Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders

Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 250 and 6000C. Three positron lifetimes were found in these systems: a short-lived component (proportional0.2 ns) due to positrons in the bulk; a component (proportional0.45 ns) due to surface-trapped positrons; and a long-lived component (proportional2 ns) of o-Ps in the voids or the interfacial spaces of powders. Both bulk and surface positron lifetimes increase as a function of temperature. Correlations between the intensity of surface-trapped positrons and the surface area and between Ps formation and the surface area of graphite are found. The Ps formation increases as a function of temperature. A thermal desorption model interprets the emission process of Ps atoms from the surface of graphite to the vacuum and gives an activation energy of 0.23+-0.02 e.V. (orig.)