High-flux hard X-ray microbeam using a single-bounce capillary with doubly focused undulator beam

A pre-focused X-ray beam at 12 keV and 9 keV has been used to illuminate a single-bounce capillary in order to generate a high-flux X-ray microbeam. The BioCAT undulator X-ray beamline 18ID at the Advanced Photon Source was used to generate the pre-focused beam containing 1.2 × 10¹³ photons s⁻¹ using a sagittal-focusing double-crystal monochromator and a bimorph mirror. The capillary entrance was aligned with the focal point of the pre-focused beam in order to accept the full flux of the undulator beam. Two alignment configurations were tested: (i) where the center of the capillary was aligned with the pre-focused beam (`in-line') and (ii) where one side of the capillary was aligned with the beam (`off-line'). The latter arrangement delivered more flux (3.3 × 10¹² photons s⁻¹) and smaller spot sizes (≤10 µm FWHM in both directions) for a photon flux density of 4.2 × 10¹⁰ photons s⁻¹ µm⁻². The combination of the beamline main optics with a large-working-distance (approximately 24 mm) capillary used in this experiment makes it suitable for many microprobe fluorescence applications that require a micrometer-size X-ray beam and high flux density. These features are advantageous for biological samples, where typical metal concentrations are in the range of a few ng cm⁻². Micro-XANES experiments are also feasible using this combined optical arrangement.