This review is concerned with the utilization of the positron-electron annihilation phenomenon in studies of the physics of condensed matter. The theory of the annihilation process is outlined, the importance of the two-photon mode of decay is established, and it is described how the observable results depend on the initial positron and electron states of the system. A brief account of the principal experimental techniques is included. Discussion of the complex nature of positron behavior in molecular substances suggests the particular value of investigations directed at the two photon pick-off mode of decay of the orthopositronium atoms that are formed in these materials. The possibility of positronium formation in ionic and metallic materials is also considered. The many important electron aspects of positron annihilation in metals are dealt with. The independent particle approach to the analysis of two photon angular distributions is discussed and illustrated. A survey of electronic structure investigations includes studies of polycrystalline and single crystal specimens. The relevance of angular correlation measurements in investigations of the Fermi surfaces of metals and alloys is discussed. More recently developed applications to the study of defected and disordered systems are dealt with. A preliminary account of the interpretation of multicomponentmore » lifetime spectra in terms of varying numbers of distinguishable positron states provides the basis for a discussion of studies of positron trapping by defects, voids. and surfaces in ionic and metallic solids, liquids and powders. (auth)« less

Positron studies of condensed matter

Positron annihilation spectroscopy is particularly suitable for studying vacancy-type defects in semiconductors. Combining state-of-the-art experimental and theoretical methods allows for detailed identification of the defects and their chemical surroundings. Also charge states and defect levels in the band gap are accessible. In this review the main experimental and theoretical analysis techniques are described. The usage of these methods is illustrated through examples in technologically important elemental and compound semiconductors. Future challenges include the analysis of noncrystalline materials and of transient defect-related phenomena.

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Defect identification in semiconductors with positron annihilation: Experiment and theory

Positron annihilation : A probe for nanoscale voids and free volume ?

The implantation and propagation of positrons in solids are discussed. Several examples for the application of the positron method in the study of solid state properties are presented.

Positron dynamics in solids

Theoretical positron lifetime values have been calculated systematically for most of the elements of the periodic table. Self-consistent and non-self-consistent schemes have been used for the calculation of the electronic structure in the solid, as well as different parametrizations for the positron enhancement factor and correlation energy. The results obtained have been studied and compared with experimental data, confirming the theoretical trends. As is known, positron lifetimes in bulk show a periodic behaviour with atomic number. These calculations also confirm that monovacancy lifetimes follow the same behaviour. The effects of enhancement factors used in calculations have been commented upon. Finally, we have analysed the effects that f and d electrons have on positron lifetimes.

https://iopscience.iop.org/article/10.1088/0953-8984/19/17/176222/pdf

Positron lifetime calculation for the elements of the periodic table

Theoretical positron-lifetime spectra are derived for targets with structures such that positrons can diffuse into interstices where they remain trapped until, after a distinct lifetime, they annihilate with an electron into $\ensuremath{\gamma}$ quanta. Exact expressions and convenient approximate formulas for different structural geometries are presented. The contribution of trapped positrons to the angular correlation between the two annihilation $\ensuremath{\gamma}$ quanta is derived. The propagation of errors in determining positron-diffusion constants or, conversely, parameters of the target microstructure from positron-annihilation characteristics is discussed.

Positron Diffusion in Solids

A method is developed to measure positron implantation profiles in solids and the changes induced by anisotropies or externally applied electric fields through the dependence on depth, $x$, of positron-electron annihilations. The implantation profiles, ${P}_{+}(x)$, of positrons from $^{64}\mathrm{Cu}$ sources, with maximum kinetic energy 0.65 MeV, in 14 different solids, ranging in density $d$(g/${\mathrm{cm}}^{3}$) from 0.9 to 9, can be described by ${P}_{+}(x)=\mathrm{exp}(\ensuremath{-}\frac{x}{{R}_{+}})$, with a mean implantation range ${R}_{+}(0.65 \mathrm{MeV})=(345\ifmmode\pm\else\textpm\fi{}25){d}^{\ensuremath{-}1}(\ensuremath{\mu}\mathrm{m})$. Compared to polycrystalline samples, ${R}_{+}$ increases by 10% in Cu single crystals under channeling conditions along the 110g direction. The measured displacements of ${P}_{+}(x)$ in $C$ (diamond) and Si crystals induced by electric fields up to 5 kV/cm give positron mobilities 200 to 400 ${\mathrm{cm}}^{2}$/Vsec, which are 5 to 10 times smaller than the corresponding electron mobilities. Profiles of lattice defects induced in NaCl crystals by x rays of various energies are measured through positron trapping, as signified by $x$-dependent changes of the angular correlation between the annihilation $\ensuremath{\gamma}$ rays.

Positron implantation-profile effects in solids

Positronium Diffusion in Solids.

The dimensions of the electronic lattice structure of the hexagonal quartz crystal and the fcc Ca${\mathrm{F}}_{2}$ crystal have been measured by resolving characteristic markings on the angular-correlation curves as observed for the two gamma quanta emitted in the annihilation of positrons with electrons in these crystals.

Positron annihilation and electronic lattice structure in insulator crystals.

Positron lifetime spectra and angular correlation curves for seven fine-grained powders of Fe, Co, Ni, and W are analyzed. From the lifetime data, the positron diffusion constant in metals atT=300°K was found to beD +=(1.0±0.5)×10−2 cm2 sec−1. Evidence is presented that positrons are trapped in metal surface states.

Robert Paulin

Papers

Positron Diffusion in Solids

Positron implantation-profile effects in solids

Positronium Diffusion in Solids.

Positron annihilation and electronic lattice structure in insulator crystals.

Diffusion constant and surface states of positrons in metals