Unit b
Name barn
Category Area
Details A barn (symbol: b) is a unit of area used in nuclear and particle physics to quantify the cross-sectional area of interaction probabilities during collisions between particles. It is a non-SI unit (not part of the International System of Units) but is accepted for use within the system. One barn is equal to 10^-28 square meters (m²) or 100 square femtometers (fm²).

The barn provides a convenient measure for expressing the probability of interaction or scattering between particles during high-energy experiments, such as those carried out at particle accelerators. It is particularly useful for working with very small quantities, as it helps to avoid large numbers and scientific notation in calculations.

The term "barn" was coined by American physicists in the 1940s as a humorous reference to the proverbial expression "as big as a barn," signifying the surprisingly large size of the effective target area for nuclear reactions compared to the actual size of atomic nuclei.