Positron Annihilation In Copper Research Articles

Investigation of shock wave effects on positron annihilation in copper and tungsten

Doppler broadening of positron annihilation spectroscopy was performed during high-power short-pulsed laser irradiation to investigate the effects of the induced shock waves on the measured positron annihilation parameters of Cu and W. The measurements were performed using the recently developed technique of accelerator-based γ-ray induced positron annihilation spectroscopy. The Doppler broadening was measured in coincidence with nanosecond laser pulses. Preliminary results show no significant effects in these measurements. These experiments have, however, demonstrated the potential of measuring positron annihilation spectra in coincidence with shock waves.

Positron Annihilation in Copper Hydrogen-Charged Electrochemically

Atomic–electronic structure of copper foils is studied by a positron annihilation method directly in the course of electrochemical hydrogen sorption and after switching the current off. The shape of the curve of angular correlation of the annihilation radiation (ACAR) alters: the intensity of the broad Gaussian component, which is connected with the positron annihilation on the ionic core, is found to increase. The simultaneous increase in the Fermi energy points to an increase in the concentration of free electrons in the conduction band. The latter results from an increased number of electronic states following the formation of a solid solution of incorporation of hydrogen into the crystal lattice of copper. After switching the current off, the ACAR curve resumes its former shape, indicating that hydrogen leaves the crystal lattice and undergoes molization in caverns and on the grain boundaries. The discovered phenomena explain the increase observed in the size of cathodes—the “swelling” of copper foils during hydrogen sorption.

The application of pseudo-potentials to the detailed analysis of positron lifetimes in helium bubbles contained in metals

Abstract A pseudo-potential technique has been applied to calculate lifetimes for positron annihilation in copper and aluminium defected with bubbles of He at various gas densities and temperatures. The method was found to produce results in close agreement with the method of Jensen and Nieminen (1987). The pseudo-potential technique did however show that at high helium densities, where the He correlation potential becomes significant, the positron wavefunction deviates from its almost perfect surface state and becomes slightly extended into the bubble. This effect while being small in He bubbles may have greater importance in bubbles of heavier noble gases such as krypton and xenon which have much stronger correlation potentials.

Pseudopotential calculations of positron annihilation rates in bubbles of the heavier noble gases contained in copper

A pseudopotential technique has been applied to calculate lifetimes for positron annihilation in copper defected with bubbles of the noble gases Ne, Ar, Kr and Xe at various gas densities. The positron in each case was found to be in a surface state of the metal on the interior surface of the bubble, the wavefunction deviating slightly from a perfect surface state according to the strength of the attractive polarization potential from the gas atoms in the bubble. This effect was particularly noticeable in the case of xenon where the surface annihilation rate was reduced to about 40% of the perfect surface state.

Self-consistent LCGO calculation of positron annihilation in copper

Two-photon momentum density and angular correlation of positron annihilation radiation have been calculated using self-consistent, linear combination of gaussian orbitals band structure method within the independent particle approximation. The resulting anisotropies show very good agreement with the experimental ones.

New high precision lifetime measurements in pure copper and a 0.3 at.% AgSb alloy with a BaF2 lifetime spectrometer

AbstractThe temperature dependence of positron annihilation in copper and a 0.3 at.% AgSb alloy has been investigated by measuring the positron lifetime. In both cases the vacancy lifetime is nearly independent on temperature while the bulk lifetime shows a rapid increase after the onset of detectable vacancies. This increase can be explained by trapping during thermalization with an enhanced trapping rate.

Positron lifetime spectroscopy in copper

The temperature dependence of positron annihilation in copper was investigated by measuring the positron lifetime between 20 and 1070 degrees C. A high-resolution lifetime spectrometer (FWHM 170 ps) with a stability better than +or-1.5 ps over a period of four weeks was used. Data treatment in terms of the two-state trapping model showed the strong temperature dependence of the bulk lifetime tau 'f above 620 degrees C. the rapid increase in tau 'f can be explained by the assumption of trapping during thermalisation of the positrons. The evaluation of vacancy-formation enthalpies is only weakly influenced by such effects if one uses the I2( lambda 1- lambda 2) or the tau methods. For copper the extracted vacancy-formation enthalpy is Evf=1.28+or-0.04 eV.

