The first γ γ -quantum ( γ γ 1 , k 1 ) will be emitted isotropically. Detecting this quantum in a detector selects a subset with an orientation of the many possible directions that has a given. The second γ γ -quantum ( γ γ 2 , k 2 ) has an anisotropic emission and shows the effect of the angle correlation. The goal is to measure the relative probability W ( Θ Θ ) d ( Ω Ω ) with the detection of γ γ 2 at the fixed angle Θ Θ in relation to γ γ 1 . The probability is given with the angle correlation (perturbation theory):