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

Ge
InSb
PET
AdP
TIAP
OVO-55
OVO-N
OVO-C
OVO-B
Germanium
Indiumantimonide
Pentaerythite
Ammoniumdihydrogenphosphate
Thalliumhydrogenphthalate
Multilayer [W/Si]
Multilayer [Ni/BN]
Multilayer [V/C]
Multilayer [Mo/B4C]
II.3.4 Dispersion, Line Separation
P, S, Cl
Si
Al – Ti
Mg
F, Na
O – Si (C)
N
C
B (Be)
0.653
0.7481
0.874
1.0648
2.5760
5.5
11
12
20
The extent of the change in angle ∆θ upon changing the wavelength by the amount ∆λ
(thus: ∆θ/∆λ) is called “dispersion”. The greater the dispersion, the better is the separation of
two adjacent or overlapping peaks. Resolution is determined by the dispersion as well as by
surface quality and the purity of the crystal.
Mathematically, the dispersion can be obtained from the differentiation of the Bragg
equation:
∆θ n
=
∆λ 2dcosθ . (IV.4)
It can be seen from this equation that the dispersion (or peak separation) increases as the
lattice plane distance ‘d’ declines.
II.3.5 Synthetic Multilayers
Multilayers are not natural crystals but artificially produced ‘layer analyzers’. The
lattice plane distances ‘d’ are produced by applying thin layers of two materials in alternation
on to a substrate (Figure IV.9). Multilayers are characterized by high reflectivity and a
somewhat reduced resolution. For the analysis of light elements the multilayer technique
presents an almost revolutionary improvement for numerous applications in comparison to
natural crystals with large lattice plane distances (e.g. RbAp, PbST, KAP).
Figure IV.9: Diffraction in the layers (here: Si/W) of a multilayer.