Positron annihilation with an electron produces what?

• A. A single 1 MeV photon
• B. Three photons
• C. Two 511-keV photons emitted at 90 degrees apart
• D. Two 511-keV photons emitted 180 degrees apart

Answer: (D)

When a positron meets an electron and annihilates, their rest-mass energy is converted into gamma rays. If the pair has negligible kinetic energy, the total initial momentum is essentially zero, so the resulting photons must carry equal and opposite momentum. Each photon has an energy of 0.511 MeV, the rest-mass energy of the electron (and positron), giving two photons of 511 keV each emitted back-to-back (180 degrees apart). The total energy released is 1.022 MeV. A single 1 MeV photon wouldn’t conserve momentum in this scenario, and three-photon outcomes occur only in a different quantum state with a different energy distribution.