Rayleigh and Compton Scattering

Total cross section (in units of 1 barn = 10 m ) of photon interaction in carbon and lead as a function of photon energy and the contributions of the following processes photoelectric effect, Rayleigh and Compton scattering, pair production in the field of the nucleus (nuclear p.p.) and of the shell electrons (electron p.p.). Note the resonances ( edges" as they are called) in photoelectric effect. The total cross section curve is flat for another two orders of magnitude in energy beyond the plot... 

Interactions of photons (including both X-rays and y-radiation) with matter may be classified according to (a) the kind of target, such as electrons, atoms or nuclei, with which the photons interact, and (b) the type of event, such as absorption, scattering or pair production, that takes place. Possible interactions are summarized in Table 1, where r is the total photoelectric absorption cross-section per atom, (Tr and (Tc are the Rayleigh and Compton collision cross-sections, respectively, and K is the cross-section for pair production. The sum of all these cross-sections, normalized to a per atom basis, is the probability cTjot that the incident photon will have an interaction of some kind while passing through a very thin absorber that contains one atom per unit of area normal to the path of the... 

Practical X-ray energies do not exceed 100 keV. The primary beam is mainly attenuated by the photoelectric effect. Scattering, both elastic (Rayleigh) and inelastic (Compton), represents a minor contribution to attenuation at energies below 100 keV. 

In traversing through matter. X-rays are attenuated by coherent (Rayleigh) and incoherent (Compton)scattering and are absorbed by the photoelectric process (6, 7). X-rays of energy below 100 keV are mainly absorbed by the photoelectric process with a cross section (i.e., the probability for absorption) proportional to (6), where E is the X-ray energy and Z... 

Rayleigh scattering occurs mostly at low energies and for high-Z material (see Figure 1), whereas Compton scattering shows a complementary behavior. 

The total atomic cross-section <7totai can be expressed as the sum of the cross-sections of the different elementary processes - that is, the photoelectric effect (r), coherent (Rayleigh) scattering (cTooh). incoherent (Compton scattering) (Omooh). photonudear absorption Oph.n and pair production, which is to be distinguished in nudear-field pah-production (k- ) and dectron-field pair production (iCe) (see Equation 4.3). 

The interpretation given above is simplified, since fluorescence is not the only process that allows the atom to lose its excess energy. Other phenomena such as Rayleigh scattering (elastic scattering) and the Compton effect (inelastic scattering with release of Compton electrons) can complicate the X-ray emission spectrum. 

Thomson and Rayleigh scattering are the classical equivalents of the Compton effect with the exception that no (internal) energy is transferred to the atoms, they are neither ionized nor excited, just the direction of the photon is changed. 

As radiation sources. X-ray tubes or radionuclides are applied. The polychromatic radiation of the tube is scattered by the Compton or Rayleigh effect towards the detector, which results in a high bremsstrahlung background and line interferences caused by the characteristic lines of the anode material and the resulting Compton peak. 

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