barn ['bärn n. (1942)] Unit of area equal to 10^–28 m2, used to express cross sections in nuclear reactions.
To physicists Marshall Halloway and Charles Baker, uranium nuclei were “as big as a barn.” That’s why they decided—over dinner one evening—to use the term as a unit of measurement. These atomic bomb researchers set the barn equal to the cross section of a typical atomic nucleus, more or less, and during World War II its meaning was classified information. After the government declassified the term in 1948, barns spread throughout nuclear and particle physics.
