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 95 G magnetic fields. Gamma-ray sources with energies above the electron-positron pair formation threshold were used to make positrons.