General description

The wavelength dispersive spectrometer of ID21 employs polycapillary optics for x-ray fluorescence detection and is operated in a flat crystal geometry. The schematic geometry of the WDS configuration is shown in Fig.1. The polycapillary optics is placed at a distance of a few millimetres from the sample. This optics converts the divergent x-ray fluorescence emitted from the sample into a quasi-parallel beam, which is then directed onto the flat crystal at the required Bragg theta angle. The x-rays diffracted by the crystal are counted by a detector placed at the 2-theta angle. In order to cover an x-ray energy range between 0.52 keV and 9.2 keV the spectrometer is equipped with five different crystals. The available crystals are listed in Table 1 together with 2d lattice constant and energy range accessible for each of the crystal. For x-ray detection, a gas-flow proportional counter is employed. This detector has an ultra-thin polymer window 22 mm in diameter (Moxtek, 2009), and is operated with an anode wire voltage of 1.7-1.9 kV. A standard P10 gas counter mixture (10% methane and 90% argon) is flowed through the detector at atmospheric pressure at a rate of about 25 sccm. For the online signal control, the same electronics as for EDS detectors is used. In order to remove the higher orders of reflection from the crystal and spurious low energy events from the gas detector noise, only the x-ray events within a suitable energy window are counted. The appropriate energy window is determined based on the energy spectrum recorded by the gas detector at the Bragg angle corresponding to the energy of the x-ray fluorescence line of interest.

The detailed specification and description of the WDS of ID21 can be found in the following article: J. Szlachetko, M. Salome, J. Susini, J. Morse, M. Cotte, J.-Cl. Dousse, J. Hoszowska, Y. Kayser, Wavelength-dispersive spectrometer for micro-fluorescence analysis at X-ray microscopy beamline ID21 (ESRF) Journal of Synchrotron Radiation 17, 400 (2010).


WDS geometry design

 Figure 1. Schematic drawing of the parallel beam wavelength-dispersive arrangement employing polycapillary optics for x-ray fluorescence collection.


Crystal 2d (A) Energy (eV)
 TlAP(001)  25.900  520-1400
 ADP(101)  10.648  1270-3400
 Si(111)  6.27  2150-5780
 Ge(220)  4.00  3370-7500
 LiF(220)  2.84  4800-9500


Table 1. List of crystals, 2d lattice constants and energies range for WDS of ID21.

Spectrometer performances

 In comparison to the solid state detectors employed at ID21, the WDS system provides better energy resolution for detected x-rays. The obtained experimental resolution versus the measured photon energy is presented on Figure 2. As shown, depending on measured energy and on employed crystal for x-ray diffraction the energy resolution of the WDS is in the range between 5 eV to 35 eV. For demonstration, several x-ray fluorescence spectra from reference standard samples are also presented in Figure 3.

 In terms of detection limits (DLs), the latter were determined for several elements using NIST-610 and -612 glass-matrix standard samples. DLs of 35 ppm for Fe (Kα=6.404 keV, Ebeam=7.2 keV); 23 ppm for Ti (Kα=4.511 keV, Ebeam=5.5 keV); and 46 ppm for K (Kα=3.314 keV, Ebeam=4.6 keV) were obtained, all at 1 s exposure time and with incident photon flux in the range of 2 to 5 x 1010 ph/s. Please note that the reported DLs can drastically change depending on Z average of  matrix samples. 



Figure 2. Measured energy resolution of the spectrometer equipped with a TlAP(001) (in black), ADP(101) (in red), Si(111) (in green) and Ge(220) (in blue) crystals.


Emission spectra

Figure 3 Examples of x-ray fluorescence spectra recorded with the WDS spectrometer for different reference samples.