Environmental Science Program at the ALS

The ALS Core Environmental Beamlines

Environmental and Materials Science, Micro X-Ray Absorption Spectroscopy (MicroXAS) Beamline 10.3.2 Specifications

Analysis of Baltic Sea ferromanganese nodule Reference MicroXAS beamline 10.3.2 at the ALS is a bend magnet beamline that has been designed for spatially-resolved heavy metal speciation investigations. The beamline has unique capabilities to perform elemental x-ray fluorescence mapping (mXRF), x-ray microspectroscopy (mXAFS), and micro x-ray diffraction (mXRD). A pair of Kirkpatrick-Baez optics provides a variable spot size of 5m - 17m  (trading flux for spot size) in the 2.5 keV to 17 keV energy range. The virtual-source optics allows the variable spot size also help stabilize the spot position against source variations, thus improving the quality of EXAFS data. Having mXRF, mXAFS, and mXRD at a single end station provides an opportunity to probe different aspects of a sample. Together, these three microscale techniques permit a more complete analysis of speciation in environmental systems than possible with fewer techniques.

For mXRF and mXAS, the fluorescence signal is detected by a Canberra seven-element detector  with XIA electronics (DXP2X Model T) that operates at 250 eV to 300 eV resolution (FWHM at Mn Ka). With deadtime correction, EXAFS may be acquired at up to 300 kHz and fluorescence maps up to 600 kHz total count rates. XRF maps are collected using the mapping mode of the detector electronics, in which the sample stage moves continuously while pixels are filled in with energy-binned counts, a mode which is much more efficient than the conventional step-and-repeat method. XRD data are obtained using a CCD detector mounted on a rail which allows the sample to detector distance to be set to capture a range of d spacings, down to 1 Å at 15 keV. At the other end of the length scale, rings have been observed up to  40 Å. A representative exposure is about a minute at 14 keV, which provides a suitable signal from poorly crystallized or nanophase materials. However, Beamline 10.3.2 currently cannot perform XRD mapping;  mXRD mapping  experiments are conducted at Beamline 7.3.3. The mXRD mapping data is registered to the mXAS results by mXRF performed on both beamlines. The beamline control system is straightforward, well-integrated, and user-friendly. The full, detailed description of beamline capabilities and characteristics can also be found at http://xraysweb.lbl.gov/uxas/Index.htm.

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