1. The problem statement, all variables and given/known data A given pozitron with T kinetic energy is approaching an electron bound to a hard nucleus(in the K energy level, and the ionization energy for this level, Ek(>0), is given). The energy of the two resulting photons is to be determined. 2. Relevant equations Conservation of energy. 3. The attempt at a solution So I wrote the conservation of energy as follows: 2m0c2 + T = 2ε + Ek ( ε is the energy of one photon ) Let me explain my way of thinking: After the anihilation, the electron has vanished, so the atom has become a ion(so it's energy has increased by Ek). However, the proposers of the problem consider the conservation of energy as follows: 2m0c2 + T + Ek = 2ε; So who is right?