Positron Annihilation Investigations Research Articles

Positron annihilation investigation for an estimation of the dislocation density and vacancy concentration of plastically deformed polycrystalline Ni of different purity

From the angular correlation measurements of the positron annihilation radiation on cold rolled, polycrystalline Ni of purity 99.99% and 99.84% the curve-shape parameters S are determined for treating the as-deformed and annealed samples up to a thickness reduction of 90%. The trapping rate of the positrons can be determined for a thickness reduction up to 15 or 18% for the dislocations and for the vacancies as well. Using known relations for the hardening within range 2 the specific trapping rates are estimated for the dislocations, μd = 2.9 × 10−15 s−1, and for the vacancies, μv = 2.2 × 10−15 s−1. Estimated dislocation densities and vacancy concentrations vary between 108 and 1011 cm/cm3 and 1016 and 1019 cm−3, respectively. Both of them show a remarkable dependence on the purity of the samples. Ni of low purity exhibits formation of vacancy clusters after annealing. Aus Winkelkorrelationsmessungen der Positronenannihilationsstrahlung an kaltgewalztem, polykristallinem Ni der Reinheit 99,99% und 99,84% werden die Kurvenformparameter S fur die Probenbehandlungen, deformiert sowie getempert bis 90% Dickenreduktion, bestimmt. Die Positroneneinfangraten konnen bis 15 bzw. 18% Dickenreduktion sowohl fur Versetzungen als auch fur Leerstellen ermittelt werden. Unter Benutzung von bekannten Beziehungen fur die Verfestigung im Bereich 2 werden die spezifischen Positroneneinfangraten fur Versetzungen zu μd = 2,9 × 10−15 s−1 und fur Leerstellen zu μv = 2,2 × 10−15 s−1 bestimmt. Die ermittelten Versetzungsdichten variieren von 108 bis 1011 cm/cm3, die Leerstellenkonzentrationen von 1016 bis 1019 cm−3. Beide zeigen eine markante Abhangigkeit vom Verunreinigungsgrad der Proben. Fur das verunreinigte Ni wird nach der Temperung eine Bildung von Leerstellenclustern beobachtet.

Electron liquid in collective description. Ii. Application to positron annihilation

For pt.i, see helsinki univ. Res. Inst. For theoretical phys., preprint 7-74. Positron annihilation is studied by using the boson description of the electron gas. The electron-electron interactions are taken in rpa, whereas the positron-electron interaction is more accurately treated. The scheme corresponds to calculating certain very large sets of perturbation diagrams. The authors give results for the positron annihilation probability, the pair-correlation function and the positron kinetic energy and chemical potential for various electron densities. New insight into the nature of the low-density divergencies occurring in many calculations of positron annihilation.

Complex energy bands inα-brass

The average-$t$-matrix approximation (ATA) is used to evaluate complex energy bands in the disordered alloy $\ensuremath{\alpha}$-CuZn over a range of Zn concentrations between 0 and 30 at.%. The calculations are based on the Korringa-Kohn-Rostoker equations of band theory and on the use of atomic phase shifts obtained from renormalized-atom muffin-tin potentials. We examine the effects of charge transfer between the atomic constituents on the basis of two empirical models. Both models involve a single free parameter whose value is adjusted to guarantee agreement with the experimental shift in the optical-absorption edge as a function of Zn concentration. The effects of lattice expansion in $\ensuremath{\alpha}$-brass are also included and are shown to have a significant effect on the energies of various states the vicinity of the Fermi level. The present calculations are compared with the available experimental results on $\ensuremath{\alpha}$-brass. Good agreement is found with experiments sensitive to the real parts of the energy bands, for example, the concentration dependence of both the splitting of the 5-eV peak in the optical-absorption spectrum and the neck radii as determined by some of the positron-annihilation investigations. The theory agrees only qualitatively with the results of measurements sensitive to the imaginary parts of the bands such as Dingle temperatures and residual resistivities.

Effect of Voids on Angular Correlation of Positron Annihilation Photons in Molybdenum

POSITRON annihilation investigations of defects in crystals have shown that for sufficiently high defect concentrations (typically above about 10−6) all positrons become trapped in the defects before annihilation, thus changing the characteristics of the annihilation process. For example, trapping of positrons may result in the increase in the positron lifetime, a narrowing of the 2-γ angular correlation distribution, and a reduction in the Doppler broadening of the annihilation line. Vacancies in metals1,2, deformation effects in metals3, and defects (F-centres or cation vacancies) in ionic crystals4 have all now been studied by this technique. The trapping of positronium (Ps) in defects in quartz5 and ice6 has also been investigated. Here we report the study of another crystal defect, the void, a small vacancy of diameter few tens of Angstroms. In practice voids may contain gas atoms and may be disturbed either in a random fashion or arranged in a macro lattice7.

Positron annihilation in alloys with structural vacancies

In NiAl alloys with a CsCl lattice comprising structural vacancies an abrupt change in the value of the probability of positron annihilation with conduction electrons is observed.