Theoretical calculation shows that suitable approximations of the line‐shape asymmetry in powder diffraction profiles should be represented by functions having limited codomains.
XPAD3S is a single-photon-counting chip developed in collaboration by SOLEIL Synchrotron, the Institut Louis Néel and the Centre de Physique de Particules de Marseille. The circuit, designed in the 0.25 microm IBM technology, contains 9600 square pixels with 130 microm side giving a total size of 1 cm x 1.5 cm. The main features of each pixel are: single threshold adjustable from 4.5 keV up to 35 keV, 2 ms frame rate, 10(7) photons s(-1) mm(-2) maximum local count rate, and a 12-bit internal counter with overflow allowing a full 27-bit dynamic range to be reached. The XPAD3S was hybridized using the flip-chip technology with both a 500 microm silicon sensor and a 700 microm CdTe sensor with Schottky contacts. Imaging performances of both detectors were evaluated using X-rays from 6 keV up to 35 keV. The detective quantum efficiency at zero line-pairs mm(-1) for a silicon sensor follows the absorption law whereas for CdTe a strong deficit at low photon energy, produced by an inefficient entrance layer, is measured. The modulation transfer function was evaluated and it was shown that both detectors present an ideal modulation transfer function at 26 keV, limited only by the pixel size. The influence of the Cd and Te K-edges of the CdTe sensor was measured and simulated, establishing that fluorescence photons reduce the contrast transfer at the Nyquist frequency from 60% to 40% which remains acceptable. The energy resolution was evaluated at 6% with silicon using 16 keV X-rays, and 8% with CdTe using 35 keV X-rays. A 7 cm x 12 cm XPAD3 imager, built with eight silicon modules (seven circuits per module) tiled together, was successfully used for X-ray diffraction experiments. A first result recently obtained with a new 2 cm x 3 cm CdTe imager is also presented.
The authors have studied the structural evolution of the fragile glass-forming liquid CaAl2O4 during supercooling from the stable liquid phase to the cold glass below Tg. The evolution is characterized by a sharpening of the first diffraction peak and a shortening of the average nearest-neighbor bond length around 1.25Tg, indicating an increase in the degree of both intermediate-range and short-range orders occurring close to the dynamical crossover temperature. The cooling curve developed a kink at this temperature, indicating a simultaneous change in thermodynamic properties.
The element and electronic shell selectivity of x-ray resonant magnetic scattering ͑XRMS͒ has been used to investigate the profile of the spin polarization of the 5d electronic states of Ce and La across the rare-earth layers in Ce/Fe and La/Fe multilayers. The magnetic contributions to the diffracted intensities have been measured at low angles, at the L 2 edge of the rare earth. In agreement with previous results from x-ray magnetic circular dichroism ͑XMCD͒ experiments, the La 5d polarization is found to be localized right at the interfaces with the Fe layers, as it is expected from a direct hybridization with the Fe 3d states. In the case of Ce/Fe multilayers where Ce is in an ␣-like electronic state with a complex behavior of the 5d magnetic polarization, the XRMS results obtained for two samples with 10 and 22 Å thick Ce layers indicate that the Ce 5d polarization decreases slowly from the interfaces towards the center of the layers. This is in agreement with previous XMCD results. However, at least for the two samples which have been investigated, XRMS also suggests that the Ce 5d polarization oscillates across the Ce layer with a period equal to the ͑111͒ interplanar distance in ␣ fcc Ce. Though compatible with the XMCD findings, this oscillating behavior cannot be derived from its dependence on the Ce layer thickness because of the decrease of the magnetic polarization which prevents us from observing changes in the sign of the XMCD amplitude. ͓S0163-1829͑99͒04637-8͔ PHYSICAL REVIEW B 1 OCTOBER 1999-I VOLUME 60, NUMBER 13 PRB 60 0163-1829/99/60͑13͒/9662͑13͒/$15.00 9662
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