Barn (nuclear cross-section unit)

The barn was coined at Purdue in 1942 by physicists working on the Manhattan Project, who needed a code word for a cross section around 10⁻²⁴ cm² and settled on the joke that such a target was "as big as a barn". It stuck, was declassified after the war, and is now accepted for use with the SI. One barn is 100 fm², about the cross-sectional area of a heavy nucleus.

Cross sections are effective target areas, not physical ones, and they vary wildly with energy. Thermal-neutron fission of U-235 has σ ≈ 585 b; the control-rod absorber ¹³⁵Xe reaches 2.6 million barns, which is what poisons a reactor after shutdown; and neutrino cross sections are around 10⁻¹⁴ b, hence the light-year of lead. Submultiples get their own puns — the millibarn, microbarn and the femtobarn used for LHC luminosities, where an integrated 150 fb⁻¹ means 150 inverse femtobarns of accumulated collisions.