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, electromagnetic absorption, etc. (Note that light in this context is described as consisting of particles, i.e., photons.) In honor of the fundamental contribution of Maria Goeppert Mayer to this area, the unit for the two-photon absorption cross section is named the "GM". One GM is 10 cm s photon. 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 formula_1 of photons absorbed, between the points formula_2 and formula_3 along the path of a beam is the product of the number formula_4  of photons penetrating to depth formula_2 times the number formula_6 of absorbing molecules per unit volume times the absorption cross section formula_7: 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: where: This is also commonly expressed as: where: