Positron Annihilation Angular Correlation Research Articles

Effect of CDW on positron annihilation characteristics in alkali metals

The existence of charge density waves in alkali metals can cause smearing at the Fermi momentum of the positron annihilation angular correlation profile. It is suggested that a significant part of the apparent non-thermalisation of positrons could be a reflection of the charge density wave ground state.

Statistical analysis of positron annihilation and Compton profile experiments using normal probability plots

Abstract The use of normal probability plots as a sensitive method for statistical processing of experimental data from positron annihilation angular correlation and Compton profile experiments is suggested. Examples showing the superiority of this method over the x2 method used hitherto, are discussed.

Experimental tests of theoretical band structure of intercalation compounds of graphite

Experimental techniques which have been recently applied to study the electronic band structures of graphite intercalation compounds are discussed. These techniques include electron energy loss spectroscopy, angle resolved photoemission spectroscopy, and angular correlation of positron annihilation. Results are compared with theoretical calculations.

Effet du phosphore et du bore sur la concentration de lacunes à l'équilibre dans des austénites 14–18

Resume Using positron annihilation (angular correlation method), we have observed that there is a great interaction between phosphorus and the equilibrium concentration of vacancies in a pure stainless steel 316L. On the contrary, boron as an impurity has no such detectable effect in this material.

On the determination of core and valence electron enhancement factors in positron annihilation studies

A new method is proposed by means of which the core and valence electron enhancement factors can be extracted from experimentally measured positron annihilation angular-correlation curves in conjunction with theoretical calculations. As an example, the method is applied to obtain the average enhancement factors for aluminium. However, the method is generally applicable to all classes of materials.

Investigation of the electronic structure of F.C.C. Co92Fe8 by measuring the angular correlation of positron annihilation δ‐quanta

Investigation of the electronic structure of F.C.C. Co₉₂Fe₈ by measuring the angular correlation of positron annihilation δ‐quanta

Positron Annihilation Rates in the Metals of Ag and Au Series

AbstractThe effect of electron–electron and electron–positron correlations on the positron annihilation rates in the metals of the Ag and Au series are investigated and discussed with the help of positron annihilation angular correlation results. The correlation effects are found to have little variation for the metals in the same series. However, they decrease for the higher Z series metals. The calculated annihilation rates obtained using the simple proposed model are found to be in good agreement with those observed experimentally.

The Fermi Surfaces of Cu–Pd Alloys Studied by Positron Annihilation

The Fermi surfaces of Cu–Pd alloys below 63% Pd are studied by means of the angular correlation of positron annihilation with crossed- and long-slit geometries. The - and -radii decrease with increasing palladium content up to 30% in agreement with a calculation based on the rigid band model. The neck radius also decreases with increasing composition, and it is suggested that the Fermi surface detaches from the hexagonal face of the Brillouin zone around 20% Pd, but remains to bulge in this direction at higher compositions. No detectable change is found in the correlation curves of the ordered and disordered states at 15 and 19% Pd. The origin of stabilization of the long period ordered structure near Cu 3 Pd is discussed in the light of the present results.

Positron Annihihtion in Colour Centres

AbstractThe angular correlation of positron annihilation in NaCl and KCl is compared for various concentrations of F‐centres produced by γ‐irradiation as well as additive doping. The intensity and the “full width at half‐maximum (FWHM)“ ‐Γ1/2 of the narrow component are slightly different for each of the X‐irradiated, electron‐irradiated, γ‐irradiated (light and heavy doses), and addi‐tively doped crystals. The possible role of additional defect centres in trapping positrons is discussed. Also angular correlation measurements of positrons annihilating in the reversible F ⇌ F' conversion centres in equilibrium in additively coloured KCI at −100 °C are given.

Evaluation of positron annihilation angular correlation curves and line profiles of compton scattering in noble metals

Angular correlation curveN (θ) as well as the Compton line profile I(δλ) of Cu, Ag, Au and Pt have been calculated by the assumption, that the coincidence counting rate and Compton line shift intensity are proportional to the area of cuts through the body defined by the Fermi surface. Parametrical expression of the Fermi surface were employed and the results were used to fit some measured curves on Cu, Ag and Au.

