Vacancies studied by positron annihilation with high-momentum core electrons

Positron annihilation with high-momentum core electrons is studied with a recently developed two-detector Doppler technique. As shown by the examples Al and Cu at room temperature, the independent-particle model satisfactorily reproduces both the shape and the absolute intensities of the experimental high-momentum tails beyond approx.2 a.u. Owing to the simplicity of the theoretical description, the high-momentum spectra should be ideal for the characterization of vacancies. The procedure to be applied is illustrated through the analysis of the temperature dependence of the high-momentum events in Al. The vacancies are identified as to the type (monovacancies), and the formation energy agrees with the value we obtain by measuring the average lifetime as a function of temperature. While this paper is limited to a case study, situations are anticipated in which the high-momentum tails may be unique in providing information on trapping sites