barn

Name	Symbol	Quantity	SI units
barn	b	area	10⁻²⁸  m²
The barn, symbol b, is a non-SI unit of area. The barn is not accepted for use with the SI. One barn is equal to 10⁻²⁸ m², or 100 fm².
Definition			c² Δν_Cs^-2
$1 \mspace{4mu} \text{b} \mspace{6mu} = \dfrac{(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^2}{(10^{28})(299 \mspace{4mu} 792 \mspace{4mu} 458)^2} \mspace{6mu} \dfrac{c^2}{{\Delta \nu _{Cs}}^2}\\ \\ \\ 1 \mspace{4mu} \text{b} \mspace{6mu} \approx 9.402 \mspace{4mu} 391 \mspace{4mu} 313 \mspace{4mu} 904 \mspace{4mu} 046 \mspace{4mu} 861 \times10^{-26} \mspace{6mu} c^2 \mspace{4mu} {\Delta \nu _{Cs}}^{-2}$

The barn is a non-SI unit of cross-section used in nuclear and particle interactions.