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The valence electron and positron charge densities in InAs and InSb are obtained from wave functions derived in a model pseudopotential band-structure calculation. It is found that the positron density is maximum in the open interstices and is excluded not only, as usual, from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001–110) plane. The results are used to analyze the positron effect in narrow-gap semiconductors.

Positron annihilation measurements made on Mg-Cd alloys are considered. Some pronounced effects at the order-disorder transition were found when the alloy composition was close to stoichiometry. No apparent effects of the order-disorder transformation were exhibited when the alloy was far from the stoichiometric composition. The measured mono-vacancy energies of the system at the stoichiometric composition showed a pronounced decrease when the alloy passed from an ordered to a disordered state.

The advent of synchrotron sources has led to an increasing availability of high resolution Compton profiles J(pz ) and a consequent renewed interest in the reconstruction of the corresponding full momentum densities p(p). We present results of applying a new method in which the radial parts of p(p) and the measured profiles are expressed in terms of the Jacobi polynomials. The technique is demonstrated using model projections that correspond to Mg and Gd spectra. Reconstructed densities, being in very good agreement with model ones, are a very good performance of our new reconstruction algorithm. PACS numbers: 71.20.—b, 78.70.Bj

Recent developments in the field of two-dimensional angular correlation (2D ACAR) studies of the Fermi surfaces of metallic solids are reviewed. The current power of the technique to image the Fermi surfaces of electronically complex systems is discussed and illustrated and the further and dramatic improvement that must result when the present radioisotope positron sources are replaced by intense monoenergetic positron beams is demonstrated.

A new approach to the analysis of positron-annihilation long-slit angular distributions which can provide a more direct reflection of Fermi surface profile has been described in a recent publication. The analysis involves the periodic superposition of angular distributions which has the effect of converting a momentum distribution into a distribution in reduced Bloch wave vectorsk. When applied to data for a Cu crystal, having the resolved momentum component parallel to the [110] direction, the analysis results in excellent agreement with the computed surface of Halse and also provides a guide to the form and intensity of the angular distribution for core electron annihilation.

Studies were made of the interactions of positrons with three low index surfaces of aluminum [(100), (110), and (111)] at room temperature with the aluminum surface at elevated temperatures and with oxygen adsorption on the aluminum surface. Our measurements using the improved two-dimensional angular correlation of annihilation radiation technique on surfaces support three established processes: spontaneous positronium (Ps) formation and emission, positrons bound in a surface state, and surface-state positrons thermally desorbed as Ps atoms. A method has been developed to accurately separate these components. The positron surface-state annihilation spectra are nearly isotropic for all three Al surfaces, which indicates a lateral localization and is inconsistent with either the ideal image-potential-induced bound state or the physisorbed Ps state. The thermal Ps momentum distributions agree well with the thermostatistical emission mechanism. The momentum distributions of the directly emitted Ps atoms are found to be very sensitive to the surface conditions, and to reflect the electron density of states near the surfaces, thus suggesting a new surface spectroscopy, angle-resolved Ps spectroscopy.

It has been established that, during heat treatment of an Al-Mg alloy, the presence of any oxygen in the annealing atmosphere causes a profound change in the recovery behaviour. This creation of substitutional MgO complexes has been suggested as the reason behind this anomaly. These complexes cause a permanent contamination of the sample.

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 authors have measured the two-dimensional angular correlation of positron annihilation radiation from single crystals of aluminium containing voids and other defects. No asymmetry was found between the (110) and the (100) directions for angles less than 10 mrad. This is contrary to what one expects if positrons are trapped in extended surface states on the void walls, which are primarily (111) planes. A tentative hypothesis involving positronium states in voids is offered.

The Doppler broadening of the positron annihilation line has been measured on polycrystal samples of quenched aluminium, neutron-irradiated aluminium, and quenched, neutron-irradiated aluminium, as a function of temperature over the range 130 K to ∼600 K. A peak in the lineshape (S) parameter is observed in both quenched samples, and is attributed to vacancy clustering. This peak is enhanced in the irradiated sample, probably b ecause of the presence of transmutation-produced silicon in this sample. The S parameter for the unquenched, irradiated sample also shows an interesting temperature dependence, some of which is produced by the temperature dependence of positron trapping into voids.

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.

