Barn (nuclear cross-section unit)
| Value | 1e-28 m² |
| Status | Exact by definition — no uncertainty |
| Source | BIPM, SI Brochure 9th edition — units accepted for use with the SI (exact by definition) |
| Categories | Universal & Atomicnuclearphysics |
| square millimeter | 1.0000000e-22 mm² |
| square centimeter | 1.0000000e-24 cm² |
| square meter | 1.0000000e-28 m² |
| hectare | 1.0000000e-32 ha |
| square kilometer | 1.0000000e-34 km² |
| square inch | 1.5500031e-25 in² |
| square foot | 1.0763910e-27 ft² |
| square yard | 1.1959900e-28 yd² |
| acre | 2.4710538e-32 ac |
| square mile | 3.8610216e-35 mi² |
Learning zone
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.