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A Greek sigma, an effective target area for a reaction.
Also known as: Reaction cross section, Interaction cross section, Sigma in nuclear physics
A cross section is the effective area a nucleus presents to an incoming particle for a particular reaction, and it is measured in barns — a unit named ironically, because 10⁻²⁸ square metres was reckoned as big as a barn door by nuclear standards.
Denotes a reaction cross section: the effective target area governing the probability of a nuclear or particle interaction.
Contexts: Laboratory
Is measured in barns, equal to 10⁻²⁸ square metres, a unit whose name was a wartime joke about how large that area seemed.
Contexts: Laboratory
Depends strongly on the incident particle's energy, so a cross section quoted without an energy is incomplete.
Contexts: Laboratory
A Greek lowercase sigma, a small loop with a horizontal stroke leading right from its top.