INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 176

In general, four different crystals are necessary and sufficient to cover the
total X-ray spectrum between 0.1 up to 40 keV. For this purpose, a crystal
changer is designed to take up to four crystals. Furthermore, a changeable
collimator is provided with, for example, two different openings or diver-
gences. And finally, provision is made to interchange a gas-filled detector and
a scintillation detector. To make use of the high performance of a WDS in the
low energy range of an X-ray spectrum, a vacuum chamber is necessary
containing collimators, crystals, and detectors. A fine vacuum with a pressure
of about 1 Pa is sufficiently low for an air path of 50 cm. In that case the
transmission is greater than 97% for photons with energies above 0.1 eV (K α
peak of beryllium).

3.7.1DispersingCrystalswithSollerCollimators

Flat-crystal spectrometers first need one or two collimators in order to align
the divergent radiation of the sample and to separate a straight beam, which
can be used for Bragg reflection at a definite glancing angle α [67]. The most
suitable collimator is the Soller-type collimator consisting of an array of
several metal sheets, each with a length l and a thickness Δ . They are arranged
parallel to each other with a closed spacing δ . Such a collimator can be passed
by X-ray beams in the direction of the collimator axis with an angular
divergence 2 δ / l whereas X-ray beams with a larger divergence are absorbed
by these plates [48,49,62-64].
Usually, two collimators that are interchangeable are provided: a coarse
collimator with a divergence of about 0.5 ° and a fine collimator with a
divergence of about 0.15 ° . The fine collimator leads to a better spectral
resolution at the expense of a higher absorption and lower intensity. The
sheets are usually made of copper or steel (covered with Bi microcrystals).
They are 3-10 cm long, about 0.1 mm thick, and spaced 0.2-0.8 mm apart. The
cross-section of the collimator should be a square of about 2 cm × 2 cm.
Consequently, a stack with 20-80 metal sheets and spacers is necessary. The
divergence of the primary collimator before the crystal is < 0.3 ° and its
absorption is up to 50%. The secondary collimator before the detector is
coarse-spaced and mainly used for the elimination of stray X-rays scattered
from the crystal or its holder and from the primary collimator.
Several flat crystals can be used as dispersing elements or Bragg crystals,
respectively, and are for sale [48,49,62-64]. A list of several different crystals
with double interplanar spacing is given in Table 3.6. In the case of inorganic
compounds, naturally occurring crystals can often be used. Other dispersing
elements are“pseudocrystals”or synthetic multilayers. Such multilayered thin
soap films (esters of carboxylic acids where heavy metal atoms are separated by
long fatty acid chains) are organic . Stacks of thin layers (heavy metals and light
spacers arranged in pairs) are inorganic . The spacing values reach from about
0.16 nm up to 20 nm; their precision is < 0.1% for natural crystals (about
3 × 10

5 % for silicon) and < 0.5% for synthetic crystals.