INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 150

proposals are normally invited twice a year. The use of a beamline is free of
charge if the results are published afterward.
3.4THEBEAMADAPTINGUNIT
The primary X-ray beam must still be adapted with respect to its geometrical
shape and spectral distribution. Shaping the beam is simply done by two
collimator slits or at least by two metallic edges acting as diaphragms. Silver,
steel, and platinum have been chosen with a thickness of about 1 mm and a
width of about 20 mm. They are either fixed in entirely self-contained devices
or have to be adjusted in modular components. In any case, their task is to mask
out a strip-like beam of some 10 μ m thickness.
Alteration of the spectral distribution is not as easy to do as is shaping. A
metal foil is usually employed as a supplementary means of accomplishing this,
but the real goal is achieved only by a first reflector. For traceanalysis , a quartz-
glass mirror is preferably used acting as a low-pass filter [28]. However, a
multilayer can also be used, acting as a monochromator or rather as a broad
bandpass filter. Even two components or combinations of them may be utilized.
The combination quartz-multilayer and multilayer-multilayer was shown
to be highly suitable [29], whereas a crystal monochromator seemed to be
unsuitable [30]. The single or double multilayer arrangement allows selection
of a specific excitation energy without the need to change the X-ray tube. This
energy tuning can be performed by a vertical shifting and/or a horizontal
displacing of the multilayers.
For surfaceandthin-layeranalyses , a natural crystal acting as a true
monochromator or small bandpass filter may be applied as a first reflector.
However, multilayers have also been used successfully. They may possibly be
chosen as a combination of two.
3.4.1Low-PassFilters
In order to achieve total reflection at the sample carrier, the primary beam must
strike this carrier as a second reflector at a small angle of incidence. For X-ray
photons of 60 keV, the glancing angle should be set at only 0.015 ° , while the
divergence of the beam must be held to an even smaller angle. Such a narrow
primary beam would provide only poor intensity. Besides, the high energy
photons would scarcely be effective since excitation is preferably induced by
lower energy photons, for example, by the characteristic radiation of the anode
material. For this reason, the glancing angle and the divergence of the primary
beam are increased threefold. In that case, the high-energy photons would not be
totally reflected but would partly be scattered, causing a significant background.
To avoid this effect, the high-energy photons must be eliminated first. This can be
done by a first reflector acting as low-pass or cutoff filter. It only transmits the low-
energy part of the primary spectrum but eliminates the high-energy part [28,31].