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

Abstract

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 x 10(13) 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 x 10(12) photons s(-1)) and smaller spot sizes (< or =10 microm FWHM in both directions) for a photon flux density of 4.2 x 10(10) photons s(-1) microm(-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.

Figure 1

Top: far-field image without beam stop; the ruler shows the actual size of the image (approximately 25 mm outer diameter). Bottom: the same rings with external lights turned off for better contrast, and its ray-tracing simulation.

Figure 2

Far-field images with beam stop in place with the direct beam completely blocked. The top images show the capillary aligned ‘in-line’ with the main beam, and its ray-tracing simulation. The bottom images show the capillary aligned ‘off-line’ with the main beam, and its corresponding ray-tracing simulation. The irregularities of the image of the outer rings are due to the irregular shape of the beam stop.

Figure 3

Horizontal beam profiles at 12 keV: (a) capillary in-line (12.3 µm FWHM); (b) capillary off-line (10.1 µm FWHM). Vertical beam profiles at 12 keV: (c) capillary in-line (10.1 µm FWHM); (d) capillary off-line (9.2 µm FWHM).

Figure 4

XANES spectra of a Cu foil (used as reference) (top) and a NIST standard (oyster tissue, NIST #1566b) (bottom), measured in transmission mode and in fluorescence mode, respectively. The microbeam position was monitored by following the far-field image during the energy scan. No change in beam position was observed during the scans.

Figure 5

Cu, Fe and Zn content of normal and tumor prostate tissue. Color scale units are µg cm⁻²; size scale: 100 µm. Samples courtesy of Dr Ping Dou and Dr Di Chen at Karmanos Cancer Institute, Wayne State University, USA.