Scattering coefficient: mass attenuation

Figure 5 X-ray mass attenuation coefficients for aluminum as a function of photon energy. At low energies, photoelectric absorption predominates. At higher energy, incoherent (Compton) scatter becomes almost the exclusive contributing mode. Eventually, pair production dominates at very high energies (above 10 MeV).

Fig. 1(a). Mass attenuation coefficient of X-ray and 7-photons in water, aluminium, iron and lead [2], (Note the relatively small dependence on chemical structure except at low energies, due to the photoelectric effect and, at high energies, to pair production, (b) Total energy absorption due to photoelectric effect, Compton scattering and pair production [2],... 

The m values for a tissue vary in the range 4-5 for the photoelectric effect 2-3 for the coherent (Rayleigh) scattering 1 for the incoherent (Compton) scattering. K E) is a function of photon energy for each partial process. For a specific interaction the mass attenuation coefficient for the individual element is given by... 

For accurate quantitative woik, the mass attenuation coefficient is used in place of the mass absorption coefficient. The mass attenuation coefficient takes into account both absorption and scattering of X-rays by the sample. 

It is important to rcali/c that the X-rays produced in the fluorescence process are generated not only from atoms at the surface of a sample bui also from atoms well below ihc surface. Thus, a part of both the incideiU radiation and the resuliiiigfluorescence iraverse a significani thickness of sample within which absorption and scattering can occur. T he extent either beam is attenuated depends on the mass absorption coefficient of the medium. which in turn i.s determined by the absorption coefficients of all of the elements in the sample. T here-... 

Where lo and I represent the irradiance (sometimes called intensity, that is the energy per unit area of a unidirectional beam). The transmitted intensity can be experimentally measured with a spectrometer. Equation 3.1 is identical to Beer-Lambert law except for the interpretation of the attenuation coefficient t. In the Beer-Lambert Law, t corresponds to the mass or molar extinction coefficient depending on the units of the concentration of the analyte. The attenuation coefficient or turbidity is directly related to the absorption and scattering characteristics of the particles. For a very dilute system of polydisperse particles, the turbidity can be expressed as an integral and is sometimes called the turbidity equation. ... 

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