An electromagnetic shower begins when a high-energy electron, positron or photon enters a material. At high energies (above a few MeV, below which photoelectric effect and Compton scattering are dominant), photons interact with matter primarily via pair production — that is, they convert into an electron-positron pair, interacting with an atomic nucleus or electron in order to conserve momentum. High-energy electrons and positrons primarily emit photons, a process called bremsstrahlung. These two processes (pair production and bremsstrahlung) continue until photons fall below the pair production threshold, and energy losses of electrons other than bremsstrahlung start to dominate.