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

Mineralogical effect were discussed by Claisse (1957a, 1957b), who tried to explain
them starting from an hypothesis according to which the fluorescence intensity would depend
on the interatomic distances separating the excited atoms in the crystal lattice. This hypothesis
might be corrected in those cases where any changes in the interatomic distances lead to
marked variations of the crystal density, and consequently, of the linear absorption coefficient
for a given radiation in the crystal. It seems, however, that the main reason for the
mineralogical effects is simply the different absorption of the fluorescent radiation in the
particles of minerals of different chemical composition.
II.5 Surface Effects
Figure VI.6: Schematic illustration of mineralogical effects.
In X-ray fluorescence analysis of solid samples, such as alloys, one observes some
effects due to surface irregularities. These effects are caused by the influence of the surface
coarseness (or surface finish) on the intensity of the detected fluorescent radiation. The
coarseness of the surface may be defined quantitatively by the dimensions of the microirregularities,
i.e., micro-protuberances and micro-cavities which occur at the sample surface.
These surface effects have been studied by, among others, Gunn (1961) and Michaelis and
Kilday (1962). Their occurrence may be attributed to the shielding (absorption) effects taking
place in individual protuberances at the sample surface with the fluorescence radiation
emerging from the sample at different angles. According to the authors cited above, the
surface irregularity effects may be of importance where both the primary and secondary
radiation beams are collimated.
The magnitude of the surface irregularity effects should depend on the energy of
fluorescence radiation and on the chemical composition of the sample (i.e., the sample
absorption coefficient). Such dependence has actually been established.
III. Mathematical Models
III.1 Sherman Equation
Use of X-ray fluorescence to determine chemical composition of unknown specimens
became more common in the following decade. With this came the need to better understand
X-ray absorption and enhancement. Sherman derived a more specific equation for the