Focused beam powder diffraction with polycapillary and curved crystal optics

Abstract

Focusing x-ray optics can be used to increase the intensity onto small samples, greatly reducing the data collection time for powder diffraction. Typically, the beam convergence is restricted to avoid loss of resolution since the focused beams broaden the resulting powder diffraction rings. However, the resolution, as defined by the uncertainty in peak location, can be much less than the peak width. Two types of x-ray optics, polycapillary and doubly curved crystals, were used to focus x rays onto standard inorganic powder diffraction samples. Comparisons were made of system resolution and diffracted beam intensity using low power microfocus sources.

Focusing Polycapillary Optics and Their Applications

“…While powder diffraction measurements are most commonly performed with collimating optics to reduce beam divergence, the nearly Gaussian peaks produced by the polycapillary optics provide uncertainties in peak center localization which is much less than the peak widths [33]. Thus the peak location uncertainty for powder diffraction are much smaller than the beam angle, even for highly convergent beams [36]. Polycapillary optics are also used in synchrotron systems, for example, to evaluate stresses in steel [105] and in other confocal geometries [106].…”

Modeling of a compound imaging system with a curved monochromator and polycapillary optics

“…This is supported by powder diffraction data taken with this optic, which showed significant resolution degradation if multiple optic segments were illuminated. 5 The simulated focal spot is shown in Figure 7.…”

“…where c is the channel diameter, f is the focal length, and the critical angle of grazing incidence is 30 keV mrad/E 1.7 mrad c    (5) for Mo Kα 1 . The actual spot of the polycapillary when placed after the DCC was measured to be 120 µm x 122 µm, with uncertainties of 22 µm, the pixel size of the camera used to image the spot.…”

Analysis of focused-beam powder X-ray diffraction resolution using doubly curved crystal optics

“…The size of the irradiated sample spot (in this case the focal spot size) and the beam divergence contribute to the broadening of the diffraction rings. The peak widths were estimated using a simple divergence geometry (Bingö lbali et al, 2008;Zhou, 2009),…”

“…In general, the beam convergence is restricted to avoid loss of resolution, since focused beams broaden the diffraction rings in powder X-ray diffraction (XRD). Since doubly curved crystal (DCC) optics (Berreman et al, 1975;Wittry & Golijanin, 1988) provide focusing from conventional sources, they may be employed in powder XRD to obtain higher diffraction intensity (Bingö lbali, 2008;Bingö lbali et al, 2008). A photograph of a DCC optic is shown in Fig.…”