X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

Figure III.5: Measured primary spectrum of a fine-focus Mo diffraction X-ray tube (45
kV acceleration voltage) as typically used for TXRF. The characteristic Mo
Kα and Mo Kβ lines are superimposed on the Bremsstrahlung background.
Monochromators also can modify the primary radiation and they are usually set to the
angry of the most intense characteristic line of the anode material. For a Mo-anode X-ray tube
Mo Kα or for a W-anode W-Lβ are selected, but a part of the continuum can be
monochromatized as well. Commonly used crystal monochromators have the disadvantage of
a very narrow energy band transmitted (usually in the range of few electron volts), whereas
synthetic multilayer structures are characterized by higher ∆E/E and reflectivities of up to 75
% for premium quality materials.
IV.2 Sample Reflectors
For the trace analysis of granular residues, a carrier with high reflectivity that serves as
a totally reflecting sample support is required. Therefore, the mean roughness should be in the
range of only a few nanometers and the overall flatness should be typically be less than λ/20
(λ = 589 nm, the mean wavelength of the visible light). Furthermore, reflectors should be free
of impurities so that the black spectrum should be free from contamination peaks and the
carrier material must not have fluorescence peaks in the spectral region of interest. In
addition, the carrier material must be chemically inert (also against strong chemicals, which
are often used for the sample preparation), easy to be cleaned for repeated use.
Some of the reflector materials that are in use are: quartz glass (most common), silicon,
germanium, glassy carbon, niobium, boron nitride and (as cheapest material) Plexiglas. The
requirements for the reflector are: no interfering fluorescence or diffraction lines, high purity,
chemical resistance, hardness, machineability for polishing and an acceptable price. The
surface must be flat and the mean roughness in the range of nanometers. Usually, they are
disk shaped, with 30 mm diameter and 3 – 5 mm thickness, but also squares of 30 mm side
length and rectangular types are in use.
IV.3 Detectors
Total reflection XRF is an energy dispersive XRF method, the radiation is measured
mainly by Si(Li) detectors. A good detector offers a high energy resolution [Full Width at
Half Maximum (FWHM) in the range of 140 eV at 5.89 keV], intrinsic efficiency close to 1
for the X-ray lines of interest, symmetric peak shapes, and low contribution to the
background. Primarily, incomplete charge collection at the electrodes leads to low-energy
tailing. The detector escape effect creates escape peaks and thus an increased background in
certain spectral regions. An inherent advantage of semiconductor detectors is the possibility of
bringing the detector crystal very close to the sample, which results in a large solid angle.
Light elements emit fluorescent lines in the range from 100 to 1000 eV. The usually used Be
entrance window would completely absorb them, so new window materials, offering better
transmission characteristics, are used instead.
V. Quantification