Positron lifetimes in crystalline solids exposed to γ rays with energies above the electron–positron pair formation threshold and a weak magnetic field

Theory predicts that positrons in crossed motional electric and magnetic fields form long-lived positronium in vacuum. It follows that binding of the electron to anions of dielectric solids may prevent fast annihilation by forming electric positron–electron dipole oscillators with lifetimes of hundreds of minutes. To test this hypothesis, lifetime distributions of time-coincident, 180° γ-rays from crystalline alkali halides and a polycyclic hydrocarbon were measured in 12 and 95G magnetic fields. Gamma-ray sources with energies above the electron–positron pair formation threshold were used to make positrons. •A positron lifetime of 233±40min is measured in KCl at 12 gauss magnetic field.•Lifetimes in KCl, KBr and KI at 95 gauss are 4.1±0.4, 8.5±1.3 and 10.0±1.0min.•Lifetimes are due to electric dipole radiation of positronium bound to a halide anion.•Lifetimes vary as the inverse magnetic field squared and anion electron density.•A positron lifetime of 9.8±2.0min has been measured in naphthalene (C10H8).