Absorption cross section
From The UCSC Wikipedia Trust Project
Absorption cross section in the context of nuclear engineering refers to the probability of a particle (usually a neutron) being absorbed by a nucleus.
Analogously, 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. 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:
dN/dx = –Nnσ
