Shifts of annihilation radiation energy from the electron rest mass energy for positons annihilating in copper and sodium

The reasons for not expecting the energy of annihilation radiation to exactly equal the electron rest mass energy when annihilation takes place in bulk matter are discussed. It is shown that directional correlation measurements on the two-photon annihilation process enable one to calculate the shift from the electron rest mass energy for positons annihilating in simple metals. Specific results are presented for positons annihilating in copper and sodium. The Doppler broadening of the annihilation radiation is also discussed. © 1969