Positron trapping at dislocations in metals

The trapping rate of positrons at dislocations in metals, and its temperature dependence, are calculated. Two different trapping processes, with the excess energy absorbed in either electron-hole pair formation or by phonon creation, are considered and the former is found to be the most important. An extension of the theory to include depletion of the positron density around the dislocations in a diffusion approximation is included. The trapping is found to be transition limited if the temperature is low or the trap potential shallow. At room temperature diffusion is important for deep traps.     

Temperature dependence of positron trapping in deformed nickel

Non-equilibrium populations of defects, produced by plastic deformation of Ni, were used to study temperature dependence of the capture cross-section for positrons. A weak positive temperature coefficient was found in material of 99.8% purity but no temperature dependence in 99.99% purity.     

A high resolution lifetime study of positron trapping by vacancies in lead

The trapping model is tested by unconstrained analyses of 31 positron annihilation spectra from Pb_between 22 and 327°C. Temperature variations in both λ_V and λ_f, are observed and the sensitivity of E_V and μ exp (S/k) to the details of analysis is discussed.     

Hamiltonian formulation for the positron trapping model

Summary A Hamiltonian formulation is used to build up an adequate Hamiltonian for the positron trapping model. The number of positrons annihilated in the free state,n _f, or the trapped one,n _v, are considered canonical conjugate variables; this point of view provides a route to propose a Hamiltonian which leads to the previously proposed phenomenological master equations.

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Riassunto Si usa una formulazione hamiltoniana per elaborare un'hamiltoniana adeguata per il modello a trappola dei positroni. Il numero di positroni annichilati nello stato libero,n _f, o in quello intrappolato,n _v, è considerato come variabili coniugate canoniche; in questo senso si fornisce una via per proporre un'hamiltoniana che porti alle equazioni principali fenomenologiche proposte precedentemente.

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Резюме Гамильтонова формулировка используется для конструирования адекватного Гамильтониана для модели захвата позитрона. Число аннигилированных позитронов в свободном состоянии,n _f, или число захваченных позитронов,n _v, рассматриваются как канонически сопряженные переменные. Этот подход приводит к Гамильтониану. Который дает ранее предположенные феноменологические ?управляющие? уравнения.

     

Properties of positron annihilation in metals

Positron annihilation in bulk metals has been studied by examining the angular distribution of the annihilation photons in polycrystalline samples of magnesium, aluminum, copper, zinc, indium, tin, lead, and bismuth. It has been shown that conduction electrons as well as core electrons take part in this process. The conduction electron densities and Fermi energies have been determined. It is found that the electron density in the vicinity of a positron is significantly higher than the density of the free electron gas. We believe that this is due to the formation of Wheeler complexes and we estimate its charge. We have analyzed various means of measuring the conduction electron density and conclude that the positron method gives the most reliable information. Fiz. Tverd. Tela (St. Petersburg) 41, 929–934 (June 1999)     

Vacancy formation energies from positron trapping measurements

The threshold temperatureT _t at which thermally generated vacancies produce measurable positron trapping is a linear function of the energy of self-diffusionQ. SinceQ is also linearly related to the vacancy formation energyE _1v ^f , a measurement ofT _t leads directly to a determination of the latter. It is possible to make a precise determination ofE _1v ^f without approaching the melting point—a major advantage in dealing with refractories or with metals having a high vapour pressure in the solid state.     

Temperature effect on positron trapping in Fe59Ni25Cr16 alloy containing voids

Positron annihilation measurements were performed in austenitic Fe₅₉Ni₂₅Cr₁₆ alloy containing vacancy-type defects. Lifetime data were recorded between 4K and 400K. The positron trapping process was studied as a function of temperature and cluster size. The smaller defects characterized by a lifetime of 260 ps yield an unchanged trapping rate during temperature scanning. However, the trapping phenomenon is strongly temperature dependent for the larger size defects, with a typical lifetime of 500 ps.     

A new positron source for positron annihilation lifetime experiments

A method using ion-implantation of Na²² to produce source-sample systems applicable to positron annihilation lifetime experiments at elevated temperatures in metals is presented.     

Evidence for positron trapping in magnetism from dopplerbroadening measurements

Positron annihilation Dopplerbroadening measurements, display for the first time the effect of positron trapping in thermally induced vacancies in magnesium. The vacancy formation enthalpy is derived and is 0.85 eV.     

