April 2010 Archives

Kyoto University Museum is staging a special exhibition called "What are X-rays?" from April 28 to August 29. For more than 100 years, X-rays have contributed significantly to many sciences and technologies including medicine, astronomy, archaeology and even forensic science. The exhibition presents a full history of X-rays and their wide variety of applications, as well the future outlook, in a comprehensive manner that is aimed at non-specialists. Some interesting X-ray images are displayed, such as scans of a 400-year-old picture of so-called hidden Christians (who continued to practice Christianity in secret despite suppression by the government of Japan at that time) and the mummy of a bird (Crested Ibis) from ancient Egypt. During this special event, 4 lectures will be given. The museum's web page is http://www.museum.kyoto-u.ac.jp/index_e.htm

Coherent X-ray diffraction imaging is one of the hottest research topics in advanced X-ray physics. The method reconstructs a real-space image from an oversampled diffraction signal by using computer algorithms instead of lenses. So far, its application has been limited to fairly strong phase objects, mainly due to parasitic scattering from the optics used for limiting the beam. Korean researchers recently published an interesting report on its application to a nonisolated weak phase object, a one-dimensional trench structure fabricated on a Si substrate. In their discussion, the authors reported that such work was enabled by employing a special aperture with a very high aspect ratio of nearly 100 made of tantalum (1.7 μm × 2.2 μm aperture with a thickness of 130 μm). For more information, see the paper, "Coherent hard x-ray diffractive imaging of nonisolated objects confined by an aperture", S. Kim et al., Phys. Rev. B81, 165437 (2010).

A group led by Professor Ch. David (Paul Scherrer Institute, Switzerland) recently developed a synchrotron-based full-field microscope, which can work with hard X-rays, typically 10 keV. The instrument supports tomographic absorption and phase contrast imaging with a spatial resolution of 144 nm. The researchers demonstrated phase-contrast 3D imaging of a melanocortin-3 preosteoblast cell. For more information, see the paper, "Phase-contrast tomography at the nanoscale using hard x rays", M. Stampanoni et al., Phys. Rev. B 81, 140105(R) (2010).

Lens-less microscopy is now widely acknowledged to be an elegant solution to the so-called phase problem in X-ray crystallography. The method is based on the digital retrieval of the phase from the object's coherently diffracted intensity patterns, with the inversion being achieved through the use of time-consuming iterative algorithms. Fourier transform holography is a similar technique, but is essentially very quick and straightforward. Dr. V. Chamard (IM2NP, CNRS, Aix-Marseille Universite, France) and her colleagues recently demonstrated 3D imaging of a SiGe nanocrystal with Fourier transform holography. One unique point of the research is that they employed Bragg geometry, rather than forward scattering geometry, to obtain full 3D information. The technique requires that a reference crystal is placed near the object crystal to be imaged, and that the two crystals need to have comparable lattice parameters. They were successful in determining the electron density and the displacement field in 3D without suffering convergence problems, which are often the case with lens-less imaging iterative algorithms. For more information, see the paper, "Three-Dimensional X-Ray Fourier Transform Holography: The Bragg Case", V. Chamard et al., Phys. Rev. Lett. 104, 165501 (2010).