Absorption and scattering cross-sections

The interaction between electromagnetic radiation and matter represents one of the most varied classes of phenomena in the whole of experimental physics. Even within the range of energies normally associated with X-rays (itself covering several orders of magnitude of the electromagnetic spectrum) many different processes occur, all of which possess their own individual characteristics.

The nature of the matter with which the radiation interacts offers almost as wide a range of phenomena as does the nature of the radiation. This is true even within the relatively restricted domain of X-ray physics. For example, the subject of X-ray crystallography is essentially a study of the interactions between ordered matter and a radiation field, and any discussion of the absorption and scattering processes in crystals must have as its basis the collective behaviour of a large number of atoms bound by chemical bonds or other interatomic forces into a recognisable structure.

However, in the present work we are concerned mainly with situations in which the overall behaviour of an absorber or scatterer can be deduced by regarding it as a collection of individual atoms each absorbing or scattering independently of its surroundings. In such cases we can assert that interactions between X- or γ-ray photons and matter are single, identifiable, processes, each associated with an individual atom, and can therefore be characterised by a cross-section.