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A unit of area for nuclear reaction probability.
Also known as: Nuclear cross section unit, b unit physics
The barn is a unit of area, ten to the minus twenty-eight square metres, used to express the probability of a nuclear reaction. The name is a joke: a nucleus that size is as easy to hit as the side of a barn, and the unit was coined during wartime work when the terminology was deliberately obscure.
Measures nuclear cross-section, equal to ten to the power minus twenty-eight square metres.
Contexts: Laboratory
Was named as a joke during the Manhattan Project, on the reasoning that such a cross-section was as easy to hit as the side of a barn, and the obscurity of the name was itself useful.
Contexts: Laboratory
A single lowercase letter b printed after a numeral in a measurement.