1. Positrons: probes of matter

The positron (denoted e ⁺ ), the antiparticle of the electron (denoted e ^- ), has the same characteristics as the latter, with the exception of its charge and magnetic moment, which are of opposite sign. In a vacuum, the positron is stable (lifetime greater than 4.2 × 10 ²³ years) [15] whereas in matter, its interaction with electrons leads to the annihilation of pairs e ⁺ –e ^- with the emission of two or three gamma annihilation photons that recover the energy and momentum of the annihilating pair. The annihilation characteristics – the number of photons emitted, their energy, the angle between...