Michael Hart

Crystal structures of quartz and magnesium germanate by profile analysis of synchrotron-radiation high-resolution powder data.

High-resolution synchrotron-radiation powder data for quartz and Mg2GeO4, an olivine homologue, were obtained with a 365 mm long parallel-slit collimator with 0.05° aperture. The crystal structures were refined using a pseudo-Voigt profile fitting function and POWLS. Fourier maps were calculated from the measured structure factors of the germanate. The results for quartz were in good agreement with single-crystal data and R(Bragg) increased from 1.35 to 2.20% as the number of reflections included in the refinement increased from 46 to 119; the e.s.d.s of the positional parameters were 4 to 8 × 10−4. The germanate gave R(Bragg) 4.85% using 77 Miller planes and the e.s.d.s were 4 to 42 × 10−4. Special attention was given to the profile fitting function and the non-random particle distributions. The JCPDS Diffraction File Nos. will be reported in a later communication.

Refinement of simple crystal structures from synchrotron radiation powder diffraction data

X-ray powder diffractometry with storage-ring radiation was used to test various aspects of a method for refining simple crystal structures (Si, CeO2 and Co3O4). Excellent powder patterns were obtained with 0.17° resolution parallel slits and 1 A X-rays. The intensities were determined with a double-Gaussian profile-fitting function and used in the powder least-squares refinement program POWLS. Except for Si, the peaks were broadened because of small particle sizes. The R(Bragg) values were in the range 0.4–1.7%.

Diamond crystal X-ray optics for high-power-density synchrotron radiation beams

Man-made perfect single crystal isotopically-enriched diamond is demonstrated to be an excellent X-ray monochromator even when subjected to the highest incident power density expected at third-generation synchrotron source undulator beam lines. Double-crystal rocking curve tests of a diamond (400) wafer exposed to an X-ray power density of 207 W/mm2 (75 W total power) revealed just 1 arc sec of induced thermal distortion integrated across the beam footprint.

X-ray monochromators for high-power synchrotron radiation sources

Abstract Exact solutions to the problems of power flow from a line source of heat into a semicylinder and of uniform heat flow normal to a flat surface are discussed. These lead to bounds on feasible designs and the boundary layer problem can be placed in proper perspective. While finite element calculations are useful if the sample boundaries are predefined, they are much less help in establishing design principles. Previous work on hot beam X-ray crystal optics has emphasised the importance of coolant hydraulics and boundary layer heat transfer. Instead this paper emphasises the importance of the elastic response of crystals to thermal strainfields and the importance of maintaining the Darwin reflectivity. The conclusions of this design study are that the diffracting crystal region should be thin, but not very thin, similar in area to the hot beam footprint, part of a thin-walked buckling crystal box and remote from the support to which the crystal is rigidly clamped. Prototype 111 and 220 cooled silicon crystals tested at the National Synchrotron Light Source at Brookhaven have almost perfect rocking curves under a beam heat load of 1 3 kW .

Adaptive crystal optics for high power synchrotron sources

Abstract We report successful operation of the first X-ray monochromator with built-in correction for the thermal strain field created by a high power X-ray beam. The 27 pole wiggler magnet at the National Synchrotron Light Source provides a total power load of 330 W in a beam 60 mm wide. A (220) silicon monochromator system has been tested; the compensated deformation field contributes only 2 arc sec full width at half maximum intensity to the monochromator Darwin width of high order Bragg reflections.

Performance of water jet cooled silicon monochromators on a multipole wiggler beam line at NSLS

Abstract A 27 pole wiggler on the X25 beam line at the National Synchrotron Light Source (NSLS) provides a horizontal X-ray power density of 350 W/mrad with a critical energy of 4.6 keV. Jet cooled silicon crystal monochromators are shown to be capable of almost perfect performance at the experimental position where the available power density is about 45 W/cm of beam width and the total power is about 250 W. Both Si(111) and Si(220) monochromators have been tested and the Bragg angle variation across the beam thermal footprint is shown to be only 2.5 arc sec at the highest power levels.

Preserving the high finesse of X-ray undulator beams from perfect water-jet-cooled diamond monochromators

Abstract A perfect single crystal of isotopically enriched diamond is shown to be an excellent X-ray monochromator for future undulator beams when jet-cooled with water. Unprecedented power densities, up to 260 W/mm 2 with 63 W total X-ray power are provided by the focussed X-ray beam from a 27 pole hybrid wiggler on beamline X25 at NSLS. Double crystal rocking curves have full widths at half maximum intensity of 3–4 arc sec which are determined by mounting strain and coolant pressure effects rather than the thermal strainfield.

Tunable x‐ray polarimeters for synchrotron radiation sources

We describe the design and optical characteristics of a tunable x‐ray polarimeter set up at the National Synchrotron Light Source at Brookhaven. Initially, the energy range from 7.0 to 9.2 keV is covered with a polarization extinction ratio greater than 10 000 and a statistically limited sensitivity of 70 μrad in measurements of optical rotary power. First spectroscopic measurements on a wide range of cobalt compounds show optical activity, Faraday rotation, and circular dichroism in the energy region close to the cobalt K edge. More recently we have worked on a variety of samples containing iron near the iron K edge.

The refractive-index correction in powder diffraction

Throughout the history of powder diffraction practice there has been uncertainty about whether or not a refractive-index correction should be made to Braggs law. High-precision Bragg-angle measurements have been performed with synchrotron radiation on SRM640 silicon powders at glancing angles; it is found that little or no correction is necessary for the usual 20 angle range.

Adaptive crystal optics for undulator beamlines

Abstract These are the first tests of an X-ray monochromator with incident beam X-ray photon powers in excess of 100 W/mm 2 . Crystal monochromators cannot be 100% efficient unless the thermal strain within the beam is small compared with the Darwin range of total Bragg reflection. At room temperature in all materials such performance is impossible unless adaptive optics methods are used to compensate the inevitable strain field which results from the X-ray beam power footprint. Third generation synchrotron radiation sources incorporating undulators will produce X-ray beams with power densities in excess of 100 W/mm 2 . The 1.1 T hybrid 27 pole wiggler (X25) at the National Synchrotron Light Source, when doubly focussed, provides a unique test facility for optical components intended for third generation synchrotron radiation beam lines. At the focus the power density reaches 200 W/mm 2 . Normal to the beam the power density gradient is far greater than that predicted for future undulator beamlines so that this beam presents a “worst case” test for undulator beam optical components. We present a new design which copes well with these high power densities and has diffraction characteristics which agree quantitatively with design calculations. It therefore follows that we can extrapolate from these results and predict monochromator performance on future insertion devices.