Types of reactions frequently encountered are s : scattering, γ γ : radiative capture, a : absorption (radiative capture belongs to this type), f : fission, the corresponding notation for cross-sections being: σ σ s , σ σ γ γ , σ σ a , etc. A special case is the total cross-section σ σ t , which gives the probability of a neutron to undergo any sort of reaction ( σ σ t = σ σ s + σ σ γ γ + σ σ f + … … ).