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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Two apparatus have been developed for time-resolved measurements of x-ray diffraction patterns using an imaging plate detector. The first one is based on a cinema method which permits up to 40 exposures of a two-dimensional x-ray pattern (100×108 mm2) with a 0.3-s time resolution. The second one works as a 200-mm-long linear detector which enables a time resolution of 23 μs for a duration of up to 46 ms, based on a streak-camera method. These apparatus have no count rate limitation, a high detective quantum efficiency (more than 80% for 8–20 keV), a 1:105 dynamic range in x-ray intensity and a spatial resolution of 150 μm (FWHM) due to the performance of the imaging plate.
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The operation of a proportional counter filled with helium gas has been investigated at low temperatures down to 5 K. It has been found for the first time that a helium-filled counter cooled at temperatures below 30 K works well in the proportional region, but with low gas gains (≤100). The operation at low temperatures (≤30 K) is attributed to the complete removal of impurities from helium gas as well as the extreme depression of the reaction frequency of metastable heliums. The absence of impurities results in eliminating the Penning effect, i.e., an ionization process of impurity atoms or molecules by collisions with metastable helium, which spreads electron avalanches in both space and time if helium gas contains a slight amount of impurities (1–103 ppm). Continuous discharges caused by the photoeffect of ultraviolet photons, which are mostly produced in decay channels of metastable helium, are thermally quenched at low temperatures below 30 K. The helium-filled counter provides us with a new method to detect nuclear radiations at very low temperatures. The present counter technique has been successfully applied to the resonance-electron Mössbauer spectroscopy near liquid-helium temperature (∼5 K).
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  • 3
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structure of heptabismuth tritantalum octadecaoxide, Bi7Ta3O18, has been solved and refined using single-crystal X-ray diffraction data collected at a synchrotron source in conjunction with unit-cell and symmetry information derived from electron diffraction. The space-group symmetry is triclinic C1 but is very close to monoclinic C2/m. A twin component observed during data collection was successfully modelled in the refinement. The C2/m prototype fitted all the Rietveld-refinable features of a medium-resolution neutron powder diffraction pattern. The metal-atom array is approximately face-centred cubic (fluorite type), punctuated by regularly spaced displacement faults perpendicular to the [111]fluorite direction every 2.5 fluorite unit cells. The metal-atom populations and O-atom positions are fully ordered. The Ta5+ cations are octahedrally coordinated, with TaO6 octahedra forming columns. The remaining O atoms occupy distorted fluorite positions. The Bi3+ cations occupy octahedral, square pyramidal or trigonal prismatic sites within the O-atom array; strain in the latter coordination environment appears to be responsible for the lowering of symmetry from monoclinic to triclinic.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 1006-1009 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The triclinic superstructure of a small crystal of LiCa2Nb3O10, lithium dicalcium triniobium decaoxide, has been investigated by synchrotron X-ray diffraction. The unit cell is an almost rectangular parallelepiped, although there is a 0.245° offset from orthogonality for β. The structure essentially belongs to a homologous series of Li[Nan−3Ca2NbnO3n+1] with n = 3, where the moiety in square brackets has a perovskite-type slab structure. The superstructure has a doubled unit-cell volume with respect to the tetragonal aristotype. The NbO6 octahedra are rotated about axes parallel to [110] by approximately 10°. Adjacent slabs are connected by Li atoms and are geometrically related by 42 pseudosymmetry lying parallel to c. There are twice as many sites as Li atoms, providing a variation of population at these Li sites.
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  • 5
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Single-crystal X-ray diffraction data [synchrotron radiation; λ = 1.2682 (4) Å] are used to solve and refine the crystal structure of heptacalcium hexatantalum heptazirconium hexatriacontaoxide, Ca7Zr7Ta6O36. The structure adopts space group Fddd with cell dimensions a = 36.394 (1), b = 7.3674 (5), c = 31.006 (2) Å. The structure was solved by direct methods. Refinement using 1299 unique reflections leads to final values of R = 0.031 and wR = 0.034. The refined metal-atom ordering scheme is far from fully ordered and reminiscent of the A/B metal-atom ordering characteristic of the pyrochlore structure type. Bond-valence sums are calculated to confirm the plausibility of the crystal chemistry of Ca7Zr7Ta6O36.
