A wavelength dispersive detector for synchrotron x-ray fluorescence microprobe analysis (abstract)
Authors: Rivers Mark L. , Sutton Stephen R. 
Published: July 2002

The synchrotron x-ray fluorescence (SXRF) microprobe has proven to be a valuable tool for trace element research. It permits analysis down to a few parts per million of many elements in a spot size of less than 10 μm. Existing SXRF microprobes are using energy dispersive detectors (EDS), either Si(Li) or intrinsic Ge diodes. Such detectors have the advantage of collecting the entire fluorescence spectrum at once. They can also be positioned to collect a relatively large solid angle. However, EDS detectors suffer from several significant problems: resolution at Fe Kα is about 150 eV, which is roughly 60 times the natural linewidth; the maximum count rate is less than 20 000 counts/s in the entire spectrum; there is significant low-energy background due to scattering and incomplete charge collection in the device. For geochemical analyses these limitations preclude trace element analyses in the presence of a large amount of a high atomic number element: for example, trace element studies of galena (PbS) and zircon (ZrSiO4), or measurements of Cr or Ti in minerals with more than a few percent Fe or Mn. The poor energy resolution prevents the measurement of small amounts of rare-earth elements in samples with significant concentrations of first-row transition elements. Wavelength dispersive spectrometers, based upon Bragg diffraction from a bent crystal, have several distinct advantages over EDS detectors. The resolution at Fe Kα is about 10 eV, or only 4 times the natural linewidth. This permits the analysis of rare-earth elements and also lowers the background which improves detection limits to the 0.1 ppm range.

The WDS spectrometer only detects a single energy at once, so it is possible to measure trace elements in the presence of intense fluorescence of a major element. We have installed a commercial wavelength dispersive spectrometer (model WDX-3PC from Microspec Corp., Fremont, CA) on the X-26A microprobe beamline at the NSLS. The spectrometer can scan the range from 33° to 135° 2θ. It contains four analyzing crystals (TAP, PET, LiF200, LiF220) mounted on a motor-driven turret, which cover the energy range from 1 to 17 keV. The detector is equipped with tandem proportional counters: a thin-window flow counter (P-10 gas) followed by a Be-windowed sealed Xe counter. A remotely adjustable exit slit is located just before the flow counter. This slit can be used to trade off count rate for energy resolution. Measured resolution at Fe Kα is 11 eV. The peak/background ratio on Fe metal is 105, which is roughly 100 times better than with a Si(Li) detector. The measured collection efficiency varies from roughly 10−3 to 10−4, which is a factor of 3–10 lower than that for the Si(Li) detector as it is normally used at X-26A. The X-26A microprobe has been configured to allow simultaneous use of both the WDS and Si(Li) detector. The detectors complement each other nicely, with the Si(Li) providing an overview of the entire spectrum and the WDS available to study selected peaks with significantly better energy resolution and sensitivity.