August 2009 Archives

Chandrayaan-1 was a lunar probe launched by the Indian Space Research Organization (ISRO). It was equipped with advanced X-ray spectrometers for investigation. After suffering from several technical problems including failure of the star sensors and insufficient thermal shielding, Chandrayaan stopped sending radio signals on August 29, 2009 shortly after which the ISRO officially declared the mission over. Chandrayaan operated for 312 days from October 2008. For more information, visit the Web page,http://www.isro.org/Chandrayaan/htmls/home.htm

When a strong laser beam hits the surface of a material, plasma is produced there, subsequently leading to the emission of a short burst of X-rays. It is believed that the electrons in the surface plasma are accelerated by the strong electric field of the laser and then penetrate the solid behind. There, they knock out electrons from inner electronic shells, which subsequently undergo inner-shell recombination, leading to characteristic line emissions such as Kα and Kβ spectra. A research group led by Professor U. Teubner (University of Applied Sciences, Emden, Germany) has reported detailed experimental results on copper and titanium K X-rays. Particular attention has been paid to the interplay between the angle of incidence of the laser beam on the target, as well as the influence of prepulses. For more information, see the paper, "Optimized K x-ray flashes from femtosecond-laser-irradiated foils", W. Lu et al., Phys. Rev. E 80, 026404 (2009).

In X-ray diffraction experiments, one measures the intensity (amplitude) of the diffracted X-rays as a function of position in the reciprocal space, and the information on the phase is always missing. For many years, this so-called phase problem has been thought as one of the biggest problems in X-ray crystallography. Professor E. Wolf (University of Rochester, New York) has recently published a very interesting and inspirational paper. He is famous for several important textbooks on optics and also for his presidency of the Optical Society of America. The present paper is theoretical, and starts with a criticism of basic understanding of the problem. The author says that trying to measure the phase is rather meaningless. Almost all scientists assume that the incident X-ray beam is monochromatic in the data analysis, but the author points out that a monochromatic beam is not possible in reality. Any beam that can be produced in a laboratory is, at best, quasimonochromatic and, therefore, even if both the amplitudes and the phases are given, it is still not possible to solve the problem. Alternatively, the author proposes the measurement of certain correlation functions, with the use of spatially coherent beams. While it is extremely important to think about a future strategy regarding the final solution of the phase problem as discussed in the paper, the author makes no mention of the recent significant strides in coherent X-ray scattering. For more information, see the paper, "Solution of the Phase Problem in the Theory of Structure Determination of Crystals from X-Ray Diffraction Experimentst", E. Wolf, Phys. Rev. Lett. 103, 075501 (2009).

X-ray phase-contrast imaging is extremely powerful for visualizing internal structures with low-Z matrices, which are most likely in bio-medical specimens. The use of an X-ray interferometer is one of the most promising ways forward for this imaging technology, but resolution has been limited to the micrometer scale so far. A research group led by Dr. A. Snigirev (European Synchrotron Radiation Facility, Grenoble, France) has recently developed a novel type of X-ray interferometer employing a bilens system with two parallel arrays of compound refractive lenses, each of which creates a diffraction limited beam under coherent illumination. The energy of the X-rays is 10-20 keV and the material used in the refractive lenses is silicon. When the two beams overlap, they produce an interference pattern with fringe spacing ranging from tens of nanometers to tens of micrometers. Readers may notice that the system is similar to the model of a Billet split lens in classical optics (See Fig.7.8, page 263 in "Principle of Optics", M. Born and E. Wolf, 6^th Ed, Pergamon Press (1988)). The use of a modern synchrotron source and this novel optical device thus opens up a new field and could revive old theorems. Coherent moiré imaging or radiography are promising straightforward applications. For more information, see the paper, "X-Ray Nanointerferometer Based on Si Refractive Bilenses", A. Snigirev et al., Phys. Rev. Lett., 103, 064801 (2009).