Röntgenabsorption

Röntgenabsorption, or X-ray absorption, describes the reduction of an X-ray beam as it passes through matter due to interactions between X-rays and atomic electrons. The extent of absorption depends on the X-ray energy, the thickness of the material, and its chemical composition.

The dominant processes are the photoelectric effect, Compton scattering, and, at higher energies, pair production. At low to moderate energies, especially in materials with high atomic number, photoelectric absorption dominates and leads to strong attenuation. At intermediate energies, Compton scattering is the primary mechanism; at energies above about 1 MeV, pair production becomes noticeable. The relative importance of these processes shapes how X-rays penetrate different substances.

Attenuation is described quantitatively by the Beer-Lambert law: I = I0 exp(-μx), where I0 is the incident intensity, I is the transmitted intensity, μ is the linear attenuation coefficient, and x is the material thickness. The mass attenuation coefficient μ/ρ, which factors out density ρ, is often used and depends on both energy and material composition. Absorption coefficients vary strongly with energy and exhibit edges (such as the K-edge) corresponding to binding energies of electron shells; near these edges absorption rises abruptly.

Applications and implications are wide-ranging. Röntgenabsorption measurements underpin X-ray radiography and computed tomography, where contrast arises from density and composition differences. In X-ray absorption spectroscopy, features such as X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) reveal electronic structure and local geometry around specific elements. The concept is also central to shielding design and radiographic dosimetry, guiding safety and exposure planning. The term reflects the German-language usage tied to Wilhelm Röntgen’s discovery of X-rays.