Question

An electron and an antielectron (positron) each have a rest energy of 0.511 MeV , or approximately 8.2×10^-14 J . When an electron and a positron are both stationary and located next to each other during an annihilation process, their mass energy converts to electromagnetic energy released as photons, electromagnetic particles that have momentum but no mass and that travel at the speed of light. What is the minimum number of photons that could be released, and how much energy would each photon posses?

A. one photon with energy 0.511 MeV

B. one photon with energy 1.022 MeV

C. two photons, each with energy 0.511 MeV

D. two photons of any energy, as long as their total energy equals 1.022 MeV

Answer 1

Correct answer: C. Two photons, each with energy 0.511MeV

Minimum 2 photons will be released. This is because, initial momentum of electron and positron is zero. Hence final momentum should also be zero. If only one photon is released , final momentum cannot be zero. So two photons with equal and opposite momentum will be released such that the final momentum is also zero. So if the magnitude of the momentum of both photons is the same, their energy is also the same.