Positron-electron pair production
Quantum Mechanics allows ultra shorts transformations of a gamma into a pair of particle-antiparticle, such as an electron and a positron. If during this short instant, the pair finds itself in the vicinity of the strong electric field of a nucleus it interacts with it (B). The interaction with the nucleus enables the creation of the electron and positron to remain permanent. The "mass energy" (twice 511 keV) necessary for the formation of the two particles is taken from the energy of the gamma that must be must greater than a 1.022 MeV threshold.