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Positron is an excellent probe of lattice defects in solids. A thermallized positron delocalized in lattice can be trapped at open volume defects, e.g. vacancies, dislocations, grain boundaries etc. Positron annihilation spectroscopy is a non-destructive technique which enables characterization of open volume lattice defects in solids on the atomic scale. Positron lifetime and Doppler broadening of annihilation photo-peak are the most common observables related to positron annihilation process. Positron lifetime spectroscopy enables to identify defects in solids and to determine their concentrations while coincidence measurement of Doppler broadening provides information about local chemical environment of defects. This article provides a review of the state-of-art of defect characterization in bulk metallic materials by positron annihilation spectroscopy. Advanced analysis of positron annihilation data and recent developments of positron annihilation methodology are described and discussed on examples of defect studies of metallic materials. Future development in the field is proposed as well.

A size effect in nonstoichiometric titanium monoxide TiOy and vanadium carbide VCy nanocrystals containing high concentrations of structural vacancies was detected by electron-positron annihilation. The effect consists of a considerable increase in the positron lifetime with the specific surface area. An analysis of the intensity of a long lifetime surface component in the positron annihilation spectra and the average positron lifetime for different nonstoichiometric compounds indicated a universal character of the observed size effect.

The objective of the study was the structural analysis of the 70TeO2-5XO-10P2O5-10ZnO-5PbF2 (X = Mg, Bi2, Ti) tellurite glasses doped with ions of the rare-earth elements Er3+, based on the PALS (positron annihilation lifetime spectroscopy) method of measuring positron lifetimes. Values of positron lifetimes and the corresponding intensities may be connected with the sizes and number of structural defects, the sizes of which range from a few angstroms to a few dozen nanometers. Experimental positron lifetime spectrum revealed existence of two positron lifetime components τ 1 andτ 2. Their interpretation was based on two-state positron trapping model where the physical parameters are the positron annihilation rate and positron trapping rate.

The influence of the positron distribution on the electron-positron (e-p) momentum densities (MDs) is studied in terms of the l (l = s, p, d, f) character of the initial electronic state. The effect of the positron is discussed for momenta in the extended and reduced zone scheme on the example of Al (simple metal), Cu (metal with almost filled d shell), and Cr (transition metal). Present result indicates a weak sensitivity of the e-p MD inside the central Fermi surface to the positron charge density for the delocalized s and p electrons. In the case of d states, the effect of the positron on the relevant contribution to the e-p MD directly reflects the degree of localization of d electrons in the electron density of states.

The objective of the study was the structural analysis of the TeO2-P2O5-ZnO-LiNbO3 tellurite glasses doped with ions of the rare-earth elements: Er3+, Nd3+ and Gd3+ based on the PALS (Positron Annihilation Lifetime Spectroscopy) method of measuring positron lifetimes. Values of positron lifetimes and the corresponding intensities may be connected with the sizes and number of structural defects, such as vacancies, mono-vacancies, dislocations or pores, the sizes of which range from a few angstroms to a few dozen nanometres. Experimental positron lifetime spectrum revealed existence of two positron lifetime components τ1 and τ2. Their interpretation was based on two-state positron trapping model where the physical parameters are the annihilation velocity and positron trapping rate.

$\gamma$ spectra for positron annihilation in noble-gas atoms are calculated using many-body theory for positron momenta up to the positronium-formation threshold. This data is used, together with time-evolving positron-momentum distributions determined in [arXiv:1706.01434 (2017)], to calculate the time-varying $\gamma$ spectra produced during positron cooling in noble gases. The $\gamma$-spectra and their $\bar{S}$ and $\bar{W}$ shape parameters are shown to be sensitive probes of the time evolution of the positron momentum distribution, and thus provide a means of studying positron cooling that is complementary to positron lifetime spectroscopy., Comment: This version contains minor update to references, otherwise unchanged

The annihilation processes of positrons with medium electrons are investigated in details. We solved the main problem of creation of nanoundulators, which have very large lifetimes, and by which we can control. The lifetime of a positron in the regime of channeling is estimated s, which on a times is bigger than at usual cases.

