Abstract
In anticipation of the increased use of coherent X-ray methods and the need to upgrade beamlines to match improved source quality, here the coherence properties of the X-rays delivered by beamline 12ID-D at the Advanced Photon Source have been characterized. The measured X-ray divergence, beam size, brightness and coherent flux at energies up to 26 keV are compared with the calculated values from the undulator source, and the effects of beamline optics such as a mirror, monochromator and compound refractive lenses are evaluated. Diffraction patterns from slits as a function of slit width are analyzed using wave propagation theory to obtain the beam divergence and thus coherence length. Imaging of the source using a compound refractive lens was found to be the most accurate method for determining the vertical divergence. While the brightness and coherent flux obtained without a monochromator (`pink beam') agree well with those calculated for the source, those measured with the monochromator were a factor of three to six lower than the source, primarily because of vertical divergence introduced by the monochromator. The methods described herein should be widely applicable for measuring the X-ray coherence properties of synchrotron beamlines.
Highlights
Coherent X-ray methods are providing revolutionary new capabilities for observing nanoscale dynamics and imaging atomic structure in materials
In the far field of the source, the ideal divergence of the X-ray beam is determined by the size of the source, and the ideal size of the X-ray beam is determined by the divergence of the source (Borland, 1989)
(gap = 20.3 mm), giving a total power of 431 W and central power density of 10.6 kW mradÀ2, the horizontal divergence measured is larger than that calculated from the source, and the measured beam size is smaller, indicating a focusing effect of the beamline mirror when no heating is applied
Summary
Coherent X-ray methods are providing revolutionary new capabilities for observing nanoscale dynamics and imaging atomic structure in materials. Previous studies have characterized the divergence (and the transverse coherence length) of the X-rays at synchrotron beamlines by measuring the apparent size of the source, using either a small aperture as a ‘pinhole’ camera (Borland, 1989; Mills et al, 1990; Elleaume et al, 1995; Cai et al, 1996; Sandy et al, 1999) or focusing optics to produce an image (Cai et al, 1997) We use both of these methods to determine divergences: measurements of vertical and horizontal diffraction patterns from slits, and measurements of the vertical focus size produced by a compound refractive lens. The methods we describe to characterize coherence properties are of general applicability to synchrotron X-ray beamlines
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