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##\frac{d\sigma}{dx}=\frac{A}{X_0N_A}\bigg[1-\frac{4}{3}x(1-x)\bigg] ##

## \sigma_{pair}\approx \frac{7A}{9N_AX_0}##

These equations are supposed to describe pair production. However, I simply don't understand what the sigma is meant to be. I think it is something along the lines of a distribution of resulting particles from the pair production given an intensity of incident particle, but the same ##\sigma## is used to describe things like photoelectric effect and compton scattering. Any assistance would be greatly appreciated! Thanks!