FUNDAMENTALS OF X-RAY FLUORESCENCE - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 69

Figure1.35. Penetration depth of X-rays hitting three different media at a variable glancing angle.
The curves were calculated for X-rays of Mo-K α with a photon energy of 17.44 keV. The critical
angles are marked by dashed vertical lines. The dashed oblique straight line represents the
penetration depth in a roughened silicon surface for which total reflection disappears. Figure
from Ref. [8], reproduced with permission. Copyright1996, John Wiley and Sons.

depth linearly decreases with the glancing angle even below the critical angle,
as is demonstrated in Figure 1.35 for silicon.
The curves of Figure 1.35 were calculated for the photon energy of the
chosen Mo-K α radiation. The influence of the different photon energies on the
penetration depth is shown in Figure 1.36—here for silicon. The points of
inflection shift to lower critical angles with increasing photon energy. Further-
more, the curves are stretched to higher depth values for normal reflection
while the depth values for total reflection remain constant at

s

A
Z

1
ρ

z 0 ≅ 3 : 424 (1.80)

This minimum is a material constant that is only dependent on the quantity C m
but is independent of the photon energy and is listed in Table 1.11 for various
media. This table also gives critical penetration depths calculated for α crit and
three different photon energies according to Equation 1.79.
In comparison to the penetration depth, a further quantity is important—the
information depth. It is the depth of a sample from which secondary radiation
emerges and reaches the detector for X-ray fluorescence analysis. This infor-
mation depth is always smaller than the penetration depth. For high-energy
peaks (energy of the emitted radiation is only a little smaller than the photon
energy of the primary radiation used for excitation) it is of the order of the