Annihilation photon
= Annihilation photon
{title2=$E_\gamma\simeq511\,\mathrm{keV}$}
For two-photon <positron annihilation> with negligible initial total momentum, each <photon> has energy $m_ec^2$ and the two propagate oppositely. Their common energy differs from the positron's continuous emission-energy spectrum before slowing in matter.