X-Ray Fluorescence Analysis
X-Ray Fluorescence - Intensity
The absorption of X-Rays by a particular material varies according to energy of the X-Rays. As a rule of thumb, low energy X-Rays are absorbed more than high energy photons.
In order to expel an electron from one of the orbitals, the X-Ray energy must exceed the binding energy of that electron – however, if the X-Ray energy is too high, then the coupling between X-Ray and electron is inefficient, and only a few electrons will be knocked out. As the X-Ray energy reduces, and approaches the electron binding energy, so the yield of expelled electrons increases. Just below this binding energy, a drop in absorption is observed, since the energy is not sufficient to emit electrons from that shell, and is too high in energy to emit electrons from the lower energy shells.
As explained in a previous slide, not all the incident X-Rays result in fluorescence. The fluorescence yield is the ratio of fluorescence X-Rays to incident X-Rays. The diagram below illustrates the K and L fluorescence yield as a function of atomic number, Z. It is clear that the yield for the light elements is very low, and this is reflected in achievable sensitivity for these elements.