Materials research using Positron Annihilation Spectroscopy (PAS)

Positron Annihilation Spectroscopy (PAS) is a well-established method used in the fields of condensed matter physics, nuclear physics, chemistry, materials science and materials engineering [1]. In PAS, positrons are typically injected into the bulk of a material where they rapidly thermalize and are likely to be trapped in neutral or negatively charged crystallographic defects. After a time interval that can vary from~ 100ps to few ns, depending on the material’s type and its defects, the positrons annihilate with that material electrons, each such process results in the emission of two almost back to back photons carrying each approximately 511 keV. Due to momentum and energy conservation, the momentum distribution of the annihilating electrons reflects in angular distribution around the 180o between the two emitted photons (ACAR–Angular Correlation of Annihilation Radiation) and in the broadening of their energy distribution (DB–Doppler broadening). The electron density in the positron proximity before annihilation defines the lifetime of positron in the material [2], with measurable differences between mean lifetimes in the bulk region and in defect traps.
The relatively high energy photons emitted from annihilation events penetrate the material under study, allowing non-destructive measurements. The distinguishable differences between the electron characteristics at point defects as opposed to the undamaged bulk region, and the high affinity of positrons to the defects, make them useful as a probe for point defects, the smallest of which are single atoms vacant from their lattice sites (mono-vacancies), down to very low concentrations, of~ 10-6 a-1 and up [3]. Comparison to sensitivity regions of other research methods is shown in Fig. 1, taken from [4]. The very good resolution of ACAR measurements allow detailed exploration of Fermi surfaces in single-crystal materials.