2.6. Primary Absorption

When any radiation (specifically X-rays) passes through a material, its intensity will be decreased (attenuated) by absorption from the elements in the material. Attenuation is due to two effects:

• Compton scattering (s) of the beam, which diverts photons in directions different from the primary beam
• Photoelectric absorption (t), which produces fluorescence or Auger electrons (Figure 2.6).

 

These effects may be combined into a single bulk mass absorption coefficient (m). This coefficient is different for each element and wavelength and is defined as a the sum of photoelectric and Compton scattering:

However, in most geological materials photoelectric absorption accounts for about 95 percent of the absorption and Compton scattering can be ignored. Additionally, Compton scattering is not important at wavelengths less than 1 Å.

Figure 2.6. Schematic representation of the major processes by which X-rays interact with the atoms of an absorbing medium (after Williams 1987).