Performance of nearly fixed offset asymmetric channel-cut crystals for X-ray monochromators

X-ray double-crystal monochromators face a shift of the exit beam when the Bragg angle and thus the transmitted photon energy changes. This can be compensated for by moving one or both crystals accordingly. In the case of monolithic channel-cut crystals, which exhibit utmost stability, the shift of the monochromated beam is inevitable. Here we report performance tests of novel, asymmetrically cut, channel-cut crystals which reduce the beam movements by more than a factor of 20 relative to the symmetric case over the typical energy range of an EXAFS spectrum at the Cu K-edge. In addition, the presented formulas for the beam offset including the asymmetry angle directly indicate the importance of this value, which has been commonly neglected so far in the operation of double-crystal monochromators.

Inelastic scattering and solvent scattering reduce dynamical diffraction in biological crystals

Multi-slice simulations of electron diffraction by three-dimensional protein crystals have indicated that structure solution would be severely impeded by dynamical diffraction, especially when crystals are more than a few unit cells thick. In practice, however, dynamical diffraction turned out to be less of a problem than anticipated on the basis of these simulations. Here it is shown that two scattering phenomena, which are usually omitted from multi-slice simulations, reduce the dynamical effect: solvent scattering reduces the phase differences within the exit beam and inelastic scattering followed by elastic scattering results in diffusion of dynamical scattering out of Bragg peaks. Thus, these independent phenomena provide potential reasons for the apparent discrepancy between theory and practice in protein electron crystallography.

Exploring the unlimited possibilities of modular aspheric Gauss to top-hat beam shaping

Beam shaping is a field of research with growing importance. Therefore, a new refractive beam shaping system is presented. The knowledge gained from analyzing patent systems was used to derive our own improved design. It is compared to a patent system, and some selected results are presented in this work. Furthermore, possibilities to scale the entrance and exit beam diameters with the help of SPA™ Beam Expander Kit and SPA™ AspheriColl (both from asphericon GmbH, Jena, Germany) are shown, so that a modular top-hat generation is achievable. Additionally, the large spectral range in which the beam shaping system is applicable is demonstrated, and it is demonstrated how the beam shaping system can be used to improve the performance of other optical elements that require a top-hat beam profile.

Diffraction imaging forin situcharacterization of double-crystal X-ray monochromators

Imaging of the Bragg-reflected X-ray beam is proposed and validated as anin situmethod for characterization of the performance of double-crystal monochromators under the heat load of intense synchrotron radiation. A sequence of images is collected at different angular positions on the reflectivity curve of the second crystal and analyzed. The method provides rapid evaluation of the wavefront of the exit beam, which relates to local misorientation of the crystal planes along the beam footprint on the thermally distorted first crystal. The measured misorientation can be directly compared with the results of finite element analysis. The imaging method offers an additional insight into the local intrinsic crystal quality over the footprint of the incident X-ray beam.

X-ray monochromator combining high resolution with high intensity

A two-germanium-crystal four-220-reflection (+ - - \,+) monochromator, combining high intensity with high resolution, is proposed in this work. The main characteristic is that only the first reflection is asymmetric. The asymmetry factor was chosen so as to allow mixing of asymmetric and symmetric reflections in a monolithic channel-cut crystal without the need for rotation of the two monolith components to correct for the different refraction-induced angular shifts of the reflection pair. It is demonstrated that the exit-beam divergence in the diffraction plane and the fractional wavelength band-pass are smaller by 40% than those of the widely used germanium 220 Bartels monochromator, while the photon flux collected from the source is larger by a factor of five. The optical features and performance of the monochromator are discussed and compared with those of other (+ - - \,+) monochromators reported in the literature.

Design of a time-compensated EUV and soft X-ray monochromator with multilayer mirrors for high-order harmonics

A time-compensated monochromator for ultrashort high-order harmonics in the EUV and soft X-ray regions is presented. The system consists of two grazing-incidence toroidal mirrors used, respectively, as collimating and refocusing elements and of two multilayer normal-incidence plane mirrors illuminated in parallel light that rotate along a vertical axis to select the operative wavelength, but remain parallel to guarantee the constant direction of the exit beam. This is performed by simultaneously rotating and translating one of the two mirrors along an axis parallel to the exit direction. The pulse time duration is not altered up to few femtoseconds.

High-flux and high-resolution spectroscopic facility in the VUV region at Super-ACO

A new spectroscopic facility, consisting of a planar/helical undulator and a 6.65 m off-plane Eagle monochromator, has been designed. It can supply high-flux and high-resolution photons with `exotic' polarization in the 5–40 eV energy range. The astigmatism can be compromised by horizontally focusing the incident radiation on the grating. The rotation of a post-focusing toroidal mirror compensates for the deviation of the exit beam caused by the grating translation. The whole system will be installed at beamline SU5 of SUPER-ACO (0.8 GeV) at the beginning of 1998. With a 4300 grooves mm−1 grating, a resolving power of around 105 is expected at 20 eV.