J. Hoszowska

Diamond planar refractive lenses for third- and fourth-generation X-ray sources

The fabrication and testing of planar refractive hard X-ray lenses made from bulk CVD diamond substrates is reported. The lens structures were generated by electron-beam lithography and transferred by reactive-ion etching into the diamond. Various lens designs were fabricated and tested at 12.4 and 17.5 keV photon energy. Efficiencies of up to 71% and gains of up to 26 were achieved. A line focus of 3.2 micro m (FWHM) was measured. These lenses should be able to withstand the extreme flux densities expected at the planned fourth-generation X-ray sources.

Deep reactive ion etching of silicon and diamond for the fabrication of planar refractive hard X-ray lenses

We report on the fabrication of diamond and silicon refractive X-ray lenses. These are made using e-beam lithography and deep reactive ion etching. The diamond lenses are promising candidates for optics meeting the extreme requirements of planned X-ray sources based on the free electron laser principle. The silicon lenses are well suited for micro-focusing applications at hard X-ray energies above 20 keV. We measured efficiencies up to 78% and a width of the line-focus down to 3 µm for these lenses. The possibility to compensate for lens aberrations using stacked lenses is discussed. A simple method for the stacking of two silicon devices (lenses) by means of micro-mechanical alignment marks is shown, giving an alignment accuracy in the micron range.

Characterization of synthetic diamond crystals by spatially resolved rocking curve measurements

For several years the ESRF, the University of the Witwatersrand and De Beers Industrial Diamonds (PTY) Ltd through their Diamond Research Laboratories have pursued a development programme to improve the quality of synthetic diamond crystals. Recently, in an effort to study the influence of nitrogen impurities on the defect structure, x-ray excited optical luminescence and spatially resolved double-crystal diffractometry were employed as new techniques. Finally, the impurity concentration distribution obtained by optical spectroscopy was superimposed on the defect structure determined by x-ray diffraction. The correlation between nitrogen impurities and the raw defect structure was clearly visible. It was confirmed that concentration variations are related to lattice imperfections, where tilts are much superior to lattice parameter variations.

Diffractive refractive optics: the possibility of sagittal focusing in Laue-case diffraction.

The sagittal deviation of a Laue-diffracted X-ray beam caused by the inclination of an exit crystal surface with respect to an entrance crystal surface has been studied both theoretically and experimentally. The use of this effect for sagittal focusing of X-ray synchrotron radiation diffracted by a Laue crystal is suggested. The focusing is based on the refraction effect due to the parabolic profile of an exit or/and entrance surface. The crystal is not bent. In order to achieve a reasonable focusing distance, the crystal should be cut asymmetrically. The experiment was performed at beamline BM5 at the ESRF.

Performances of synchrotron X-ray monochromators under heat load. Part 2. Application of the Takagi–Taupin diffraction theory

Abstract The aim of this work is to generate the rocking curves of monochromators exposed to heat load in synchrotron radiation beams with a computer code performing diffraction calculations based on the theory of Takagi and Taupin. The model study starts with the calculation of deformation by finite element analysis and from an accurate characterization of the incident wave and includes the simulation of the wavefront propagation between the first and the second crystal (analyzer) of a double crystal monochromator. A monochromatic plane wave as well as a polychromatic spherical wave approach is described. The theoretical predictions of both methods are compared with experimental data measured in Bragg geometry and critically discussed.

Sagittally focusing diffractive—refractive x-ray lens with a large acceptance and a long focusing distance

The high resolution, high asymmetric diffractive-refractive x-ray lens was tested at BM5 beamline in ESRF. The lens consists of two Si (111) channel-cut crystals in dispersive arrangement with the angle of asymmetry of 12.7°. The channels have a circular profile with the diameter of 22 mm. The test was performed for the energy of 7.8 - 8.1 keV and the focusing distance of 19 - 20 m. At the place of the focus the beam was squeezed horizontally from 8.8 mm (unfocused beam) to 0.4 mm, i.e. more than 20 times. To get a good reflectivity in such a highly asymmetric diffraction, the cylindrical surface had to be mechanically-chemically polished.

The Quest for Diamond Crystal Perfection to Meet the Needs of Physics: Unrealistic Dream or Attainable Target?

The ongoing need for single crystal diamond of high quality in physics research and application, has been amply demonstrated. This need cannot be satisfied by access to natural diamonds. In recent years much progress has been made in the synthesis of diamond at high temperature and high pressure. Several interrogation methods were deployed to ascertain the crystal quality attainable for HPHT diamond. It is shown that the best of such materials can approximate in quantitative terms closely to the theoretical predictions for perfect diamond.

Characterization of high-quality synthetic diamond crystals by μm-resolved x-ray diffractometry and topography

We have conducted a systematic characterization of (111)- and (100)- oriented synthetic diamond crystals comparing the best presently available specimens of two types (Ib and IIa). The samples were grown by the two major diamond producers, namely the De Beers Industrial Diamonds (Pty) Ltd. in South Africa and the Sumitomo Electric Industries Ltd. in Japan. Double-crystal x-ray diffractometry with microscopic spatial resolution and x-ray topography were employed. The type IIa crystals showed much less pronounced defect structure than the Ib crystals for the (100)- orientation, but the (111) samples were comparable. A clear correlation between the distribution of nitrogen impurities in the Ib crystals and the defect structure was observed. The rocking curve widths from small regions of all specimens were very close to theoretical values on the arcsec level, whereas for larger sample areas they were broadened due to both local defects and crystal curvature. The quality of the IIa crystals from De Beers and Sumitomo was comparable.

Influence of crystal surface roughness on the angular spread of x-ray diffracted beam

One of the factors influencing the focus size in diffractive-refractive optics is the quality of diffracting surface. If the surface is uneven, then the diffraction at each point of the surface is a combination of an asymmetric and inclined diffraction (general asymmetric diffraction). This somewhat deviates and spreads the diffracted beam. The integration over the surface hit by an incident beam gives the angular spread of the diffracted beam. It is shown that in some cases (highly asymmetric, highly inclined cut) the etched surface may create the spread of the diffracted beam, such that it causes a significant broadening of the focus. In this case a mechanical-chemical polishing is necessary.

Performance of synchrotron x-ray monochromators under heat load: How reliable are the predictions?

With the ongoing development of insertion devices with smaller gaps the heat load generated by modern synchrotron sources increases continuously. To predict the overall performance of experiments on beam lines it is of crucial importance to be able to predict the efficiency of x-ray optics and in particular that of crystal monochromators. We report on a detailed comparison between theory and experiment for a water-cooled silicon crystal exposed to bending magnet radiation of up to 237 W total power and 1.3 W/mm2 power density. The thermal deformation has been calculated by the code ANSYS and its output has been injected into a finite difference code based on the Takagi-Taupin diffraction theory for distorted crystals. Several slit settings, filters and reflection orders were used to vary the geometrical conditions and the x-ray penetration depth in the crystal. In general, good agreement has been observed between the calculated and the observed values for the rocking curve width.