Water-adsorption and desorption processes in the modified functional elements based on humidity-sensitive MgO-Al 2 O 3 ceramics for microelectronics were studied using specialized positron annihilation lifetime system. It is shown that adsorption of water leads to transformation of positron annihilation spectra in the MgO-Al 2 O 3 ceramics and reflects increasing of positron trapping near grain boundaries of ceramics and ortho-positronium decaying in nanopores. Fixation of positron lifetime components results in changes in positron trapping rate.

A new photon-induced positron annihilation lifetime spectroscopy approach has been developed using an S-band compact electron linac at the National Institute of Advanced Industrial Science and Technology (AIST). The high energy ( 42 MeV ), intense (105 photons pulse−1), and ultra-short pulse (3 ps pulse width) photon beam creates positrons throughout an entire sample via pair production. A positron lifetime spectrum can be obtained by measuring the time difference between the accelerator's RF frequency and the detection time of the annihilation gamma rays. The positron lifetimes for lead and yttria-stabilized zirconia samples have been successfully measured.

A positron annihilation lifetime spectrometer using a magnetically guided continuous positron beam and the signal from secondary electrons (SE) emitted from the sample is presented. The positron and secondary electrons trajectories as well as the physics of SE were simulated using GEANT4 program. A special attention was paid to the consequences of positron backscattering, in particular on the shape of the lifetime spectrum. A very good agreement was obtained between simulation and measurements concerning the time resolution (~300–400 ps FWHM) for the studied range of incident positron energies. Due to the complex shape of the resolution function, the present design of the spectrometer is not suited to study of short lifetimes, but can be used for depth profiling porosimetry of materials with Ps formation with the an excellent peak to background ratio of ~105.

Ortho-positronium pick off annihilation lifetime gives the information of the microscopic free volume of polymers. Polyimide polymers such as Kapton are applied in many fields, but it was impossible to apply the positron lifetime method for free volume investigation because of no positronium formation. Here, we apply the idea of the free positron annihilation probability that is sum of the probability of annihilation by the positron and electrons on the molecular chain where the positron localized and that for the annihilation with the electrons on the neighboring molecular chains. The second term is probably affected by the free volume change. We have successfully shown the temperature dependence and the electron beam irradiation effect on the free volume change by observing the free positron annihilation lifetime for Kapton.

The Low Energy Positron Toroidal Accumulator (LEPTA) at JINR proposed for generation of positronium in flight can be used for positron annihilation spectroscopy (PAS) [1]. The positron injector of the LEPTA facility can generate continuous a slow positron beam with the intensity up to 1∙107s-1 at the energy in the range of a few eV to 100 keV and width of the spectrum 1 – 2 eV. The injector is based on radioactive 22Na isotope. The solid neon is used as a moderator to generate monochromatic beam. The parameters of the positron beam allow scanning the condensed matter in depth up to 10 microns with resolutions less than 10 nanometers and investigating layered structures for microelectronics and properties of a surface.

We describe the recent installation of a new slow positron beamline at AIST and our plans to develop a dedicated superconducting accelerator for positron production. The new positron beamline is already installed and should be operational by the end of this fiscal year (March 2012). Initially positrons will be generated using a 70 MeV electron beam from the existing accelerator directed onto a newly installed converter and moderator assembly. The beamline has two experimental ports both dedicated to positron lifetime spectroscopy, one port with a focused beam (diameter ~ 30 microm) and the other unfocussed (~ 10 mm). A superconducting accelerator for positron production is currently under development. When completed, it will deliver a high frequency (~ MHz), high current (~ mA), short pulse length (< 100 ps) beam to the positron production target. We investigate the possibility of transporting the positron pulses thus produced directly onto samples for lifetime measurement. Such a scheme would remove the necessity for pulse stretching and chopping which is required with the existing LINAC and should allow for greatly increased slow positron transport efficiency.

At present time the Low Energy Positron Toroidal Accumulator (LEPTA) at JINR is under commissioning with circulating positron beam. The LEPTA facility is a small positron storage ring equipped with the electron cooling system and positron injector. The maximum positron energy is of 10 keV. The storage ring is aimed for generation of direct fluxes of ortho-positronium (o-Ps), produced in recombination of the positron beam circulating in the ring with single pass cooling electron beam. The project has few goals: annihilation spectroscopy (PAS) to monitoring defects of nanometer sizes in materials as a function of depth managed by the positron energy in a rage of a few eV to 100 keV.

