Vacancy in Graphite: Positron Srudies

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Abstract
The positron techniques of lifetime and Doppler-broadening were applied to vacancies in graphite. Vacancies were introduced into highly-oriented pyrolytic graphite by irradiation with 2 MeV electrons and fission reactor neutrons. The lifetime of positrons trapped at the vacancies was determined as 245±5 ps. Lifetime studies revealed two distinct vacancy annealing stages at 600–900°C and at 1300–1500°C. The Doppler-broadened lineshapes of the annihilation gamma-ray were taken in the directions perpendicular and parallel to the c -axis of the graphite lattice. Momentum distribution for an electron-positron pair at a vacancy was deduced and compared with the results of calculation based on the defect molecule model in the framework of LCAO-MO theory.