POSITRON-ANNIHILATION STUDY OF IRRADIATED HOPG-TYPE GRAPHITE

Positron annihilation angular correlation measurements (ACAR : angular correlation of annihilation radiation) were made for HOPG (highly ori ented pyrolytic graphite) and PGCCL (HOPG-type graphite, Le Carbone-Lo rraine) irradiated by various particles, such as electrons, neutrons a nd deuterium particles. The ACAR measurements made under the condition of P-z // c-axis for unirradiated specimens showed a clear minimum at theta = 0 degrees, but P-z perpendicular to c-axis measurement did no t show this minimum. This minimum is considered to be due to the annih ilation of positrons with pi electrons the momentum distribution of wh ich is extended to the direction parallel to c-axis. On the other hand , by neutron irradiation this minimum disappeared, which suggests that positrons are trapped at radiation-induced defects, probably vacancy sites, and the probability of the annihilation with pi electrons was d ecreased to the large extent. But, in the case of hydrogen irradiation the minimum was not affected so much, which suggests hydrogen atoms d o not disturb the pi electrons, probably because hydrogen atoms are tr apped at boundary regions, such as those between adjacent crystallites or region of stacking disorder.