This chapter reviews the steps required to capture and accumulate copious numbers of low-energy positrons, and to form positronium (Ps) atoms at low energies. We then relate the few results that have so far been achieved using large pulses of positrons at high densities, include observing Ps–Ps spin exchange quenching, formation and optical excitation of the Ps 2 molecule, and production of a fully spin-polarized ensemble of Ps via Ps–Ps spin exchange quenching. In addition there are many ongoing experiments that are benefiting from the availability of large positron pulses at lower densities, including studying antihydrogen, doing optical spectroscopy on Ps and its negative ion ( Michishio et al., 2015 ) with pulsed lasers, and eventually for measuring Ps free fall in Earth's gravitational field. Meanwhile a number of interesting possibilities involving positron–positron and Ps–Ps interactions are on the horizon, including forming Ps Bose–Einstein condensates and dense collections of positrons having a nonnegligible positronic Fermi energy. In particular, collecting positrons at high density on a tungsten tip from which they would be field emitted could be the basis for a valuable single-molecule positron microscope suggested by Platzman ( Barbiellini and Platzman, 2006 , Mills and Platzman, 2001 ), in which the damage that would ordinarily be sustained by a molecule is mitigated by the rapid transfer of the molecular excitations to the metallic surface.

Functional materials which have excellent performances in electrics, magnetics, optics, calorifics, acoustics, mec- hanics, chemistry and biomedicine are widely used in the various high-tech field. Positron annihilation spectroscopy, including positron annihilation lifetime spectroscopy, Doppler broadening spectroscopy and slow positron beam technique, which is especially sensitive to detect various defects, vacancies and microvoids, is a very effectual method to probe the microstructure of materials. Defecs distribution information from the surface to the inner and microstructure variation due to external physical and chemical condition gradual change can be obtained by refined analysis of positron annihilation parameters. Microstructure of Several types of functional materials was studied by positron annihilation spectroscopy. It’s proved that positron annihilation spectroscopy is a unique technique to study the microstructure of materials.

It is described that the proton beam induceds micro-size defects and electronic deep levels in n or p type single crystal silicon. Positron lifetime and Coincidence Doppler Broadening Positron Annihilation Spectroscopy were applied to study of characteristics of p type and n type silicon samples. In this investigation the numerical analysis of the Doppler spectra was employed to the determination of the shape parameter, S, defined as the ratio between the amount of counts in a central portion of the spectrum and the total counts of whole spectrum. The samples were exposed by 3.98 MeV proton beams ranging between 0 to particles. The S-parameter values strongly depend on the irradiated proton beam, that indicated the defects generate more. Positron lifetime shows that positrons trapped in vacancies and lifetime increased according to proton irradiation.

A new type of a positron annihilation lifetime spectroscopy (PALS) system has been set up at the superconducting electron accelerator ELBE [ at Helmholtz-Zentrum Dresden-Rossendorf. In contrast to existing source-based PALS systems, the approach described here makes use of an intense photon beam from electron bremsstrahlung which converts through pair production into positrons inside the sample under study. The article focusses on the production of intense bremsstrahlung using a superconducting electron linear accelerator, the production of positrons inside the sample under study, the efficient detector setup which allows for annihilation lifetime and Doppler-broadening spectroscopy simultaneously. Selected examples of positron annihilation spectroscopy are presented.

Positron annihilation lifetime spectroscopy (PALS) is an experimental technique whereby the lifetime spectrum of positrons injected into a material is measured. Analysis of this spectrum can be used to characterize defects in the material. While radioisotope positron sources are often used for PALS, there are several advantages of using mono-energetic, slow positron beams. In order to measure lifetime spectra with such beams it is necessary to produce a pulsed beam with an extremely short pulse length (~ 200 ps). In this article we discuss the advantages of using pulsed, slow-positron beams, the various methods of beam production and pulsing methods. We focus in particular on intense beams generated by electron accelerators and describe the current status and future plans for the accelerator based facility at AIST.

