A study of the temperature dependence of anisotropy effects characteristic of positron annihilation in single-crystal α-Zr

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Abstract
Positron annihilation measurements on single-crystal alpha -Zr show that the Doppler broadening of the annihilation photopeak changes with the crystallographic direction in which the photons are emitted. Conventional peak-height (S) and wing (W) parameters have been measured with the c axis of the specimen either parallel (S/sub ///, W/sub ///) or perpendicular (Sperpendicular to , Wperpendicular to ) to the cylindrical axis of the coaxial Ge(Li) detector. From 295-600K, S/sub ////Sperpendicular to approximately 1.026; from 600-1130K, S/sub ////Sperpendicular to decreases linearly to 1.017; at 1179K, S/sub ///=Sperpendicular to within the experimental uncertainty ( approximately 0.12%). The low-temperature results are associated with extrinsic behaviour (positron-defect interactions); in the intermediate range intrinsic (or bulk) behaviour dominates; the loss of anisotropy at 1179K is associated with the HCP to BCC transformation. Anisotropy is not apparent in measurements of W. Preliminary results for Ti show similar low-temperature behaviour.