Positronium is converted into

Positronium is a bound state of an electron and a positron. When it annihilates, the total rest mass energy (2 × 0.511 MeV = 1.022 MeV) is converted into photons. Momentum conservation requires at least two photons, and the most common decay mode (para-positronium) is into two photons each of energy 0.511 MeV. One-photon decay is impossible due to momentum conservation, and two photons of 1.02 MeV each would violate energy conservation.