Unit used in nuclear physics to indicate probability of a reaction between nuclear particles and atomic nuclei or even other nuclear particles. The dimension of the Barn is cross sectional area and 1 Barn is equal to 10-28 m². The ‘physical cross section area’ of the nucleus of an atom is approximately 1 Barn but depending on the reaction involved the actual ‘reaction cross section area’ can vary over many decades of Barns in size and are very dependent on particle energy.