September 2009 Archives

Some readers might remember the news article, "A compact synchrotron light source driven by pulse laser", in X-ray Spectrometry, Vol. 37, No.2 (2008). The essential point is that a table top pulse laser can be used as an alternative to a linear or circular electron accelerator. The article above reported the first successful synchrotron radiation generation from laser-plasma-accelerated electrons, but the wavelength was only in the visible to infrared region. Recently, an international team led by Professors S. Karsch and F. Grunera achieved a new breakthrough. The team belongs to Munich's Cluster of Excellence "Munich Centre for Advanced Photonics" (MAP), in the Laboratory for Attosecond Physics (LAP) of Ludwig-Maximilians-Universitat (LMU) in Munich and the Max Planck Institute of Quantum Optics (MPQ) in Garching. In their experiment, the electron accelerator is driven by pulses from a 20 TW (850 mJ in 37 fs) laser system. Focused into a hydrogen-filled gas cell with a length of 15 mm, the laser pulses produce stable electron beams showing a quasi-monoenergetic energy spectrum with a stable peak in the range of 200-220 MeV and 7 pC of charge in the whole spectrum. In order to transport the electron beam from the plasma accelerator, the scientists employed a pair of miniature permanent-magnet quadrupole lenses, which have been found to be critical for stability. The spectrum of their 30cm-long undulator typically consists of a main peak at a wavelength of 18 nm (fundamental), a second peak near 9 nm (second harmonic) and a high-energy cutoff at 7 nm. For more information, see the paper, "Laser-driven soft-X-ray undulator source", M. Fuchs et al., Nature Physics. 5, 826 (2009).

Professor D. Sparks (University of Delaware, USA) and his colleagues have reported an interesting application of quick X-ray absorption spectrometry to environmental science. The experiment is basically a continuous monochromator scan (0.3-0.6 sec for each spectrum) at the synchrotron beamline at Brookhaven National Lab. The main interest here is the initial oxidation rate of As(III) to As(V) by hydrous manganese(IV) oxide, because the toxicity and availability of arsenic to living organisms depends on its oxidation state at the interface to the water. The research team found that the initial apparent As(III) depletion rate constants are nearly twice as large as those measured with conventional, but much slower techniques. This indicates the necessity of further studies using such a rapid analytical method. For more information, see the paper, "Quantification of rapid environmental redox processes with quick-scanning x-ray absorption spectroscopy (Q-XAS)", M. Ginder-Vogel et al., Proc Nat Aca Sci, 106, 16124 (2009).

It is known that sulfide sometimes play a significant role in the geochemistry of arsenic under reducing conditions. So far, it has been assumed that sulfide primarily reduced the solubility and mobility of arsenic by precipitation of arsenic-sulfide minerals, As₂S₃, but recent studies indicate that under certain conditions, significant concentrations of soluble As-S compounds can exist in sulfidic waters. Thus, the question is whether they are As(III)-S species ("thioarsenites") or As(V)-S species ("thioarsenates"). A research group led by Dr. B. Planer-Friedrich (University of Bayreuth, Germany) has recently reported that use of X-ray absorption spectroscopy (XANES and EXAFS) can determine the concentration ratio of each species. The experiment was done at beamline BM20, ESRF. For more information, see the paper, "Discrimination of Thioarsenites and Thioarsenates by X-ray Absorption Spectroscopy", E. Suess et al., Anal. Chem., Article ASAP (2009), DOI: 10.1021/ac901094b

At Stanford's linac coherent light source (LCLS), a great deal of effort has been devoted since April this year to initial scientific tests of an X-ray laser. In September, scientists attempted to strip all ten electrons from an atom of neon. They were able to adjust the proportion of different neon species, from non-ionized Ne (no missing electrons) to Ne¹⁰⁺ (lacking all 10 electrons), by fine-tuning the powerful LCLS X-ray beam. For more information, visit the Web page, http://today.slac.stanford.edu/

Dr. C. T. Chanter and his colleagues have published a paper on the unresolved quantitative discrepancies between experimental and theoretical Cu Kα spectra. For more information, see the paper, "Theoretical Determination of Characteristic X-Ray Lines and the Copper Kα Spectrum", C. T. Chantler et al., Phys. Rev. Lett. 103, 123002 (2009).

Nearly $19 million in funding through the American Recovery and Reinvestment Act is supporting the Cornell High Energy Synchrotron Source (CHESS), Cornell Electron Storage Ring (CESR) and ongoing efforts to plan and build a new linear accelerator, the Energy Recovery Linac (ERL). So far, Cornell has received more than 90 ARRA grants, totally about $76 million. For more information, visit the Web page, http://www.news.cornell.edu/

Japan is celebrating the 1,300th anniversary of Kohfukuji Temple in Nara. The temple's sculpture of Ashura, one of the greatest treasures of the early to mid-7th century, is on exhibition at Kyushu National Museum in September. The exhibition features some X-ray imaging results of non-destructive observation of the interior of the sculpture. The images establish that the sculpture is still in good condition and also give a lot of information on the materials and methods used in its creation. Information on Ashura is available at the following Web page, http://en.wikipedia.org/wiki/Asura_(Buddhism)

So far, diffusion in solids has been investigated by profiling the depth dependence of tracer atoms diffused into the sample. Although one can obtain the diffusion constant from this, the question is how diffusion takes place on the atomic scale, rather than on the micron scale. Sometimes quasielastic neutron scattering as well as Mobauer spectroscopy can be used in a very limited number of fortunate cases. A research group led by Professor G. Vogl (University of Vienna, Austria) recently reported the use of X-ray photon correlation spectroscopy (XPCS) to observe the dynamics of diffusing atoms. The research was done for intermetallic alloy Cu₉₀Au₁₀, at temperatures of around 540 K, where the system is a substitutional solid solution, that is, the Au atoms statistically occupy sites in the Cu fcc lattice. The research gives the dynamical behavior of single atoms as a function of their neighborhood, and confirms quantitatively that Au atoms have a tendency to locally order on a certain set of sites in the crystal. Photon correlation spectroscopy is based on analysis of 'speckle' patterns, which are fine-scale diffraction patterns that appear in the scattering of coherent light from a disordered system. Speckle patterns are sensitive to the exact spatial arrangement of the disorder. By observing the intensity fluctuations in the speckle pattern, the characteristic times of fluctuations in the system can be determined. For more information, see the paper, "Atomic diffusion studied with coherent X-rays", M. Leitner et al., Nature Materials,8, 717 (2009).

In the film Star Trek IV (1986), transparent aluminum is used for the exterior portals and windows of spacecraft. Now transparent aluminum has become a hot topic for real, rather than in science fiction. An international team, led by Oxford University scientists, has recently reported that a short pulse from the FLASH laser (wavelength 13.5 nm) knocks out a core L-shell electron from every aluminium atom in a 50 nm Al thin film without destroying the metal's crystalline structure. This rendered the aluminium almost invisible for this wavelength. This phenomenon is called saturable absorption. The transient state of aluminium produced in this way is as dense as ordinary matter but can only exist for an extremely short period of time of 40 femtoseconds. For more information, see the paper, "Turning solid aluminium transparent by intense soft X-ray photoionization", B. Nagler et al., Nature Physics 5, 693 (2009).