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Abstract: Single and polycapillary optics has obtained in recent times increasing attention for its potential high efficiency as device for focusing high-energy photons and thermal neutrons. The actually developed and future applications depend strongly on a detailed knowledge of the optical characteristics of such devices. Nevertheless, an exhaustive theoretical treatment of the polycapillary properties is extremely complicated due both to the intrinsic complexity of the structure and of the interaction between the X-ray photon beam and the mono- and polycapillary surfaces.
Using PolyCAD, a ray-tracing original package developed by our group, here we report a wide study concerning single and polycapillary optics properties. The software allows easily to simulate the optical path inside cylindrical, conical and curved mono- and polycapillary structures; their focusing properties are also presented and discussed.
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Jul 2007
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Abstract: The development of high-reflectivity devices for soft X-rays at quasi-normal incidence is a challenging research for the development of synchrotron radiation optics, particularly for soft X-ray microscopy and X-ray microprobe spectroscopy. Here we present data concerning the deposition of the first Ni/Ti and Ni/TiO2 multilayers grown at the INFN Legnaro Laboratories (LNL). These multilayers have a lattice spacing in the order of 14 Å and more than 100 of bilayers. Experimental tests on these multilayers have been performed by a vacuum compatible ?–2? reflectometer, set up at the INFN Frascati Laboratories (LNF), where their characterization has been accomplished by means of synchrotron radiation.
The first multilayer mirrors tailored in order to work at quasi-normal geometry have been measured in the lower X-ray energy domain using both white-beam and monochromatic radiation at about 1 keV.
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Jul 2007
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Optics
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Abstract: X-ray compound refractive lenses (CRL) are becoming a widespread tool for the generation of microfocus spot sizes at synchrotron beamlines. The calculation of their performance by means of ray-tracing is useful for a rapid estimation of flux, resolution and focusing properties achievable in a beamline, when other optics are present, or simply to study the lens acceptance and focusing in the presence of a particular bending magnet, wiggler or undulator X-ray source. The ray-tracing method presented in this paper has been used to calculate the efficiency of beryllium CRL's using, for the instrument layout, realistic source size and divergence, and usual optics like perfect crystal monochromators. It is shown that the intensity transmitted by the lens, the effective aperture and the gain are in good agreement with analytical formulas. Additional information provided when running the program are the precise shape of beam at the focus, and at any position along the optical axis. For instance the intensity distribution at the CRL entrance and exit planes allows a comparison between the effective and the geometrical apertures. Finally, the method provides a precise value for the lens focal distance, which depends on the CRL length.
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Jul 2007
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Abstract: The crystal structure of the high-pressure phase of potassium hydrogen carbonate, here termed KHCO3 form III, has been solved using single-crystal X-ray diffraction techniques. It adopts triclinic P1? symmetry and is formed on direct compression of the monoclinic ambient-pressure phase (kalicinite, form I) via a first-order phase transition at 3.2 GPa (?V/V ~4%). We have also used time-of-flight high-pressure neutron powder-diffraction to determine the structural changes in KDCO3 of the form I and III polymorphs to 8.3 GPa. This study indicates that the transition appears to be in response to pronounced displacements of the K+ cations and the cooperative tilting of the (DCO3?)2 dimers. The O···O hydrogen bond distance shows a monotonic compression over the entire pressure range studied with no obvious discontinuity at the phase transition. However, the O-D···O bond angle appears to exhibit an abrupt ~4° decrease across the I–III phase transition with a concomitant change in its pressure dependence. Birch Murnaghan fits to the equation of state data above and below the transition indicate that form III [B0 = 26.5(2.8) GPa, B? = 5.2(6)] is marginally less compressible than form I [B0 = 22.7(8) GPa, B? = 4.1(5)].
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Jul 2007
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Abstract: Ellipsometry and atomic force microscopy (AFM) were used to study the film thickness and the surface roughness of both ‘soft’ and solid thin films. ‘Soft’ polymer thin films of polystyrene and poly(styrene–ethylene/butylene–styrene) block copolymer were prepared by spin-coating onto planar silicon wafers. Ellipsometric parameters were fitted by the Cauchy approach using a two-layer model with planar boundaries between the layers. The smooth surfaces of the prepared polymer films were confirmed by AFM. There is good agreement between AFM and ellipsometry in the 80–130 nm thickness range. Semiconductor surfaces (Si) obtained by anisotropic chemical etching were investigated as an example of a randomly rough surface. To define roughness parameters by ellipsometry, the top rough layers were treated as thin films according to the Bruggeman effective medium approximation (BEMA). Surface roughness values measured by AFM and ellipsometry show the same tendency of increasing roughness with increased etching time, although AFM results depend on the used window size. The combined use of both methods appears to offer the most comprehensive route to quantitative surface roughness characterisation of solid films.
