Abstract
The Diamond Light Source (DLS) beamline I15-1 measures atomic pair distribution functions (PDF) using scattering of 40-80 keV X-rays. A unique focusing element was needed to condense these X-rays from an initial large cross section (11.0 mm H × 4.2 mm V) into a required spot size of FWHM ≈680 μm (H) × 20 μm (V) at a variable position between the sample and the detector. The large numerical aperture is achieved by coating a silicon substrate over 1 m long with three multilayer stripes of Bragg angle 4.2 mrad. One stripe selects X-rays of each energy 40.0, 65.4, and 76.6 keV. Sixteen piezoelectric bimorph actuators attached to the sides of the mirror substrate adjusted the reflecting surface's shape. Focal spots of vertical width < 15 µm were obtained at three positions over a 0.92 m range, with fast, easy switching from one focal position to another. Minimized root mean square slope errors were close to 0.5 µrad after subtraction of a uniform curvature. Reflectivity curves taken along each stripe showed consistent high peaks with generally small angular variation of peak positions. This is the first application of a 1 m long multilayer-coated bimorph mirror at a synchrotron beamline. Data collected with its help on a slice of a lithium ion battery's cathode are presented.
Highlights
1.1 Requirements for quality pair distribution functions (PDF) dataPair distribution function (PDF) data provides information about local, medium, and average long-range atomic structures simultaneously, unlike powder diffraction which can only be applied to materials that have long-range order
The geometrically demagnified full width at half maximum (FWHM) source size will be approximately 1.9 μm, much smaller than the image blurring caused by even the lowest possible slope errors
A unique, 1 m long bimorph mirror with three stripes of coatings, each composed of a different multilayer structure, has been manufactured and installed at the Diamond Light Source X-ray Pair Distribution Function beamline I15-1 to focus X-rays of 40.0, 65.4, and 76.6 keV
Summary
Pair distribution function (PDF) data provides information about local, medium, and average long-range atomic structures simultaneously, unlike powder diffraction which can only be applied to materials that have long-range order. A wiggler was chosen as I15-1’s source because, unlike standard synchrotron bending magnets or undulators, it can produce a large flux of X-rays of high photon energy even at DLS’s 3 GeV storage ring, allowing collection of scattering data up to a large Qmax and improving the resolution of the measured PDF. By making the graded multilayer’s period small enough, it was possible to raise the Bragg angle of the multilayer’s first-order reflection to 4.2 mrad, large enough to allow a mirror of useful optical length of 1 m to accept the central 0.13 mrad vertical aperture of I15-1. The variation of the grazing angle of incidence of the X-ray beam along the mirror’s length (3.991-4.455 mrad, i.e. a range of 0.464 mrad) is several times larger than the widths of these multilayer reflections as calculated by IMD [8]. Reports on continuing work to add functionalities that will make the HVBPS faster and more accurate for dynamical adaptive optics have recently been published [9,10]
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