(a) A particle and its antiparticle are at rest relative to an observer and annihilate (completely destroying both masses), creating two rays of equal energy. What is the characteristic -ray energy you would look for if searching for evidence of proton-antiproton annihilation? (The fact that such radiation is rarely observed is evidence that there is very little antimatter in the universe.)

(b) How does this compare with the 0.511-MeV energy associated with electron-positron annihilation?