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Jul 2007
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Abstract: Coherent x-ray diffraction has been used to image grown in antiphase boundaries (APBs) in a metal alloy, which represent pure phase objects. The fine structure within the (001) superstructure diffraction peak of a B2-ordered bulk Fe(65)Al(35) sample was inverted by means of iterative algorithms that Fourier transform between reciprocal and real space, applying appropriate constraints in each domain. Since the sample object is non-compact, bigger than the beam footprint, knowing the precise beam profile was essential to define the real space constraint. Even though a unique long range structure could not be derived, the algorithm found phase structures that were locally unique. These were identified in all reconstruction runs by means of a cross-correlation analysis. The obtained characteristic APB morphology is confirmed by transmission electron microscopy results from the same sample, revealing nearly planar APB walls on (110) planes terminating at grown-in dislocations.
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Jul 2007
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Abstract: Coherent X-ray diffraction imaging is a rapidly advancing form of lensless microscopy. The phase information of the diffraction pattern is embedded in a sufficiently sampled coherent diffraction pattern. Using advanced computational methods, this diffraction pattern can be inverted to produce an image of a sample with diffraction-limited resolution. It is attractive to use high-power coherent X-ray beams produced by future X-ray free-electron lasers for imaging nanoscale condensed matter, materials and biological samples. Here, the scientific case, requirements and the possible realisation of the coherent X-ray diffraction imaging beamlines at the European XFEL Facility are presented.
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Jul 2007
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Abstract: We have studied the magnetic and electronic properties of (Ga,Mn)As ferromagnetic semiconductor layers containing low concentrations of room temperature magnetic MnAs precipitates, using x-ray magnetic circular dichroism in x-ray absorption spectroscopy. Measurements below and above the Curie temperature of the (Ga,Mn)As show localized multiplet structures and broad metallic-like line shapes, respectively. By applying magnetic fields below the coercivity of the (Ga,Mn)As layer, the magnetization reversal behavior of the ferromagnetic semiconductor and the MnAs precipitates can be resolved and are found to be independent of each other.
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Jul 2007
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Abstract: We have studied the superconducting properties of sputtered bilayer thin films of Nb and Pd. The superconducting proximity effect suppresses the critical temperature TC of these bilayer systems. Here, we report that this suppression shows a marked layer-sequence dependence: the suppression is larger for a Nb layer capped with Pd rather than buffered by it. We interpret our data in terms of a theoretical model of the proximity effect that takes into account finite interface transparency, and explain this effect as arising due to different levels of disorder in the Pd layer. We believe the same proximity induced effect could also appear in other studies and will also affect ferromagnet/superconductor/ferromagnet structures.
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Jul 2007
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Abstract: [1] Paleoproterozioc (ca. 1.9 Ga) stromatolites from the Biwabik Iron-Formation (Minnesota, United States) contain fossil forms which in the literature have been attributed to ancient microbes including iron-oxidising bacteria and cyanobacteria. To search for valence state fossils, we measured Fe3+/?Fe (?Fe = Fe2+ + Fe3+) transverse to the laminae of a Biwabik, probably biogenic stromatolite and compared the results with a scan across the enclosing, likely abiotic sedimentary rock (or interstromatolite fill). To obtain Fe3+/?Fe and information about the site symmetry and crystal field strength, we used L3 (2p3/2 ? 3d) soft X-ray absorption spectroscopy and interpreted the measurements using our calculated spectra. We found that Fe3+/?Fe is approximately constant at 0.3 within the stromatolite, increases steeply to around 0.6 in the region of the stromatolite-fill boundary, and then reduces significantly in the interstromatolite fill. The crystal field strength for octahedral Fe3+ increased and became more irregular as the scan moved from the stromatolite into the fill. Our results tentatively suggest that a combination of ancient biological and later diagenetic processes can produce steep gradients in Fe3+/?Fe and alterations in the crystal field strength across a probable abiotic-biotic boundary. To describe the L3 absorption spectra in the stromatolite-fill region, it was necessary to include a significant Fe3+ tetrahedral component, which suggests the presence of magnetite. At other positions, fits using only octahedral Fe2+ and Fe3+ theoretical curves satisfactorily reproduce the spectra. The search for a possible new type of spatial biosignature in the valence state record for ancient stromatolites might help differentiate between the biogenic stromatolites and abiogenic stromatolite-like structures and could even be relevant to the search for fossil evidence of life in extraterrestrial rocks.
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Jul 2007
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