A slow-positron beam system for in-situ positron lifetime measurements during ion beam irradiation is currently under development in order to obtain fundamental information on radiation-induced defects. The system uses a high-intensity positron beam generated through pair-creation by a 70 MeV electron beam from a linear accelerator. The system has typical pulsing electrodes for chopping and bunching positrons. The incident directions of positrons and ions are 0° and 45° to the surface normal of samples, respectively. H, He and inert-gas ions up to 150 keV can be introduced to a target chamber with the irradiation temperature variable from room temperature to 600 °C. In a preliminary examination of the beamlines, 150 keV Ar+, electrons from a photocathode and positrons from a radioisotope were successfully transported to the target chamber from their respective sources. In addition, in-situ lifetime measurements using the existing positron beamline were demonstrated.

A new and unique setup for Positron Annihilation Spectroscopy has been established and optimized at the superconducting linear electron accelerator ELBE at Helmholtz-Zentrum Dresden-Rossendorf (Germany). The intense, pulsed (26 MHz) photon source (bremsstrahlung with energies up to 16 MeV) is used to generate positrons by means of pair production throughout the entire sample volume. Due to the very short gamma bunches (< 5 ps temporal length), the facility for Gamma-induced Positron Spectroscopy (GiPS) is suitable for positron lifetime spectroscopy using the accelerator’s radiofrequency as time reference. Positron lifetime and Doppler broadening Spectroscopy are employed by a coincident measurement (Age–Momentum Correlation) of the time-of-arrival and energy of the annihilation photons which in turn significantly reduces the background of scattered photons resulting in spectra with high signal to background ratios. Simulations of the setup using the GEANT4 framework have been performed to yield optimum positron generation rates for various sample materials and improved background conditions.

A technique for determining the sizes of vacancies and pores in metals by positron annihilation spectroscopy (PAS) in the angstrom and nanometer ranges is proposed. Following the proposed technique, the radii of vacancy-type defects in iron are estimated. The obtained results are of special interest for research programs in which properties of critical engineering materials [49–55] are studied using beams of slow positrons at positron facilities (see Research Program at the LEPTA Facility, I. Meshkov for LEPTA collaboration).

Positron annihilation technique is a well known technique to characterize the defects in a material. These defects can be identified by positron annihilation lifetime and coincidence Doppler broadening of positron annihilation radiation measurement. In this chapter we report the room temperature positron annihilation lifetime for single crystalline ZnO. From our study it is confirmed that the present crystal contains VZn–hydrogen complexes with low open volumes. Another important nuclear solid technique is the Mossbauer Spectroscopic technique which has been used to probe the local magnetic properties of a solid. Here we have summarized Mossbauer spectroscopic studies on ferrites.

Computer simulation using GEANT4 codes indicates an enhancement of the fraction of implanted positrons stopped in the denser regions of a layered sample. However, positron lifetime measurements performed for layers of pure aluminum, silver and gold foils do not reveal this effect, indicating instead that backscattering plays an important role in determining the positron implantation profile in layered and/or heterogeneous samples.

Native defects in complex oxides play a crucial role in determining their optical, electrical, and magnetic properties and it is di cult to identify and characterize them. Positron lifetime spectroscopy is a powerful technique to study vacancy defects; however its application to complex oxides has been limited. In this work we apply positron lifetime spectroscopy to study open volume defects in rare earth doped yttrium aluminum garnet (YAG) complex oxides grown in argon atmosphere. In YAG single crystals, positron lifetime measurements identi ed isolated aluminum vacancies and complexes of aluminum vacancy and neighbor oxygen vacancies. Thermoluminescence measurements were also performed to elucidate the interaction between trapping defects and luminescence centers. By combining positron lifetime and thermoluminescence, both the defect type and its e ect on the optical properties of YAG crystals were revealed.

