Observation of Long Positron Lifetimes in Crystals with a Weak Magnetic Field
Abstract
Gerald. A. Smith
Lifetime distributions of time-coincident, 180o γ-rays from KCl, KBr, KI and C8 H10 (naphthalene) in 12 and 95 gauss magnetic fields have been measured. The samples were exposed to low frequency γ-ray sources with energies above the electron/positron pair-formation threshold. According to theory, in these fields slow positrons may bind to anions and form positronium with lifetimes of tens to hundreds of minutes. At the low rates gas-filled radiation detectors worked satisfactorily. Lifetimes agree with classical electric dipole decay that is expected of long-lived positronium formed in crossed motional electric and magnetic fields. This work fills an important gap in knowledge that has existed since the discovery of the positron in 1932. The results reported herein establish the highest energy pure γ -ray emitter and longest lived excited atomic state known at this time.
