Images produced by positron emitters have become important in recent years. When the emitted…

Images produced by positron emitters have become important in recent years. When the emitted positron ($\beta^+$) encounters an electron, mutual annihilation occurs, producing two $\gamma$-rays. These rays have identical $0.511\text{ MeV}$ energies and they move directly away from one another, allowing detectors to determine their point of origin accurately. The system is called positron emission tomography (PET). It requires detectors on opposite sides to simultaneously (i.e., at the same time) detect photons of $0.511$-MeV energy and utilises computer imaging techniques similar to those in SPECT and computed tomography (CT) scans. Its resolution of $0.5\text{ cm}$ is better than that of Single-photon emission computed tomography (SPECT); the accuracy and sensitivity of PET scans make them useful for examining the brain's anatomy and function. Why energy of each produced '$\gamma$' photon is $0.511\text{ MeV}$?