The superconducting electron accelerator ELBE (Electron Linac with high Brilliance and low Emittance) at the Research Centre Dresden-Rossendorf (Germany) serves as a high-intensity bremsstrahlung photon-source delivering a pulsed beam (26 MHz) with very short bunches (

Bulk cobalt silicides Co2Si, CoSi and CoSi2 have been investigated using experimental positron lifetime, coincidence Doppler broadening and depth-resolved positron beam studies. Psitron lifetime and coincidence Doppler ratio curves have shown distinct features for the silicides as compared to that of pure Co and Si. Ab-initio calculations of positron life time have been carried out using the DOPPLER program. The experimental positron lifetime values are higher than the theoretical estimates for Co2Si and CoSi2, suggesting that these silicides contain vacancy defects. This is further supported by positron diffusion lengths deduced from the VEPFIT analysis of positron beam results. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the paper we consider application of the 48V isotope as a source in the positron lifetime spectroscopy. The isotope was produced in the 48Ti(p,n)48V reaction using 15 MeV proton beam. As a target the natural titanium thin plate was used. The measurements using the typical positron lifetime spectrometer have shown the usefulness of the source obtained for this application. Due to its properties, the source may be used for measurements of positron annihilation characteristics in high temperature or aggressive environments. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The paper presents theoretical and experimental studies of the positron implantation profiles in layered samples. The Monte Carlo simulations performed using the GEANT4 toolkit reveal accumulation of positrons in the denser layer embedded between two less dense layers. That effect is significant not only for low energy positrons at slow monoenergetic positron beams but also for high energy positrons which are emitted from radioactive nuclei in conventional experiments. Measurements of the positron implantation profile into the samples which consist of silver and aluminium foils of different thickness show profile features which correspond well with those simulated by the GEANT4 toolkit for examined cases. We propose to call this phenomenon the accumulation effect of energetic positrons.

Gamma-induced positron generation has been performed using the superconducting electron accelerator ELBE at the Research Centre Dresden-Rossendorf (Germany). It has an intense, pulsed (26 MHz) gamma source (bremsstrahlung). This source has been used to accomplish positron annihilation spectroscopy for different materials (Fe, silica, water). Since the gamma bunches are very short (

The two-component density functional theory is applied to the study of the thermalized positron state in simulated structures of hydrogenated amorphous silicon. Results show that positron properties in bulk and ad hoc defect structures are sensitive to the defect free-volume. Using the normalized positron density, it is determined that the thermalized positron state is weakly localized at hydrogen-decorated vacancy-like complexes, and not at microscopic open volume defects. These defect complexes form as clusters of hydrogen-passivated dangling bonds. It is also found that hydrogen enhances the delocalization of positron density in the simulated structures. The relevance of the present results to the interpretation of actual positron lifetime spectroscopy in real materials is discussed.

Hydrogen in nano-voids in neutron-irradiated nickel has been detected using positron annihilation lifetime spectroscopy (PALS). As positron lifetime is greatly affected by nano-voids bound with hyd...

Positron annihilation is an established technique for investigating vacancy-type defects in semiconductors. When a positron is implanted into solids, it annihilates with an electron and emits two 511 keV γ quanta. From measurements of Doppler broadening spectra of the annihilation radiation and the positron lifetimes, one can detect vacancy-type defects such as monovacancies and divacancies. We used monoenergetic positron beams to study vacancies in ion-implanted and rare-earth-doped GaN. We identified the defect species and estimated their concentrations from a comparison between the Doppler broadening spectra obtained through the experiments and those calculated using first-principles calculation (projector augmented-wave method). We have thus shown that positron annihilation is a useful tool for studying relationships between vacancy-type defects and properties of group-III nitrides.

This paper describes the development and application of an intense slow positron beam at IHEP with regard to its two main components.The Variable-Energy Positron Lifetime Spectroscopy (VEPLS) based on the pulsing system consisting of a chopper,a prebuncher and a buncher has been constructed in or...

