Positron diffusion in metals and trapping at grain boundaries

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Abstract
Positron trapping has been observed in fine-grained specimens of Zn-22 wt. % A1 by measurements of the positron lifetime and Doppler-broadened lineshape. The trapping is attributed to large-angle grain boundaries. A plot of the trapping rate versus the inverse grain size squared is approximately linear, as predicted by a motion-limited trapping model. Under the assumption of motion-limited trapping, a value of 0.6 cm2 s−1 is obtained for the positron diffusion constant in this alloy at 293 K, with a corresponding mean diffusion distance of 0.14pm. The specific trapping rate μ at the boundaries of grains with sides 1 μm is 2 × 1012 s−1(at 293 K). A decomposition of the lifetime spectra indicated a trapped-positron lifetime of about 250 ps, implying that the local electron density at the trapping sites is comparable to that at Vacancies. No additional positron trapping was observed in a sample treated to yield a high density of Zn precipitates in the Al-rich phase, indicating that these precipitate boundaries are semi-coherent.