Backscatter occurs when gammas make Compton interactions in the material that surrounds the detector. Fig. 3.5, taken from ref. 10, is a good illustration of the various events that can occur in a typical source-NaI(Tl)-lead shield arrangement. Backscattered gammas from these interactions (Eγ′) create photoelectric interactions in the NaI(Tl) when they enter the crystal. The energy of the backscattered peak can be found by solving Eq. (2).