Recent advances in the study of quasicrystals (QC's) and their related materials (crystalline approximants) by using positron annihilation spectroscopy are reviewed. We describe how a series of positron annihilation spectroscopy has been applied for the studies of aperiodic QC's. Positron lifetime measurements revealed the presence of high concentration of structural vacancies giving rise to saturation trapping for QC's and approximants studied. Chemical environments around the structural vacancies were successfully clarified by coincident Doppler broadening measurements. In addition, the concentrations of structural vacancies were estimated by analyzing the positron diffusion data obtained by a slow positron beam under the saturation trapping condition. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the paper we present positron annihilation studies of the subsurface zones created below pure magnesium surface exposed to sliding. We used the conventional positron techniques i.e. positron lifetime spectroscopy and DSIP (Depth Scanning of the positron Implantation Profile) method, and variable positron energy beam technique. The obtained results suggest the creation of dislocations zone during dry sliding in the subsurface which weakly localize positrons.

Achievements in instrumentation for positron lifetime measurements are summarized with emphasis on digital spectrometric systems. The main part of the data presented are based on the long-term exploitation of a conventional positron lifetime spectrometer developed at Charles University in early 90s, on bench-mark testing measurements with this spectrometer working in conjunction with a pair of 8-bit ultra-fast digitizers and on analogous measurements with a new, recently assembled digital positron lifetime spectrometer. Perspectives of further improvements of positron lifetime instrumentation are discussed.

A monoenergetic positron beam was employed to characterize the uniformity and the microstructural variation of thermally treated W–Si–N thin film. As the annealing temperature is increased, positrons are found to be progressively trapped in sites rich in silicon. This behavior is explained by the formation of W clusters from which positrons are favorably trapped into the Si–N amorphous matrix. Positron results are discussed together with information obtained on similar samples by Rutheford backscattering, infrared spectroscopy and transmission electron microscopy measurements.

There are many advantages in being able to perform positron annihilation lifetime spectroscopy (PALS) using a variable energy positron beam, the most obvious being the easy identification of different defect types at different depths. The difficulty in conducting variable energy (VE) PALS studies lies in the fact that a “start” signal is required to signal the entry of the positron into the target. Two methods have been used to overcome this problem, namely the bunching technique, which employs radio frequency (RF) cavities and choppers, and secondly the use of secondary electrons emitted from the target. The latter technique is in terms of experimental complexity much simpler, but has in the past suffered from poor time resolution (typically ∼500 ps). In this work, we present a series of computer simulations of a design based on the secondary electron emission from thin C-foils in transmission mode which shows that significant improvements in time resolution can be made with resolutions ∼200 ps being in principle possible.

A new type positron beam system is being constructed in Wuhan university. The goal of this project is to build a positron beam which can measure positron lifetimes and has high moderation efficiency. The system utilizes a magnetically guided incident positron beam and the sample is biased to a high negative potential to achieve the desired implantation energies. A conventional tungsten moderator is replaced by a solid Ne moderator with high moderation efficiency (about 1%). A multi-functional target chamber for slow beam studies is designed, which can be used for positron annihilation lifetime spectroscopy (PALS), Doppler broadening (DB) and coincidence Doppler broadening (CDB) measurements.

We assessed usefulness of commercially available sealed 22Na positron sources for positron annihilation lifetime measurements. Three sources with different nominal activities below 1.85 MBq were adopted for measuring positron annihilation lifetimes in silicon and annealed copper. It was found that all the sources have long lifetime components over 1 ns with different relative intensities of several percents. The result suggests that care should be taken in the analysis of the positron annihilation lifetime data obtained with these sources.

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.

Positrons may be used in many cases to determine defect densities of vacancies and dislocations. In case of saturated positron trapping, i.e. all positrons are getting trapped, only a lower-limit estimation can be given. However, a combination of positron back-diffusion measurements using a monoenergetic positron beam in combination with conventional lifetime spectroscopy can be used to overcome the problem of saturated positron trapping. As a case study, this combination was used for the determination of dislocation densities in polycrystalline nickel samples of highly varying dislocation density. Saturated positron trapping into dislocations and small voids was observed. The total positron trapping rate was calculated from the positron diffusion length obtained by back-diffusion experiments. The trapping rates of the two defects were finally obtained using the decomposition of lifetime spectra. The results were found in good agreement with those determined by the analysis of synchrotron Bragg-diffraction profiles, measured on the same set of samples. From the comparison of both techniques, the positron trapping coefficient was found to be μ disl = 3.9 ± 0.3 cm 2 /s for a high density of dislocations in Ni.

