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| [[Image:Photon Cross Sections.png|thumb|right|300px|'''Cross-sections''' values for all elements with [[atomic number]] Z smaller than 100 collected for photons with energies from 1 keV to 20 MeV. The discontinuities in the values are due to [[absorption edge]]s which were also shown.]]
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| '''Absorption cross section''' is a measure for the probability of an absorption process. More generally, the term [[cross section (physics)|cross section]] is used in physics to quantify the probability of a certain particle-particle interaction, e.g., [[scattering]], [[absorption (electromagnetic radiation)|electromagnetic absorption]], etc. (Note that light in this context is described as consisting of [[elementary particle|particle]]s, i.e., [[photon]]s.) | |
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| 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 (waves)|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:
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| :<math>\frac{dN}{dx}= -N n \sigma</math>.
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| The absorption cross-section is closely related to [[molar absorptivity]] and [[mass absorption coefficient]]. For a given particle and its energy, the absorption cross-section of the target material can be calculated from mass absorption coefficient using:
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| :<math>\sigma= (\mu/\rho) m_a/N_A</math>
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| where:
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| * <math>\mu/\rho</math> is the mass absorption coefficient
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| * <math>m_a</math> is the [[atomic mass|atomic molar mass]] in g/mol
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| * <math>N_A</math> is [[Avogadro's number]] and is the number of molecules per mole
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| This is also commonly expressed as:
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| :<math>\sigma= \alpha/N</math>
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| where:
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| * <math>\alpha</math> is the [[absorption coefficient]]
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| * <math>N</math> is the atomic number density
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| ==See also==
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| *[[Cross section (physics)]]
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| *[[Photoionisation cross section]]
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| *[[Nuclear cross section]]
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| *[[Scattering cross-section]]
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| *[[Neutron cross section]]
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| *[[Mean free path]]
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| *[[Compton scattering]]
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| *[[Transmittance]]
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| *[[Attenuation]]
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| *[[Beer-Lambert law]]
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| *[[High energy X-rays]]
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| *[[Attenuation coefficient]]
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| *[[Absorption spectroscopy]]
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| [[Category:Electromagnetism]]
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| [[Category:Nuclear physics]]
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| {{nuclear-stub}}
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| {{physics-stub}}
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