In nuclear physics, a measure of the probability that a given atomic nucleus will exhibit a specific reaction (absorption, scattering, or nuclear fission) in relation to a particular incident particle. Cross section is expressed in terms of area, and its value is chosen so that, if the bombarding particle hits a circular target of this size perpendicular to its path and centred at the nucleus, the given reaction occurs. If it misses the area, the reaction does not occur. The unit of cross section is the barn, which equals 1024 sq cm. Cross-section values for a given nucleus depend on the energy of the bombarding particle and the kind of reaction and are often different from the actual cross-sectional area of the nucleus.