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  • 6
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Magnetic circular dichroism (MCD) of resonant inelastic X-ray scattering (RIXS) of 3d5/2 → 2p3/2 decay (Ho Lα1) was measured at the Ho LIII-edge in Ho3Fe5O12. The MCD-RIXS, in which the intermediate state has the 2p 4fn + 1 configuration due to the quadrupolar transition of 2p → 4f, was also observed at the pre-edge region of the Ho LIII-edge. The obvious superposition of two peaks, which comes from the high-energy off-resonant Raman scattering and the fluorescence, could be found in both the RIXS and the MCD-RIXS when the energy of the incoming X-ray was 7eV higher than the white line. The dependence of the integration of the MCD-RIXS spectra on the incident x-ray energy could roughly reproduce the MCD of X-ray absorption spectra (XAS).
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  • 7
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The design and performance of a new spectrometer for coincidence measurements between the Compton scattered photon and the recoil electron are described. Coincidence measurements give direct information on the three-dimensional electron momentum density (EMD) of condensed matter. The present spectrometer measures energy spectra of both the photon and the electron. The energy spectrum of electrons is measured by a time-of-flight method using single-bunch operation at the Photon Factory Accumulator Ring (PF-AR). The energy resolution obtained for the recoil electron is 190 eV, which is better than that of the photon detector, so that a momentum resolution of the three-dimensional EMD of 0.3 atomic units can be achieved.
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  • 8
    Electronic Resource
    Electronic Resource
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 5 (1998), S. 1309-1316 
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Structure factors for small hydrothermally grown yttrium and holmium orthoferrites, RFeO3 (R = Y, Ho), were measured with focused synchrotron radiation at wavelengths of 0.70 and 0.84 Å using both scintillation and high-speed avalanche photodiode (APD) detectors. Resulting APD Δρ images showed striking correlations between aspherical electron densities around Fe and rare-earth metals. Approximate high symmetry in the Δρ images indicates that cations deform the electron density far more strongly than the O atoms. The Ho—Fe magnetic interactions appear to affect the electron density distribution of the Fe atoms and the magnetic phase transitions. Space group Pnma, orthorhombic, YFeO3 (APD): Mr = 192.76, a = 5.5931 (3), b = 7.6102 (4), c = 5.2806 (3) Å, V = 224.77 (2) Å3, Z = 4, Dx = 5.695 Mg m−3, μ0.84 = 15.56 mm−1, F(000) = 356, T = 293 K, R = 0.045, wR = 0.073, S = 4.83 (9) for 1282 unique reflections; HoFeO3 (APD): Mr = 268.78, a = 5.5922 (3), b = 7.6157 (5), c = 5.2798 (3) Å, V = 224.86 (2) Å3, Z = 4, Dx = 7.939 Mg m−3, μ0.84 = 61.98 mm−1, F(000) = 356, T = 293 K, R = 0.036, wR = 0.037, S = 3.07 (6) for 1284 unique reflections.
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  • 9
    Electronic Resource
    Electronic Resource
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 5 (1998), S. 275-279 
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: An avalanche photodiode (APD) detector provides a sub-nanosecond time resolution and an output rate of more than 108 counts s−1 of synchrotron X-rays. Moreover, the APD has the advantage of low noise. A review of recent developments of detectors using APD devices designed for X-ray experiments is presented in this paper. One of the detectors has an excellent time response of 100 ps resolution and a narrow width on its response function, 1.4 ns at 10−5 maximum. The other consists of a stack of four diodes and has a transmission structure. The stacked detector improved the efficiency for X-rays, e.g. 55% at 16.53 keV. The output rates reached more than 108 counts s−1 per device.
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  • 10
    Electronic Resource
    Electronic Resource
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 5 (1998), S. 883-885 
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A cooled avalanche-photodiode (APD) detector has been developed for X-ray diffraction experiments. Although an APD is normally used at room temperature and a high internal gain, the energy resolution can be improved by decreasing temperature and gain. The APD device was cooled to 253 K by a thermoelectric cooler. When the gain was M = 13, the energy resolution was 5% (FWHM) at 16.53 keV with a charge-sensitive preamplifier. By scanning the discriminator threshold level of a fast-counting system, energy spectra were obtained at M ∼ 50 and count rates of up to 4.7 × 107 s−1.
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