Solvated positron chemistry. The reaction of hydrated positrons with chloride ions

The reaction of hydrated positrons ( c aq + with cloride ions in aqueous solutions has been studied by means of positron annihilation angular correlation measurements. A rate constant of k = (2.5 ± 0.5) × 10 10 M −1 s −1 was found. Probably the reacting positrons annihilated from an e + Cl − bound state resulting in an angular correlation curve 8% narrower than for the hydrated positron. Carbontetrachloride in benzene seems to give similar, but smaller effect.

The effect of divacancies on positron annihilation in Cu

The effect of divacancies has been observed on the temperature dependence of gamma - gamma angular correlation of positron annihilation in copper. The peak counting rates of the angular correlation are higher at temperatures near the melting point than those predicted by the trapping model assuming that only single vacancies exist. Numerically analysing the results by the trapping model including divacancies, the formation energy of a single vacancy and the binding energy of a divacancy were estimated to be 1.28+or-0.06 eV and 0.3+or-0.1 eV, respectively.

Effect of Chemisorbed Water on the Magnetic Quenching of Orthopositronium in Silicon Dioxide Powders

Positron-annihilation angular-correlation measurements have been made on compressed pellets of powdered Si${\mathrm{O}}_{2}$. The results show an anomolously strong magnetic quenching of orthopositronium which disappears with the removal of chemisorbed water.

Study of Single Crystals of Magnesium and Zinc by Positron Annihilation

A point geometry positron annihilation angular correlation apparatus has been used to study oriented single crystals of magnesium and zinc. The results are compared with the theoretical predictions obtained from a simple picture of the electronic wave functions combined with two models for the positron wave function. The agreement between theory and experiment is considerably improved in the case of magnesium by the use of an enhancement factor.

On the determination of monovacancy formation energies from the temperature dependence of positron annihilation in metals

Abstract The effect of the temperature dependence of the mobility of thermalized positrons on the determination of the formation energies of vacancies from the temperature variation of the positron life-time or the angular correlation of positron annihilation is calculated.

Electron momentum distribution in silicon

Abstract X-ray Compton profiles of silicon were measured, using Mo Kα (Λ = 0·709 A) and Ag Kα (0·559 A) along the [100], [110] and [111] directions. After subtracting the 2s 22p 6 Hartree-Fock core contributions in the impulse approximation (1δ2 not excited the Compton profile for the four valence electrons averaged over these three directions agreed with the averaged theoretical profile calculated by Stroud and Ehrenreich (1968) and with the averaged profile determined by Erskine and McGervey (1966) from their positron annihilation angular correlation studies. The small departures from spherical symmetry in the Compton profiles did not agree with the theoretical calculations nor with the positron measurements (which agree with each other) but rather appeared to be similar to the anisotropies in the diamond Compton profile. The disparity between the x-ray and positron work may be due to the local positron shielding of the electron-electron correlation effect.

The angular correlation of positron annihilation of Cu — The influence of plastic deformation —

The effect of plastic deformation in Cu crystals on the angular correlation of positron annihilation was studied. A remarkable change of the correlation curve was mainly caused by clustered vacancies and dislocations.

Vacancy-impurity complexes and positron annihilation

The angular correlation of positron annihilation in quenched samples of Cu-0.5 at.% Ge, Cu-0.5 at.% Zn and Cu-0.5 at.% Ni alloys has been measured. The vancancy-impurity binding is very important and it alters the general features of the vacancy effect.

Systematic Angular-Correlation Studies of Positronium Formation in Silicon Dioxide Powders

Positron-annihilation angular-correlation measurements have been made on compressed silicon dioxide powders having mean particle diameters of 50, 120, and 400 \AA{}. The narrow component in the momentum distribution is much more pronounced in the 50- and 120-\AA{} powders than it is in the 400-\AA{} powder. Measurements made in vacuum reveal a narrow component in the momentum distribution which increases with increasing temperature until a saturation temperature is reached. A positronium diffusion constant has been calculated for five temperatures. Similar measurements made in oxygen show an increased narrow component with a nonsaturating temperature dependence. From these measurements a temperature-dependent oxygen enhancement factor has been determined. Both vacuum bakeout and compressing the powder were found to increase the amount of narrow component.

Apparent extinction of the (111) neck effect in the angular correlation of positron annihilation in a dilute Cu-Mn single crystal by quenching

The effect of quenching on positron annihilation in single crystals of pure copper and Cu-0.5 at% Mn was studied. From the results, the nature of electrons at the vacancy sites and the vacancy-impurity binding were discussed.