Measurement of rocking curve wings at high x-ray energies

Measurements done recently at the NSLS have indicated that the level of intensity found in the wings of diffraction peaks from silicon at higher x-ray energies (>20keV) far exceeds the value which would be predicted based on the dynamical theory. We have measured Si(220) double crystal rocking curves at the 40keV fundamental and harmonics with various crystal scattering geometries: Bragg-Bragg, Laue-Bragg, Laue-Lauel. The comparison of the Bragg and Laue case diffraction geometries was done to determine scattering volume effects. Comparisons with dynamical theory calculations will be discussed. These measurements have been carried out in order to assess the level of harmonic contamination which will be present from a double crystal monochromator being designed for the X17 Superconducting Wiggler Beamline

CT imaging of small animals using monochromatized synchrotron x rays

Rats and chicken embryos were imaged in vivo with a prototype Multiple Energy Computed Tomography (MECT) system using monochromatized x rays from the X17 superconducting wiggler at the National Synchrotron Light Source. The CT configuration coated of a horizontal low-divergence, fan-shaped beam, 70 mm wide and 0.5 mm high, and a subject rotating about a vertical aids. A linear-array high-purity Ge detector with 140 elements, each 0.5 mm wide and 6 mm thick, was used with a data acquisition system that provides a linear response over almost six orders of magnitude of detector current. The dual photon absorptiometry (DPA) algorithm was applied to images of the rat head acquired at 20 and 45 keV to obtain two new images, one representing the low-Z, and the other the intermediate-Z clement group. The results indicate that the contrast resolution and the quantification accuracy of the images improve stepwise; first, with the monochromatic beam and, second, the DPA method. The system is a prototype for a brain scanner

Novel Temperature Dependence of the Smectic−C Tilt in the Polymeric Liquid Crystal, C−6 Polysiloxane: A High−Resolution X−Ray Study

High-resolution x-ray scattering measurements of the smectic-C phase of the polymeric liquid crystal, C-6 polysiloxane, show that the smectic-C tilt angle φp decreases continuously upon decreasing temperature. This behavior, opposite to that observed in monomeric liquid crystals, is caused by stiffening in the polymeric backbones and spacers

High resolution X-ray measurements of the nematic-smectic C phase transition in the polymeric liquid crystal C-6 polysiloxane

We report on high-resolution X-ray measurements of the nematic to smectic C phase transition in the polymeric liquid crystal, C-6 polysiloxane. Preliminary results show that smectic fluctuations measured in the nematic phase are roughly consistent with a Chen and Lubensky type of theory, as was developed for monomeric liquid crystals. The phase transition in C-6 polysiloxane shows pre-transitional smectic fluctuations at least 30°C above the phase transition temperature, TNC, where TNC = 37.84 ± 0.05°C. At TNC, there is a weak first order phase transition to the smectic C phase

High-Resolution X-Ray Measurements of the A Transition in the Polymeric Liquid Crystal P4.1 Polysiloxane

We report on the first high-resolution x-ray measurements of a second-order nematic to (bilayer) smectic-A phase transition in a polymeric liquid crystal, P4.1 polysiloxane. The critical exponents of the parallel and perpendicular correlation lengths and susceptibility were ν∥=0.77±0.05, ν⊥=0.57±0.08, and γ=1.33±0.10, respectively. The bare correlation lengths, ξ0∥q0 and ξ0⊥q0, were 3.27 ± 0.08 and 1.09 ± 0.14, respectively, and are unusually large in comparison to monomeric liquid crystals

Nematic−Smectic−C phase transition in the polymeric liquid crystal C−6 polysiloxane: A high resolution x−ray study

We report on the first x-ray scattering measurements of pretransitional smectic-C fluctuations in the nematic phase of a side-chain polymeric liquid crystal, C-6 polysiloxane. The data are well described by the mean-field theory of Chen and Lubensky suggesting that the mesogenic side groups are only weakly coupled to the polymeric backbone. Because of disorder in the polymer backbone, the correlation lengths, in the vicinity of the nematic-smectic-C phase transition, are weak functions of temperature and their magnitudes are smaller than those of conventional monomeric liquid crystals

Measurement of Rocking Curve Wings at High X-Ray Energies

Measurements done recently at the NSLS have indicated that the level of intensity found in the wings of diffraction peaks from silicon at higher x-ray energies (>20keV) far exceeds the value which would be predicted based on the dynamical theory. We have measured Si(220) double crystal rocking curves at the 40keV fundamental and harmonics with various crystal scattering geometries: Bragg-Bragg, Laue-Bragg, Laue-Lauel. The comparison of the Bragg and Laue case diffraction geometries was done to determine scattering volume effects. Comparisons with dynamical theory calculations will be discussed. These measurements have been carried out in order to assess the level of harmonic contamination which will be present from a double crystal monochromator being designed for the X17 Superconducting Wiggler Beamline