I was wondering wether the γ-ray energy produced from the annihilation of a positron with a electron is always given only by the rest mass E=mc^2, that is 1022keV in total. I've read that the positron will slow down almost to rest before annihilation takes place. I've also read in http://prola.aps.org/abstract/PR/v77/i2/p205_1" [Broken] that the thermalization time for positrons is small compared to the annihilation process. Nevertheless this does not prove that annihilation cannot take place in higher kinetic energies of the electron-positron pair. If that happens is it possible to get γ-rays with energies given by E=sqrt(p^2c^2 + m^2c^4). Experimentally this does not seem to be the case bacause only the photopeak coming from the pair annihilation appears exactly at 511keV whereas if the the γ-ray could also get part of the kinetic energie of the positron and the electron one would see a whole continuum of energies above the cutoff of 511keV.