The barn (symbol b) is an special unit employed in Nuclear physics and Physique of the particles to express the cross sections.

It appears in table X of the international Système, Unités to temporarily maintain with the International System .

This unit is homogeneous on a surface; its value is of 10^-28 m².

This unit is of the same order of magnitude as the geometrical section of the core of a Atome, the ray of the Proton being of 1,5 × 10^-15 m, that of the Neutron of 10^-15 Mr. However, the values of the cross sections differ notably from their geometrical values and also vary in an important way according to nature, of the energy of the flow of particle S and of the interactions which they undergo while crossing material considered.

For example, the total cross section of interaction of a core of atom of Hydrogène with a flow of neutrons whose energy is of 1 eV is of 20 B (2000 × 10^-30 m ²), whereas the geometrical section of a proton is approximately three hundred times smaller: ~0,07 B (7,06 × 10^-30 m ²).

As comparison, the geometrical section of a hydrogen atom is approximately 6,25 Mb (6,25 × 10^-22 m ²), three hundred and thousand times larger.