EFFICIENCY AND EVALUATION - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 412

Several manufacturers produce and supply energy-dispersive detectors only,
in recent years mainly silicon-drift detectors. Such companies are Amptec,
Bruker, Canberra, EDAX, KETEK, Kevex, Link, Ortec, Oxford Instruments,
Princeton Gamma Tech, SGX Sensortech, Thermo Scientific, and Vortex. The
XR100 detector of Amptek already collected spectra on the surface of Mars in
1997. Today, KETEK has a market share of 75%. The main advantage of SDDs
in comparison to SiLis is the thermoelectric cooling instead of liquid nitrogen
cooling. Detectors can be cooled down to-40 ° C or even-60 ° C with an electric
energy of only 2 W up to 7 W. They are available with an active area of 10 mm 2
up to 150 mm 2 and a thickness of 500 μ m. The spectral resolution measured by
the FWHM of the Mn-K α peak of iron-55 is about 125 eV at a shaping time of
8 μ s. By reducing this time to only 0.2 μ s, the input count rate can reach the level
of 10 6 cps. A new SDD can simply be adapted to a complete TXRF system with
only little mechanical and electrical effort and leads to an easy upgrade [104].
For particular signal processing, application-specific integrated circuits
(ASICs) are developed that can contain several millions of logic gates for
digital electronics.
Multilayers are provided by other small companies such as AXO,
Dresden, and incoatec, Geesthacht, Germany. Such layers are produced
by physical vapor deposition (VPD) of heavy metals and light elements in
alternation. Between ∼ 10 and 500 different double layers are deposited
on Si substrates or quartz glass. Prominent examples are W/C, Mo/Si, Ta/Si,
Ni/C, Pt/SiC, La/B 4 C, and WSi 2 /C. Such multilayered coatings can be used
(1) as beam adapters for X-ray sources, (2) as monochromators for energy
tuning, and (3) as reference samples for thin-layer analysis. In comparison
to natural crystals, synthetic multilayers can be fabricated as plane or even
bent mirrors. Kirkpatrick-Baez and Göbel mirrors are well-known exam-
ples of X-ray optics with multilayered coatings of substrates that are
elliptically or parabolically bent or prefigured. They can collimate and
monochromatize a divergent beam simultaneously and increase the inten-
sity of X-radiation. In order to focus the X-ray beam, two mirrors can be
cross-coupledorthogonaltoeachother.Suchdevicesthatweighseveral
tons and cost $100 000 and more are usually installed at synchrotron
beamlines. Apart from such big devices, simple monolayers can be pro-
duced with a thickness smaller than 1 nm by means of VPD. Such layers of a
single element show a good lateral homogeneity down to the pm range, that
is, with a density of submonolayers [105].
Thin windows suitable for X-ray tubes, detectors, and vacuum chambers
can be purchased from special companies, such as Moxtec Inc., Orem, UT.
The windows mainly consist of beryllium or polymer foils, and their trans-
mittance depends on the thickness and the energy of X-ray photons. Ultra-
thin windows have a thickness of about 0.3 μ mandarecoatedwithathinAl
layer or a multilayer in order to absorb visible, UV, or IR light and to prevent
charging. They are supported by a silicon or carbon grid with about 75%
transmittance.