3. Measurement of two-photon gamma annihilation characteristics

Since positron annihilation with an electron is a statistical process, it is necessary to record the annihilation of at least 10 ⁶ positrons to determine annihilation characteristics. In materials where only positron annihilation takes place, for example the annihilation of e ⁺ –e ^- quasi-free pairs, the annihilation characteristics commonly measured to study defects in solids are the positron lifetime distribution and the energy distribution of annihilation gamma photons in the two-gamma annihilation process. In insulating solids, where a fraction of positrons can form positronium (Ps) atoms, the pick-off process