• Spokesperson: Steve Sutton (U. Chicago)
• Scheduling and Scientific Oversight: Tony Lanzirotti (U. Chicago)
• Local Contact: Sue Wirick (U. Chicago)
• Safety Coordinator: Sue Wirick
• Training Coordinator: Sue Wirick
• Beamline Staff: William Rao (U. Kentucky)

       The x-ray microprobe at beamline X26A is primarily operated in focused, monochromatic mode utilizing microfocusing optics consisting of two, 100 mm long, dynamically bent silicon mirrors arranged in a Kirkpatrick-Baez (KB) geometry (designed and built by P. Eng, CARS-University of Chicago). The KB mirrors (40:1, 9 meters from the source) focus a 400 x 400 µm beam down to about 5-10 µm (FWHM) resulting in a gain in flux/µm² of about 1500 over a pinhole. Photon Flux at 18 keV is roughly 1x10⁹ photons/sec in focused monochromatic mode. The focusing mirrors are Rh coated and reside inside a He enclosure. The incoming white beam is collimated using a water-cooled, copper 'ladder and slit' assembly. Typically this beam is collimated to 400 µm in the vertical to reduce effects of thermal heating on the monochromator. A second set of tantalum, motor-driven 4-jaws are then used to collimated the beam entering the hutch (either monochromatic or white).

       Two channel-cut, silicon crystal monochromators are available to monochromatize the x-ray beam for XAFS applications, one with a (111) lattice cut and the other with a (311) lattice cut located 6 meters from source. Both crystals are cut to a 7 mm gap and are rotated using a Huber 410 one-circle goniometer (40:1 gear reducer). Crystals are cooled to 9°C using a Neslab chiller. The crystals can be translated into position using a motorized translator. To minimize backgrounds from Compton scattering, the X-ray detectors reside in the storage ring plane and at 90° to the incident beam. Currently the detectors we have available include a Canberra 9-element HPGe Array detector, Canberra SL30165 Si(Li) detector (rarely used), two Radiant Vortex-EX Silicon Drift Diode (SDD) detectors, and a Microspec Wavelength Dispersive Spectrometer (requires prior notification for use). The HPGe and SDD solid state detectors are the default systems in daily use and utilize the XMap series of compact PCI based digital spectrometers produced by XIA. Custom made mini ion chambers and PIN diode detectors are also available for transmission analysis. The sample stage sits at 45° to the incident beam, allowing an optical microscope with a gigabit ethernet CCD attachment to be mounted horizontally to view the sample surface in normal incidence.

       The entire microprobe apparatus, including final beam collimator, sample stage, optical microscope and X-ray detector, rests on a 107 x 107 cm breadboard, which in turn sits on a motor-driven lift table. The lift table allows the entire instrument to be positioned vertically and horizontally to intercept the most intense and most highly polarized portion of the synchrotron radiation profile and also pivot about an arbitrary fixed point in space. The X26A x-ray microprobe is currently capable of trace element analyses with ~ 1 ppm sensitivity and X-ray absorption near edge spectroscopy (XANES) analyses with 10-100 ppm sensitivity. Microdiffraction analysis is also available utilizing Bragg techniques, instrumentation includes Rayonix SX-165 and MAR 345 array systems. Both systems offer large input active area and highest available spatial resolution, optimized for collection of data out to higher 2 theta angles, and on very weakly diffracting samples. Since X26A is optimized for spatially collimated microbeams, we have been able to obtain high-resolution microdiffraction data on very small (5 µm) crystals. A fluorescence microtomography setup is also available, but arrangements must be made in advance.