R N West

POSITRON ANNIHILATION AND DEFECTS IN METALS.

Abstract Measurements of the temperature dependence of positron lifetimes in some metals show a marked increase in lifetimes below the melting point. A simple theoretical treatment assuming “trapping” of positrons by defects reproduces the essential features of these measurements.

Positron annihilation and Fermi surface studies: a new approach

A new approach to the analysis of positron annihilation long slit angular distributions is presented. The analysis, which provides a more direct measure of the Fermi surface profile, is applied to data for Cu and the results are found to agree well with the computed surfaces of Roaf and Halse. The same analysis provides a guide to the form and intensity of that part of the angular distribution arising from the annihilation of electrons occupying states in the filled Brillouin zones.

The effects of vacancies on positron annihilation in cadmium

The strong temperature dependence of positron lifetimes in some metals has been explained in terms of the trapping of positrons by vacancies. To test the model further, precision measurements of the angular distribution of annihilation photons have been performed on samples of carefully annealed cadmium metal. The results, which are analysed in terms of the trapping model, provide a value of 0.52 ± 0.05 eV for the activation energy for vacancy formation in cadmium and emphasise the importance of core annihilation in the study of positron behaviour in metals.

Positron annihilation in disordered binary alloys

Measurements of the concentration dependence of the form of the long slit angular distributions for solid solutions of the PbIn, MgIn, CuAl and CuNi alloy systems are reported. A simple theoretical model provides an interpretation of the results in terms of the spatial distribution of positron and electron density within the alloy, and suggests a basis for the qualitative prediction of positron behaviour in any alloy system.

Positron annihilation in quenched cadmium metal

Abstract Positron annihilation experiments in quenched samples of cadmium are described. The results give further support to a theory of trapping of positrons by defects.

Positron studies of vacancy clustering in quenched aluminium and dilute aluminium alloys

Doppler broadening positron studies of annealing in quenched aluminium and dilute Al-Mg and Al-Zn alloys show that the effects of Mg and Zn on stage III annealing processes are essentially similar. Comparative analyses of the various measured annealing curves suggest that both microvoid and vacancy-loop formation are involved in stage III, the detailed balance being determined by the impurity concentration.

A simple correction technique for positron annihilation lineshape measurements

Abstract A simple correction technique for positron annihilation photon lineshape measurements which can significantly decrease systematic errors arising from resolution changes is described.

Positron lifetimes in solid and liquid mercury

Abstract Consistent and plausible values for positron lifetimed in solid and liquid mercury are obtained with both conventional sandwich and amalgam source-sample arrangements.

The effect of precipitation on positron annihilation in Pb-Sn alloys

Positron trapping at structural defects in lead, tin and some lead-tin alloys has been studied by measurements of the temperature dependence of the Doppler broadening of the annihilation photon line. Monovacancy formation enthalpies have been deduced from the data. In a Pb-8 at.% Sn alloy a sharp transitory peak in the peak height to area ratio or S parameter is observed some 20 degrees below the solvus temperature. An interpretation is offered in terms of enhanced positron trapping at grain boundary sites associated with the initial stages of tin precipitation.

The temperature dependence of positron lifetimes in cadmium

Positron lifetime studies in cadmium disclose a pattern of temperature dependence similar to that previously found in angular correlation and Doppler broadening studies. A value of 0.47+or-0.03 eV for the monovacancy formation energy is deduced.