Correlation of annihilation γ-ray polarization

The angular correlation of Compton-scattered photons coming from positron annihilation in copper has been measured. As is well known, such an experiment may test the validity of certain hiddenvariable theories. Our experiment differs from previous ones of the same type by the use of semiconductor detectors. These allow an accurate determination of the sum of the energies deposited by the annihilation photons in the scatterer and in the absorber of the Compton polarimeter. Our result is in complete agreement with the prediction of quantum mechanics.

Observations of the prevacancy temperature dependence of positron annihilation in copper

The temperature dependence of the Doppler-broadening lineshape parameter has been studied for Cu in the temperature range 30-1040 degrees C, with special attention given to the prevacancy region (T<500 degrees C). Effects attributable to positron trapping were observed in the prevacancy region for samples annealed at temperatures T<or=950 degrees C. Measurements at T>500 degrees C allowed a quantitative comparison to be made between the prevacancy traps and thermally generated vacancies. This comparison permitted a determination of both the fraction of positrons trapped in the prevacancy region and the lineshape parameter characteristic of annihilation in the prevacancy traps. It was found that the fraction of positrons trapped in the prevacancy temperature region depends upon the thermal and mechanical history of the sample, and possibly also on its impurity content. In addition, two possible explanations of the prevacancy trapping are discussed.

Source effects on positron annihilation in copper

It is shown that in sample-source sandwich arrangements the fraction of positrons annihilating in the source material itself contributes mainly to the source term which is also sensitive to the thermal treatment. The diffusion of the direct deposited source produces some type of surface defects which act as positron trapping sites.

An investigation of positron annihilation in copper between 93 and 665K

The temperature dependence of positron annihilation in copper between 93 and 665K has been investigated by simultaneous measurements of positron lifetime and Doppler broadening of the annihilation spectrum. The lifetime and spectrum shape were found to vary linearly with temperature, thus giving no indication of the existence of positron self-trapping in copper.

Measurement of linear polarization of positron annihilation photons

The linear-polarization correlation of γ-rays from positron annihilation in copper and plexiglass was measured under various experimental conditions. Both the absolute degree of linear polarization and its dependence on the source-polarimeter distance were found in agreement with quantum-mechanical predictions.

On the influence of high electric currents on the positron annihilation in copper and zinc

A recently reported influence of electric current on the angular distribution of annihilation quanta in Cu and Zn could not be confirmed, even at much higher currents.

The effect of plastic deformation on positron annihilation in copper of varying grain size

Mean positron lifetimes were measured in copper of grain sizes of 169, 264 and 309 μm as a function of tensile strain. Saturation of the positron lifetime was approached at values of strain proportional to the original grain diameter. The percentage of trapped positrons was evaluated for the 264 μm material.

The effect of plastic deformation on positron annihilation in copper of varying grain size

The effect of plastic deformation on positron annihilation in copper of varying grain size

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.

Positron annihilation in 3d metals: I. Band-structure calculation of momentum densities in copper and ferromagnetic iron

A calculation has been made of the contributions of the 3d and conduction electrons to the momentum density of photon pairs from positron annihilation in copper and ferromagnetic iron. A fast approximation scheme of Hubbard gives the band structure and the wave functions, which are used to compute the overlap matrix element. The momentum density is computed along the <100 >, <110 >, and <111 > directions in copper. A selection rule prevents large parts of the band structure from being sampled by the positron. The results are in qualitative agreement with experimentally observed features such as higher-momentum components and core anisotropy. Calculations for ferromagnetic iron, based on the lattice potentials of Wakoh and Yamashita and of Wood show that positron annihilation should enable one to distinguish between the band structures resulting from these potentials. A comparison with experiment is given in the following paper.

Positron Annihilation in Copper — Comparisons of Different Results

Different positron annihilation measurements of the momentum distributions in copper are compared. A disagreement and some indications about the nature of the electron wave functions are found.Received 22 September 1969DOI:https://doi.org/10.1103/PhysRevB.2.563©1970 American Physical Society

Influence du potentiel du reseau et contribution des couches a l'annihilation du positon dans le cuivre

Positron annihilation in copper is investigated, taking into account the periodic potential of the lattice. It is found that annihilation with the conduction electrons takes place practically as if the positron were free with its ordinary mass. The contribution to annihilation due to d electrons in copper is also evaluated ; this term is relatively small, but the only one able to explain the “tails” with large momenta of the experimental curves deduced from γ-ray angular correlations. Collisions between electrons and the thermalized positron do not broaden the energy levels much. All these results justify the approximations made in a preceding article.