The absorption of X-rays by a target is described by an exponential expression akin to the Beer-
Lambert law (p. 357)
where Io is the intensity of an incident beam of monochromatic X-rays and I is its attenuated intensity
after penetrating a distance b into a target of density ρ. μ is the mass absorption coefficient and depends
upon the atomic number of the element and the energy (wavelength) of the radiation only. The value of
μ increases rapidly with the atomic number of the element e.g.
When μ for an element is plotted as a function of the incident energy or the incident radiation (Figure
8.38) it shows a smooth decrease as the energy increases (wavelength decreases). Sharp discontinuities
or absorption edges appear when the ionization energy for a K or L electron is reached. At this point a
large increase in μ occurs as the X-rays are absorbed in producing ionization. The energy of X-ray
photons emitted in L to K transitions will be less than that required for complete ionization (Figure
8.36), and the Kα line for an element will appear at an energy below that of the absorption edge
corresponding to K-ionization. Consequently the target element will have a low absorption for its own
fluorescent radiation.
The matrix however may bring about considerable attenuation, and for a multi-element target the mass
absorption coefficients are additive.
Figure 8.38
Log–log plot of mass absorption coefficient
against wavelength for lead showing K and L
absorption edges.