An Etymological Dictionary of Astronomy and Astrophysics

In nuclear physics, unit of area for measuring the cross-sections of nuclei. 1 barn equals 10^-24 sq. cm.

Etymology (EN): Barn, from O.E. bereærn “barn,” lit. “barley house,” from bere “barley” + aern “house.” The use of barn in nuclear physics comes from the fact that the term denotes also “an unexpectedly large quantity of something.” It seems that when physicists were first studying nuclear interactions, they found out that the interaction probabilities, or cross-sections, were far more larger than expected; the nuclei were `as big as a barn'.