barn
Areaexact by definition
The barn is exactly 10⁻²⁸ square metres, 100 fm², which is roughly the cross-sectional area of a uranium nucleus. It is the unit of nuclear reaction cross section, a quantity with the dimensions of area that expresses how likely an interaction is rather than how big anything physically is.
Watch out: A cross section is not a physical area. It is a probability expressed in area units, and it varies enormously with neutron energy: uranium-235's fission cross section is around 585 b for thermal neutrons and about 1 b for fast ones, which is the entire reason reactors use moderators.
| 1 | 1.0000000e-28 |
It was coined at Purdue in 1942 during the Manhattan Project, deliberately as an innocuous name that would not reveal its subject in correspondence. The joke is that a nucleus this size is "as big as a barn" to a passing neutron, which is enormous by nuclear standards.
One barn in every area unit
| 1,000,000 | microbarn (μb) | |
| 1,000 | millibarn (mb) | |
| 1 | barn (b)this unit | |
| 1.00000e-10 | square nanometer (nm²) | |
| 1.00000e-16 | square micrometer (μm²) | |
| 1.97353e-19 | circular mil (cmil) | |
| 1.97353e-22 | thousand circular mils (kcmil) | |
| 1.00000e-22 | square millimeter (mm²) | |
| 1.00000e-24 | square centimeter (cm²) | |
| 1.55000e-25 | square inch (in²) | |
| 1.00000e-26 | square decimeter (dm²) | |
| 1.07639e-27 | square foot (ft²) | |
| 1.19599e-28 | square yard (yd²) | |
| 1.00000e-28 | square meter (m²) | |
| 3.02500e-29 | tsubo (tsubo) | |
| 3.95369e-30 | square rod (rd²) | |
| 1.00000e-30 | are (a) | |
| 2.47105e-31 | square chain (ch²) | |
| 1.00000e-31 | dunam (dunam) | |
| 9.88422e-32 | rood (rood) | |
| 2.47105e-32 | acre (ac) | |
| 1.00000e-32 | hectare (ha) | |
| 1.00000e-34 | square kilometer (km²) | |
| 3.86102e-35 | square mile (mi²) |