- Absorption cross section
**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 ofquasi-particle s, i.e.,photon s.)In the context of

ozone shielding ofultraviolet light ,**absorption cross section**is the ability of amolecule to absorb a photon of a particularwavelength andpolarization .Analogously, in the context ofnuclear engineering it refers to the probability of a particle (usually aneutron ) 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 thedensity or state of the target molecule will affect theprobability 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:

:$frac\{dN\}\{dx\}=\; (mu/\; ho)\; *\; u\; m\_a$

where:

* μ/ρ is mass attenuation coefficient

* m_{a}is atomic molar mass in g/mol

* u isatomic mass unit (1/12 of the mass ofCarbon -12) equal to 1.6605402E-24g**ee also***

Cross section (physics)

*Photoemission cross section

*Nuclear cross section

*Scattering cross-section

*Cross Section

*Mean Free Path

*Compton edge

*Compton scattering

*Transmittance

*Attenuation

*High energy X-rays

*Attenuation coefficient **References****External links**

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