The electron and positron charge densities are calculated as a function of position in the unit cell for two diamond and zincblende semiconductors, using wave functions derived from nonlocal pseudopotential band structure calculations for the electronic part, and the positron wave functions are calculated for single-particle approximation in manner which is free of any theoretical assumptions. Les densites de charge electronique et positronique sont calculees en fonction de la position dans la maille cellulaire pour deux semiconducteurs de structures diamant et zinc-blende. Pour la partie electronique, les fonctions d'onde sont obtenues a partir des calculs de structure de bande utilisant le pseudopotentiel non local. Les fonctions d'onde du positron sont calculees a partir de l'approximation qui consiste a ne considerer qu'une seule particule.

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.

The Fermi surfaces of Nb, Mo and a Nb50Mo50 alloy have been studied by the two-dimensional angular correlation positron annihilation technique. The unusually high precision of the alloy data has enabled the authors to make a quantitative measurement of some major Fermi surface parameters.

A computational technique is developed to evaluate electron and electron—positron momentum densities in silicon and germanium and the use of these to provide means of interpreting the k-space densities obtained by experimentalis's using the two-dimensional angular correlation of positron annihilation radiation technique (2D ACAR). As a consequence of the good agreement observed in the momentum densities and the LCW folded data obtained for silicon and germanium the independent particle approximation (IPM) coupled with the use of electron pseudo-wavefunctions represent an excellent starting point for a systematic many-particle description of the process, since significant deviations from the experimental angular correlation distribution can be ascribed to the electron—positron and electron—electron interactions. Nous presentons dans ce papier le developpement des techniques de calcul computationnelles pour evaluer les densites de moment de l'electron et de l'electron—positron dans le cas du silicium et du germanium. L'utilisation de ces techniques donne les moyens d'interpreter les densitěs dans l'espace des k obtenues par les experimentateurs utilisant la correlation angulaire a deux dimensions de la technique d'annihilation des positrons (2D ACAR). Le bon accord observe entre les densites de moment et les donnees obtenues par le theoreme LCW pour le silicium et le germanium montre que l'approximation de la particule independante (IPM) associee aux pseudofonctions d'onde de l'electron constitue un excellent point de depart pour la description systematique du processus a plusieurs particules. les ecarts entre les resultats calcules et experimentaux de la distribution de la correlation angulaire peuvent ětre attribues aux interactions electron—electron et electron—positron.

Positron lifetime and annihilation photon angular correlation studies have been performed in glasses of molar composition xNa2O, (1-x)B2O3. A strong dependence of the lifetime parameters on composition, x, is attributed to underlying changes in both disorder and free volume. Additional investigations of the effect of vanadium doping in a sodium borate 0.2Na2O, 0.8B2O3 glass and a calcium phosphate 0.5CaO, 0.5P2O5 glass suggest a further positronium inhibition mechanism. An interpretation based on electron scavenging by the transition metal impurities is tentatively proposed.

A positron trapped at a simple metal surface is treated within the jellium model. The positron lifetime and angular correlation (ACAR) spectrum for the surface state are calculated and compared with recent measurements. Results for the lifetime are good but the anisotropy of the calculated ACAR spectra is larger than that observed. It is suggested that surface defect trapping may be responsible for the isotropy of the experimental data. Results obtained for the angular correlation in the Independent Particle Model (IPM) and in the Mixed Density Approximation (MDA) are in close agreement, supporting the use of MDA in future calculations. The anisotropy of the calculated MDA spectra is shown to be insensitive to the selvedge width of the electron density but is affected by an attempt to represent the screening of the positron through a shift in the image plane of the positron potential.

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.

The positron pseudopotential method is employed in calculations of the effects of structure and specifically the effects of volume expansion and lattice vibration on the positron density distribution in metals. Calculations of the prevacancy temperature dependence of annihilation rates in several metals provide results in good agreement with observations.

The temperature dependence of positron annihilation parameters in In–Pb alloys is studied by the long-slit angular correlation technique. A strong correlation between the results and underlying changes in lattice constants is found. The lattice effect is most marked in the In-rich face-centred tetragonal (c/a > 1) phase. Die Temperaturabhangigkeit der Positronenannihilationsparameter in In–Pb-Legierungen wird mit der Lang-Schlitz-Winkelkorrelationstechnik untersucht. Es wird eine starke Korrelation zwischen den Ergebnissen und den ihnen zugrunde liegenden Anderungen in den Gitterkonstanten gefunden. Der Gittereffekt ist am starksten ausgepragt in der In-reichen tetragonal flachenzentrierten (c/a > 1) Phase.

