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

Atomic Radial Distributions and Ion-Ion Potential in Liquid Gallium

The conscious study of lattice defects with positrons is now some ten years old. Although sharing the techniques and guided by the results of the longer established studies described in Chap.2, defect studies are based on a somewhat different philosophy. Electronic structure studies rely for their credence on a belief that, in sufficiently well-annealed single-crystal samples, the positrons annihilate from a common, well-defined, and delocalised state in which they can freely sample nonlocal properties of the medium. The perturbing effect of the positron charge on the electronic system is in general an unwelcome complication. In defect studies the-situation is radically different. Here the charged positron’s interactions with the surrounding medium play the vital role in providing for the possibility of a variety of additional localised positron states at the structural defects. The investigation of defect-induced changes in the commonly measured characteristics of annihilation (Chap.l) and the interpretat on of such changes in terms of such things as the concentration, nature, and origins of the defects responsible are the essential ingredients of our study.

Positron Studies of Lattice Defects in Metals

A precise measurement of the angular correlation of two gamma-rays emitted from copper single crystals by the annihilation of positrons has been carried out. The crystals were irradiated by neutrons to produce 1∼2 C of 64 Cu so that the positrons were confined in the crystals. This containment of the strong positron sources in the crystals allowed us to use two pairs of slits perpendicular to each other rather than the ordinary one pair of slits. With this geometry inaccuracies in determining the shape of the Fermi surface due to poor resolutions were considerably eliminated. The necks of the Fermi surface of copper were found on the curves of the angular correlation. The observed value of the radius of these necks, 2.7×10 -20 gr·cm/sec, agrees with those obtained by the magnetoacoustic effect etc.. The maximum momentum of electrons in the -directions was found to be 1.60×10 -19 gr·cm/sec, which agrees well within the experimental error with the distance of the {111}-Brillouin zone boundaries from th...

A precise measurement of the angular correlation of annihilation radiation in copper single crystals.

Positron lifetimes in solid and liquid metals

tion of photons from the annihilation of positrons with electrons in solid and liquid mercury. The angular distribution of photon coincidences can be separated into contributions arising from annihilations with the ionic-core electrons and with the conduction electrons. The angular variation of the former does not appear to change at the solid-liquid transition, but the distribution for the conduction electrons is considerably modified. The plot of the number of coincidences against angle for the conduction electrons in the solid can be fitted very well by a parabola corresponding to two free electrons per atom, which indicates that the Fermi surface in extended k space does not depart very significantly from a sphere. The relative number of annihilations from the conduction electrons in the liquid is considerably greater and the distribution departs from the free electron parabola at large angles. These effects are interpreted in terms of the distortion of the wave functions and the broadening of the electronic energy levels by the disorder in the liquid. It is concluded that the uncertainty in the wave vector of an electron at the Fermi surface in the liquid is about 20% of the Fermi wave vector. (auth)

D. R. Gustafson

Papers

Positron Annihilation in Liquid and Solid Mercury

Electronic structure of liquid gallium by positron annihilation