(1) The [[nuclear]] capture cross section for neutrons is the effective area within which a [[neutron]] passes in order to be captured by an atomic nucleus. It is a probabilistic value dependent on the nature and energy of the particle as well as the nature of the capturing nucleus. Nuclear capture cross section is often measured in barns (1 barn = 10<sup>–24</sup> cm<sup>2</sup>).

(2) "Macroscopic capture cross section" **S** is the effective cross-sectional area per unit volume of material for capture of neutrons; hence, it depends on the number of atoms pre sent as well as their nuclear capture cross sections. Thus, the macroscopic capture cross section is the sum of the various weighted capture cross sections. The unit of measure for **S** is cm<sup>–2</sup>/cm<sup>3</sup> or reciprocal cm (cm<sup>–1</sup>) is often measured in "capture units" or "sigma units." A c.u. = 10<sup>–3</sup> cm<sup>–1</sup>.

(3) The rate of absorption of [[thermal neutron|thermal neutrons]] with a velocity v is v**S**.

(4) Of gamma rays. See [[photoelectric absorption]], [[photoelectric effect]], and [[photoelectric absorption cross section index]].