Absorption cross section

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Cross-sections values for all elements with atomic number Z smaller then 100 collected for photons with energies from 1 keV to 20 MeV. The discontinuities in the values are due to compton edges which were also shown.

Absorption cross section is a measure for the probability of an absorption process. More generally, the term cross section is used in physics to quantify the probability of a certain particle-particle interaction, e.g., scattering, photoabsorption, etc. (Note that light in this context is described as consisting of quasi-particles, i.e., photons.)

In the context of ozone shielding of ultraviolet light, absorption cross section is the ability of a molecule to absorb a photon of a particular wavelength and polarization.Analogously, in the context of nuclear engineering it refers to the probability of a particle (usually a neutron) being absorbed by a nucleus. Although the units are given as an area, it does not refer to an actual size area, at least partially because the density or state of the target molecule will affect the probability of absorption. Quantitatively, the number dN of photons absorbed, between the points x and x + dx along the path of a beam is the product of the number N of photons penetrating to depth x times the number n of absorbing molecules per unit volume times the absorption cross section σ:

$\frac{dN}{dx}= -N n \sigma$ .

Concept of cross-section is closely related to mass attenuation coefficient and for a given particle and its energy cross-sections of the target material can be calculated from mass absorption coefficient using:

$\frac{dN}{dx}= (\mu/\rho) * u m_a$

where:

μ/ρ is mass attenuation coefficient
m_a is atomic molar mass in g/mol
u is atomic mass unit (1/12 of the mass of Carbon-12) equal to 1.6605402E-24g

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