INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 128

3.1BASICINSTRUMENTALSETUP

In Section 2.2, it was pointed out in detail that standing waves appear when
X-rays interfere at external total reflection. The standing waves may arise in
front of a thick and flat substrate and/or within a layered structure on top of such
a substrate. Sample material placed in the field of standing waves can be excited
to X-ray fluorescence. Two cases may be distinguished in general: (1) For a
granular residue, the fluorescence intensity will be constant, that is, independent
of the glancing angle if this angle is reduced below the critical angle of total
reflection. The spectral background arising from Rayleigh and Compton
scattering of the primary beam is likewise constant. (2) For thin layers, however,
the fluorescence intensity will be angle dependent—the layers may be buried in
or deposited on a thick substrate and may be self-reflecting or not.
In order to take such measurements, great demands are made on

instrumentation:

• The glancing angle of incidence for the primary beam must be rather small
in order to ensure external total reflection. The critical angle is on the
order of 0.1 ° for primary X-rays of some 10 keV as normally applied (see
Table 1.9).
• The primary beam should be shaped like a strip of paper, realized by an
X-ray tube with a line-focus. Apertures have to restrict the beam to some
10 μ m in height and about 10 mm in width because the detector window is
usually smaller than 1 cm in diameter.

For the examination of a granular residue, case 1, only one single measure-
ment at a fixed angle setting has to be carried out (an“angle cut”by TXRF).
Two conditions have to be met:

• The glancing angle must be set to about 70% of the critical angle of total
reflection. Depending on excitation energy and carrier material, it should
be fixed to maybe 0.07 ° . The divergence of the primary beam should be
restricted to only 0.01 ° . For a stable setting of such small angles, the
instrumental arrangement has to be very solid and compact.
• Intensive spectral peaks or a broad band of the primary brems-continuum
should first be selected for excitation. The high-energy part of the primary
spectrum must be eliminated by a filter so that total reflection can occur at
a small but not too small angle according to Equation 1.68. This high-
energy part of the spectrum would not be totally reflected under the larger
glancing angle but would lead to an increased background. A prior low-
pass or bandpass filtration prevents this detrimental effect.

For the examination of thin layers, case 2, not only a single measurement at a
fixed angle is needed but an angle-dependent intensity profile has to be