Effects of positron phonon interaction in metals

We investigate the importance of positron phonon interaction for the many body effects observed in positron annihilation in metals. The interaction is formulated as the screened Coulomb interaction of the positron with the polarization charge of the metal produced by lattice vibrations. The phonon contribution to the effective mass is found negligibly small. Due to the interaction with phonons the equilibrium positron momentum distribution deviates from a Boltzmann distribution leading to a larger thermal smearing of the annihilation curves than expected from the rather small effective mass. The positron energy loss in the thermalization process due to phonon excitation is discussed and found comparable to the energy loss due to the excitation of electron hole pairs in the experimentally interesting temperature range.     

Electron density and positron trapping at a vacancy

The redistribution of valence electrons at a vacancy in aluminium is calculated in the Thomas-Fermi approximation. This result is then used in evaluating the annihilation characteristics of positrons trapped at vacancies. A good agreement with experiments is established.     

Defect trapping by100Pd in fcc metals

Lattice defects in Al, Cu, Ag and Au were studied by the perturbed angular correlation technique (PAC) using the probe atom¹⁰⁰Pd/¹⁰⁰Rh. The comparison of data obtained on interstitial trapping in Cu and Au at different probe atoms (¹⁰⁰pd,¹¹¹In) allows defect characterisation less affected by the respective probe. The trapping efficiency of¹⁰⁰Pd for vacancy like defects is quite different to that of¹¹¹In atoms.     

Temperature characteristics of positron trapping at defects in electron-irradiated silicon

Positron-annihilation lifetime and Doppler-broadening measurements are used to investigate defects in silicon irradiated at 373 K with 6 MeV electrons to a dose of 1×l0¹⁹e/cm². In the unirradiated silicon sample (p type) a temperature-independent behaviour of the bulk-lifetime is observed in the temperature interval 110–500 K with a constant value of 220±1 ps. The slight effect observed on the S-parameter evolution is explained taking into account the thermal expansion of the lattice. The lifetime results obtained at 80 K and at 300 K after isochronal annealing as well as the behaviour of the intensity of the second lifetime componentI ₂ during lifetime measurements below the irradiation temperature in the irradiated silicon sample (n type), clearly indicate the temperature dependent characteristics of the positron trapping cross section _t(T) T ⁿ withn= –1.905±0.016. From isochronal annealing results, an annealing stage is observed in which di-vacancies agglomerate into quadri-vacancies. The mean positron lifetime in those quadri-vacancies is 350 ps.A.B.O.S., on leave from University of Kinshasa, Zaïre     

Temperature dependence of positron trapping rate at lattice vacancies

The positron trapping rate at a lattice vacancy in a metal may not vary in the manner assumed by Seeger. A correct interpretation of quenched cadmium data does not confirm a $\text{T}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}$ dependence for the trapping rate.     

Magnetic field effects on positron annihilation in metals

It is shown that the positron annihilation rate from a state with momentum k parallel to the magnetic field is periodic in $\text{1}\text{B}$, and that the periodicity is determined by the area of the section of the fermi surface whose z component of momentum is k.     

Doppler broadening of positron annihilation photons in metals

The Doppler broadening of positron annihilation photons was measured in 17 metals. A model which considers the positron lifetime spectra in metals to be composed of terms for annihilation with conduction and core electrons and surface centers of low electron momentum is used to correlate calculated core annihilation rates with the Doppler lineshape. Ta metal was doped with defects with high energy implantations of¹⁴N⁺⁴ ions at variable doses. Differences in the Doppler linewidths were ascribed as being principally a reflection of the probabilty of annihilation with core electrons relative to annihilation with conduction electrons.     

Deconvolution of Doppler-broadened positron annihilation energy spectra in metals

The development of computer programmes to deconvolute and fit the Dopplerbroadened annihilation radiation from a moving positron-electron pair is discussed. An analysis is presented for the annihilation line observed in a series of metals using a high-resolution Ge(Li) detector.     

Influence of escape from traps on parameters in the positron trapping model

Positron detrapping may explain weak trapping in some metals, and it can affect deduced vacancy formation energies. However, it appears that the recently observed additional temperature dependence of the positron capture rate cannot be attributed to escape from traps.     

Lattice location and trapping of hydrogen implanted in FCC metals

Hydrogen implantation in fcc metals is studied by ion-beam analysis methods. The lattice location and long-range migration are carried out after low-temperature implantation defects is found.