In this paper, we propose a non-destructive testing method with positron annihilation lifetimes for thick materials. The positron annihilation lifetime is sensitive to the defect density of atoms or dislocations. A positron from a radiation source is annihilated with an electron at the front surface of the materials under investigation. The application of the positron lifetime method is restricted to thin materials. A positive muon can readily penetrate thick materials and decay into a positron and two neutrinos. The positrons from the muon decays are annihilated with electrons in the materials. The positron annihilation method using muon decays can be applied to thick materials. We estimated the efficiencies and the statistical uncertainties of the lifetime measurements for this method using a GEANT simulation. Some technical issues pertaining to the estimation of the defect distribution with this method are discussed.

The positron lifetime technique was employed to study vacancy-type defects in 8 MeV electron-irradiated n-type 6H silicon carbide. A long-lifetime component having a characteristic lifetime of 223–232 ps was observed in the irradiated sample and was attributed to the VCVSi divacancy. Other positron traps, which dominated at low temperatures, were observed to compete with the VCVSi for trapping positrons. A positron trapping model involving a positron shallow trap, a negatively charged monovacancy and the VCVSi divacancy was found to give a good description of the temperature-dependent positron lifetime data of the 1200 ◦ C annealed sample. The identity of the monovacancy could not be unambiguously determined, but its lifetime was found to be in the range 160–172 ps.

A series of dense copolyimide membranes was characterized using positron annihilation spectroscopy. The positron annihilation lifetime spectroscopy performed on film with a classical positron source gives informations on the positronium fraction formed and also on the hole size within the film. The Doppler broadening spectra (DBS) of the gamma annihilation rays coupled with a variable energy positron beam allow the microstructural analyses as a function of the film depth. Experimental data were also linked to the chemical structure of the polyimides. It was found that the presence of the fluorine atoms strongly affects the positron annihilitation process and especially the DBS responses.

Our recent positron lifetime measurements for metal oxides suggest that positron lifetimes of bulk state in metal oxides are shorter than previously reported values. We have performed theoretical calculations of positron lifetimes for bulk and vacancy states in MgO and ZnO using first-principles electronic structure calculations and discuss the validity of positron lifetime calculations for insulators. By comparing the calculated positron lifetimes to the experimental values, it was found that the semiconductor model well reproduces the experimental positron lifetime. The longer positron lifetime previously reported can be considered to arise from not only the bulk but also from the vacancy induced by impurities. In the case of cation vacancy, the calculated positron lifetime based on semiconductor model is shorter than the experimental value, which suggests that the inward relaxation occurs around the cation vacancy trapping the positron.

Positron and positronium annihilation in silica-based thin films has been investigated by means of measurement techniques with a monoenergetic pulsed positron beam. The age–momentum correlation study revealed that positron annihilation in thermally grown SiO 2 is basically the same as that in bulk amorphous SiO 2 while o -Ps in the PECVD grown SiCOH film predominantly annihilate with electrons of C and H at the microvoid surfaces. We also discuss time-dependent three-gamma annihilation in porous low- k films by two-dimensional positron annihilation lifetime spectroscopy.

The Idaho Accelerator Center (IAC) has developed new techniques for positron annihilation spectroscopy (PAS) by using highly penetrating γ -rays to create positrons inside the material via pair production. Two sources of γ -rays have been employed. Bremsstrahlung beams from small-electron linacs (6 MeV) were used to generate positrons inside the material to perform Doppler-broadening spectroscopy. A 2 MeV proton beam was used to obtain coincident γ -rays from 27 Al target and enable lifetime and Doppler-broadening spectroscopy. This technique successfully measured stress/strain in thick samples, and showed promise to extend PAS into a variety of applications.

A lifetime spectrometer for use with an isotope-based DC positron beam is described. The system utilizes secondary electrons emitted from the sample and annihilation gamma rays to determine positron lifetimes. The device has been tested with porous silica films capped with a 25 nm layer of Cu. For positron implantation energies between 1.7 and 15 keV, the time resolution was found to be between 985 and 425 ps, respectively.