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Hitting the broad side of a (classified) barn
In the luminosity lexicon, a picobarn is one trillionth (10-12) of a barn, and a femtobarn is one quadrillionth (10-15) of a barn... but what's a barn? The distinctive and amusing term originated with two Purdue University physicists working on the Manhattan Project in 1942—and it was classified information by the US government until after World War II.
A History of Physics at Purdue (Gartenhaus, Tubis, Cassidy, and Bray) cites the July 1972 issue of Physics Today in which Marshall Halloway and Charles Baker write of tossing around ideas over dinner until arriving at "barn" to describe the typical nuclear cross section of 10-24 cm2, the effective target area that a nuclear particle represents in a collision. Dining in the Purdue Memorial Union, back in Lafayette, Indiana, Halloway and Baker dismissed "Oppenheimer" and "Bethe" as candidates, then considered John Manley, director of the Purdue group at Los Alamos. They decided "Manley" was too long, and then, as the authors put it in the Physics Today article to:
"'John' was considered, but was discarded because of the use of the term for purposes other than as the name of a person. The rural background of one of the authors then led to the bridging of the gap between the 'John' and the 'barn.' This immediately seemed good, and further it was pointed out that a cross section of 10-24 cm2 for nuclear processes was really as big as a barn. Such was the birth of the 'barn.'"
Because of the need for communicating project information as secretly as possible by telephone, the term "barn" was immediately classified. Halloway and Baker wrote an internal report in 1944 ("Note on the Origin of the Term 'Barn'" (LAMS 523), September 1944), and Los Alamos issued a report after the war in 1947 ("Origin of the Term 'barn'" (LAMS 523), 5 March 1947). But the term "barn" wasn't officially de-classified by the government until 1948. It is now used across nuclear and particle physics.
Mike Perricone
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