The temperature dependence of positron annihilation parameters in In-Pb alloys has been studied by the long-slit angular correlation and Doppler broadening techniques. A strong correlation between the results and underlying changes in the lattice constants has been found. Both the parameters characterising the positron states and the measured values of the monovacancy formation energy show the effect, which is most marked in the In-rich face-centred tetragonal (c/a>1) phase.

High precision measurements of positron lifetimes in indium have been carried out at five temperatures in the range 83K to 405K. Constraint free analyses of the results clearly establish the multicomponent nature of the lifetime spectra and the fitness of the 'no escape' models of positron defect trapping phenomena. The implications for positron studies of vacancy formation energy are discussed.

During annealing of Ni and Au, previously quenched into HCl at 203 K, annealing stages that cannot easily be explained in terns of the migration of open volume defects are observed. The contamination of the hot samples with hydrogen released from the quenching fluid is suggested as the origin of these additional annealing stages. Wahrend der Ausheilung von abgeschreckten Ni und Au (in HCl, 203 K) haben wir Ausheilungsstufen beobachtet, die nicht als Auswanderung von offenen Defektarten erklarbar sind. Eine Kontamination der heisen Probe mit Wasserstoff, der von HCl freigelassen wird, wird als die Ursache dieser Ausheilungsstufen vorgeschlagen.

A two-dimensional angular correlation spectrometer based on position sensitive Anger camera detectors is described. The specification of the machine includes an angular resolution of 0.65 mrad FWHM by 0.65 mrad FWHM over an angular field of order 21*21 mrad2 and an exceptionally high efficiency for counting coincidence events. The spectrometer design is discussed in the light of the basic measurement problem and its performance is illustrated with the aid of the results of explicit test measurements and some representative results.

Experiments are reported which measure the angular correlation of gamma rays from positron annihilation in solid and liquid mercury, solid and liquid bismuth and a liquid bismuth-mercury alloy. The positron wave functions and the angular correlations to be expected from positrons annihilating in the ion cores are calculated in the single-electron approximation. It is not possible to use these calculations to subtract core annihilation from the total observed annihilation because the shapes of the theoretical and experimental curves do not agree at large angles. This situation is discussed. Valence electron annihilation is shown to be more probable relative to core annihilation in the liquids than in the solids, and for liquid bismuth it is compared with theoretical calculations by Ballentine.

A positron wave function incorporating the proper crystal symmetry is calculated and applied to the determination of that part of the angular distributions arising from the annihilation of core electrons in Cu, Cd, and Zn. The core electrons are treated in the tight binding approximation while the conduction electrons are assumed to be free. The resulting distributions are found to be in better agreement with experiment than those of previous independent particle calculations.

The Fermi surface of a ${\mathrm{Nb}}_{50}$${\mathrm{Mo}}_{50}$ disordered alloy has been studied by two-dimensional angular correlation of positron-annihilation radiation. The measured Fermi surface is well defined and several dimensions of the ellipsoidal hole pocket at $N$ have been determined. The results are compared with the predictions of various alloy band calculations.

A positron lifetime spectrometer incorporating digital stabilization allows for prolonged experiments without degradation of time resolution by electronic drifts. The consequent improvement in statistical precision increases the scope for realistic numerical analysis of complex types of spectra.

Three laser scanning instruments will be described for the inspection of surfaces; particular emphasis will be put on the components to provide special optical features. The first instrument is a pinhole detector for strip materials which can also be used for surface inspection. Two independent alternate scan lines are produced to highlight holes of various angles within the strips. A large, efficient, light guide collection system collects the transmitted light, and an acousto-optic modulator reduces the laser beam power when the scan is beyond the edges of the strip. An ancillary channel of the instrument collects part of the specular light reflected from the strip surface of one of the two scans and, by means of a double reflection at the inspected surface, such effects as gross tilt and heave of the strip do not affect the inspection process. Another strip inspection instrument using two scan lines is employed to detect all classes of defect in clear strip, such as glass or plastic. The two scan lines in this case are created simultaneously and are arranged to scan, on the far side of the strip, two linear gratings which are arranged to be exactly out of phase with each other. The scan of the light transmitted by the gratings should thus normally be constant and any deviation from this indicates the presence of defects in the strip. The third instrument has been designed to inspect the internal surfaces of cylinder bores for surface defects, the position and size of ports and surface finish. Two versions will be described, one using a probe containing a single clad optical rod and a second with a more complex fibre optic assembly.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The phenomenon of low energy positron-electron annihilation provides the basis for a wide variety of sensitive and significant studies of particle, atom, and materials problems. The breadth of the field can best be gauged from the many recent and excellent books and reviews describing works concerning fundamental tests of quantum electrodynamics [1], positronatom scattering in gases [2], positron and positronium chemistry [3], solid state physics [4,5,6,7], metallurgy [8,9], and medical physics [10]. The subject is clearly too large for complete cover by a single author within these proceedings. Accordingly we shall in this paper concentrate our attention on those studies in solids where the positron has proved a particularly powerful probe of the physics of the medium. The physics of the positron will be considered only to the degree that is necessary to establish its credibility as such a probe.

