2008).

Recently, Professor I. Nakai (Tokyo University of Science, Japan) and his colleagues published a very interesting report on synchrotron X-ray fluorescence analysis of the cadmium hyper-accumulating plant, Arabidopsis halleri ssp. gemmifera. To investigate the Cd accumulation mechanism, they analyzed the spatial distribution and chemical form of Cd at a cellular level. At Japanese synchrotron facility, SPring-8, a tiny beam of 3.8 × 1.3 μm² with 37 keV X-rays was used to see Cd K X-rays. For more information, see the paper, "Micro X-ray fluorescence imaging and micro X-ray absorption spectroscopy of cadmium hyper-accumulating plant, Arabidopsis halleri ssp. gemmifera, using high-energy synchrotron radiation", N. Fukuda et al., J. Anal. At. Spectrom., 23, 1068-1075 (2008).