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GeoSoilEnviroCARS
The University of Chicago
micro-XRF, XANES, and EXAFS Measurements
with a Wavelength Dispersive Spectrometer
The GSECARS Microprobe Station
S Sutton, M Rivers,
P Eng, M Newville
The GeoSoilEnviroCARS undulator beamline (station 13-IDC at the APS) provides a
micro-beam facility for x-ray fluorescence (XRF) and absorption (XAS) studies. The
heart of the system is a Kirkpatrick-Baez focussing mirror, which can focus
monochromatic undulator radiation down to 1x1 micron with enough intensity for XRF
mapping and XAS of dilute systems. The microprobe has been successfully used for
many problems in environmental and earth sciences.
The table-top Kirkpatrick-Baez mirrors use a four-point
bender and a trapezoidal mirror to dynamically shape a flat
mirror into an ellipsis. They can focus a 300x300mm
monochromatic beam to down to 1x1mm. With a working
distance of 100mm, they give a stable, energy-independent
focal point and spot size, and are easy to use.
Matthew Newville, Stephen Sutton,
and Mark Rivers
Consortium for Advanced Radiation
Sources, The Univeristy of Chicago
Chicago, IL 60637
Using the WDS for EXAFS measurements:
The Re LIII-edge of K7[ReOP2W17O61].nH2O
with Mark Antonio (Argonne Natl Lab)
XRF and XAFS measurements in many natural and
heterogeneous samples are complicated by the prescence of
fluorescence lines from other elements near the line of
interest. A common problem is to measure a minor element
(say, Re) in the prescence of a major element just below it in Z
(say, W) whose fluorescence line will be excited at the
absorption edge of the minor element. (dilute amounts of Co,
Ni, or Zn in the prescence of Fe pose similar problems).
Shown below is the microprobe station in 13-IDC, using
Rh-coated silicon and fused-silica mirrors.
Kirkpatrick-Baez
focusing mirrors
table-top slits
ion chamber
sample positioner
(x-y-z stage)
optical microscope
(10x to 50x) with
video system
Above is the XRF spectra for this W-rich sample measured
with the WDS. Using a Ge or Si solid-state detector, the Re
La line would be a shoulder on the W La line, the detector
would tend to saturate from the unwanted W fluorescence,
and the Re XAFS would essentially impossible to measure.
The WDS allows us to isolate the Re La line and measure the
XAFS of this previously “impossible” sample:
Re LIII-edge XAFS data for K7[ReOP2W17O61].nH2O measured with the WDS.
Clockwise from lower left:
measured m(E), isolated EXAFS
kc(k), and the Fourier transform
of the EXAFS |c(R)|.
fluorescence
detector (WDS)
The Wavelength Dispersive Spectrometer (Oxford WDX-600)
This detector uses a Johansson geometry Rowland circle to analyze fluorescent
x-rays. The sample, bent crystal analyzer, and detector slits all lie on a circle, with
the angle between sample and detector and crystal lattice spacing selecting the
accepted energy. LiF(200) and (220) crystals are typically used for hard x-ray
energies. Tandem proportional counters (one sealed with 2atm Xe, and one using
flowing P-10 gas) are the detectors. The energy resolution at 6KeV is ~10eV,
and the detector will not saturate as solid-state detectors are prone to do.
Though the solid-angle is small, the dynamic range and resolution make this
detector an excellent complement to traditional solid-state fluorescence detectors.
Examples of its use are shown throughout this poster.
Cs XRF Mapping with the WDS
with J McKinley, J Zachara, S Heald (PNNL)
Here is a photograph and an x-ray fluorescence map
of Cs (the La line) of a cross-cleavage plane of biotite
exposed to a Cs-rich solution. This natural mica
contains trace amounts of many transition metals
including Ti, and large amounts of Ca and Fe, making a
solid-state detector difficult to use for such a map.
photograph of
biotite crosssection
1000ppm Au in arsenopyrite (FeAsS)
with Louis Cabri (NRC Canada), Robert Gordon,
Daryl Crozier (Simon Fraser Univ)
The understanding of the chemical and
physical state of Au in arsenopyrite ore
deposits is complicated by the proximity of the
Au LIII and As K edges and their fluorescence
lines.
Shown below is the XRF spectra
measured with the WDS. The tail of the As Ka
line is comparable to the small Au La line even
at the energy of the Au line.
On the left is the EXAFS spectra for 1000ppm Au in
FeAsS measured with a 13-element Ge detector (at
PNC-CAT: APS sector 20). On the right is the same
sample (at two different spots) measured with the
WDS. The WDS supresses the As K-edge, making the
Au LIII -edge much easier to see and analyze. The
WDS spectra shows the Au to be heterogeneous -sometimes metallic, and sometimes oxidized.
Cs fluorescece
map of biotite
cross-section
1000 x 200 mm image of Cs fluorescence in biotite,
made with a 5x5mm beam, 5mm steps, and 2sec dwell
time per point (5hr collection time). The incident x-ray
energy was 7.0KeV.
XRF Spectra of Au in FeAsS
XAFS from a 13-element Ge detector
XAFS from the WDS