Observation of positron annihilation in defect in graphite

Much attention is recently drawn to graphite as material for containing plasma in a nuclear fusion reactor because of its good high-temperature characteristics. However, the plasma particle, or hydrogen, affects the plasma-containment performance of graphite as it is adsorbed on the graphite and then desorbed from it, possibly leading to serious problems. In the study, positron annihilation in different graphite samples is observed to obtain information on microscopic structures that can provide trap sites for the hydrogen. The study focuses mainly on the measurement of their life and γ-γ angular correlation. Investigation is also made for samples to which defects are introduced by electron beam irradiation. Of the various graphite samples, the highly oriented pyrolytic one (HOPG) is the shortest in life, followed by the isotropic and glassy ones, which have longer lives. The curve for γ-γ angular correlation measured parallel to the c-axis of HOPG has a peak near ±3 mrad, which disappears gradually as the sample is irradiated with electron beam. Atomic vacancies are formed in HOPG by the electron beam and positrons are trapped in them, leading to an increased life. (N.K.)