Name given by physicists to the interaction probability of a projectile particle (gamma, neutron, etc ...) with a target (atom, nucleus, electron ...) leading to a given reaction. For instance, gamma photoelectric and Compton effects cross sections on atoms; in reactor physics, neutron radiative and fission cross section of nuclei on nuclei. This probability is equivalent to a very small area taking into account the dimensions of atomic or nuclear targets. The unit cross section is the barn (1 barn or b = 10-24 cm2), a surface of the order of a nucleus cross section.
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