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.

A series of angular correlation spectra for positrons annihilating in the noble gases and in gas mixtures has been obtained using the University of East Anglia two-dimensional angular correlation spectrometer.l Beta positrons from a sodium-22 source are guided by a O.8T magnetic field to the interaction region, which is first evacuated and then filled with gas at 1 bar pressure at room temperature. Gamma cameras placed at 3m on either side of the sample chamber record the two-photon annihilations of free positrons and positronium (Ps) atoms in a 9mm-diameter volume of gas at the centre of the cell. Events recorded include annihilation from the triplet (m = O) — singlet state, which at O.8T is 91% mixed and has a mean lifetime of 9.7ns. In contrast, a negligible number of two-photon decays are rec — orded from (triplet) o-Ps atoms quenched on the (distant) walls of the gas cell or through collisions with gas atoms within the viewed volume.

This paper is a review of the applications of laser scanning in inspection. The reasons for the choice of a laser in flying spot scanning and the optical properties of a laser beam which are of value in a scanning instrument will be given. The many methods of scanning laser beams in both one and two dimensions will be described. The use of one dimensional laser scanners for automatic surface inspection for transmitting and reflective products will be covered in detail, with particular emphasis on light collection techniques. On-line inspection applications which will be mentioned include: photographic film web, metal strip products, paper web, glass sheet, car body paint surfaces and internal cylinder bores. Two dimensional laser scanning is employed in applications where increased resolution, increased depth of focus, and better contrast are required compared with conventional vidicon TV or solid state array cameras. Such examples as special microscope laser scanning systems and a TV compatible system for use in restricted areas of a nuclear reactor will be described. The technical and economic benefits and limitations of laser scanning video systems will be compared with conventional TV and CCD array devices.

The final inspection of manufactured goods is a labour intensive activity. The use of human inspectors has a number of potential disadvantages; it can be expensive, the inspection standard applied is subjective and the inspection process can be slow compared with the production process. The use of automatic optical and electronic systems to perform the inspection task is now a growing practice but, in general, such systems have been applied to small components which are accurately presented. Recent advances in vision systems and robot control technology have made possible the installation of an automated paint inspection system at the Austin Rover Group's plant at Cowley, Oxford.

Using a high-intensity beam of 200-eV positrons the authors have measured the two-dimensional angular correlation of the 2..gamma.. annihilation radiation from a clean Al(100) surface. The momentum distribution identified with the positron surface state has a nearly isotropic conical shape and a (7.1 +/- 0.5)-mrad full width at half maximum. The data are not consistent with a simple interpretation based on either the usual model of a positron bound in a surface state by its image-correlation potential or a positronium atom weakly bound to the surface. 15 references, 3 figures.

A novel charged-particle velocity filter of particular relevance to magnetic-transport slow positron beams has been successfully built and tested. The device employs the usual crossed electric and magnetic fields but, by incorporating a cylindrical electrode geometry, avoids the distortions and the consequent losses of intensity caused by post-filter aperturing that are common in the conventional planar electrode systems.

Curves for 2D ACPAR have been computed on the basis of LAPW band structure calculations and compared with experimental data. A new approach to enhancement was proposed based on a Thomas-Fermi type approximation. This treatment resulted in improving the agreement between theory and experiment by a factor of about three (as concerns the relative contribution of d- and s-electrons to positron annihilation) in comparison with the independent particle model. In order to extract information about the physical effects responsible for the residual differences between theory and experiment the isotropic average of the electron-positron density rho 0(p) was computed from the experimental data using the Cormack method and compared with theoretical predictions.