A barn is a unit of cross section used in particle physics; it is used to indicate
the probability of a reaction between nuclear particles and atomic nuclei
or other nuclear particles. One barn is equal to 10-28 m2 (10-24 cm2 ).
Many interesting cross sections are measured in pb (picobarns), which are equal to 10-12 barns. The physical cross sectional area of the nucleus of an atom is approximately 1 barn but depending on the reaction involved the actual reaction cross sectional area can vary over many decades of barns in size and are very dependent on particle energy.