Table of contents

Preface

These proceedings arose from the 12th International Workshop on Slow Positron Beam Techniques (SLOPOS12), which was held on Magnetic Island, North Queensland, Australia, between 1–6th August 2010. Meetings in the SLOPOS series are held (roughly) every three years and have now been held on (almost) all continents, indicating the truly international nature of the field. SLOPOS12 marked the second time that the Workshop had been held in the southern hemisphere, and the first time in Australia.

SLOPOS12 attracted 122 delegates from 16 countries. Most encouraging was the attendance of 28 student delegates, and that about half of the overall delegates were early career researchers – a good sign for the future of our field. We also enjoyed the company of more than a dozen partners and families of delegates.

In a slight departure from previous SLOPOS meetings, the International Advisory Committee approved a broader scope of scientific topics for inclusion in the program for the 2010 Workshop. This broader scope was intended to capture the applications of positrons in atomic, molecular and biomedical areas and was encapsulated in the byeline for SLOPOS–12: The 12th International Workshop on Slow Positron Beam Techniques for Solids, Surfaces, Atoms and Molecules.

The scientific and social program for the meeting ran over 6 days with delegates gathering on Sunday August 1st and departing on August 6th. The scientific program included plenary, invited, contributed and student lectures, the latter being the subject of a student prize. In all there were 53 oral presentations during the week. There were also two poster sessions, with 63 posters exhibited, and a prize was awarded for the best poster by a student delegate. The standard of the student presentations, both oral and posters, was outstanding, so much so that the judging panel recommended an additional number of prizes be awarded.

Topics that were the focus of invited presentations and contributed papers at SLOPOS–12 included:

• Positron Interactions with Surfaces

• Positron Beam and Detector Technology

• Positron Interactions with Atoms and Molecules

• Positronium Science

• Defects and Vacancies in Materials

• Porosity and Open Volume in Materials

• Antimatter in Biomedical Science

• Anti-hydrogen Studies

• Positron Transport

• Annihilation

On a sad note, delegates paid tribute to the contributions of one of our colleagues, Chris Beling, who tragically passed away shortly before the meeting. Chris' contributions to positron science and to the education of young scientists were noted in a number of the invited presentations. It is an honour for our community to begin these proceedings with a short tribute to Chris' life by Professor Paul Coleman.

The Workshop could not have occurred without the generous support of our sponsors: The ARC Centre for Antimatter–Matter Studies, The Australian National University, Flinders University, James Cook University, The Institute of Physics (UK) and the Australian Government's Department of Innovation, Industry, Science and Research.

It would also not have been possible without the hard work of the Local and International Organising Committees and the friendly and efficient staff at the All Seasons Resort, Magnetic Island. We are most grateful for the on-site assistance of Gillian Drew, the CAMS student and postdoc team, the financial wizardry of Chris Kalos, and the post-Workshop editorial assistance of Julia Wee and Adam Edwards.

Finally we would like to thank all of the attendees at SLOPOS12 for their scientific contributions to the Workshop, and for the warm spirit of engagement which characterised the scientific discussions and social occasions.

SLOPOS13 will be held in Germany in 2013 and we all look forward to the occasion.

Stephen Buckman, James Sullivan and Ronald White (Guest Editors)

SLOPOS–12 Delegates, 1–6 August 2010, Magnetic Island, Australia

SPONSORS

This short tribute to Chris Beling, who died in July 2010 at the age of 54, is written on behalf of all members of the positron research community, by whom he was much loved and admired.

Chris Beling, a much respected and admired member of the positron research community who was a familiar face at SLOPOS and other positron conferences over the past three decades, suffered heart failure as he swam out to rescue his younger brother Jeremy while holidaying in his home town of Paignton, in the southwest of England, on June 18 2010.

Chris gained a first-class honours degree in physics at Keble College, Oxford, in 1977, and his PhD in Radiation Physics from the University of London in 1981. His postdoctoral research, performed with Alan Smith at St Bart's Medical College in London, focussed on positron studies of liquids [1]. His appointment as a lecturer at University College London in 1983 marked the beginning of his research involving positron beams [2] which was to continue for the rest of his life. In 1987 he moved to the University of Hong Kong (HKU), where he became professor of physics in 2007, working with Professor Steve Fung (with whom he studied at Oxford) and later with Francis Ling. During his 23 years in Hong Kong Chris developed his research interests, concentrating principally on positron beam studies of semiconductors [3]. His brother Jeremy commented that 'moving to Hong Kong was the making of Chris; he found love and happiness'.

Chris's research interests reflected the deep intellectual interest he had in his work. He maintained a strong interest in developing the capabilities of positron beam systems – initially by proposing models for field-assisted moderators to increase slow positron yields [4] and later by constructing a hybrid magnetic/electrostatic beam [5] and scanning annihilation spectroscopy [6], among other imaginative advances. His interests in semiconductor physics led him to develop a positron technique analogous to deep-level transient spectroscopy [7]. Chris was characteristically interested in all aspects of the research in which he was engaged, and invested much effort in developing theoretical models and data handling [8]. Chris published over 200 journal papers and 75 conference contributions; almost two-thirds of his publications deal with investigations and complex characterisation of semiconductors, mostly combining the results of many different techniques. 15% of his papers were concerned with the building, development and use of a mono-energetic positron beam and other spectroscopies at HKU, and almost 20% involved numerical modelling of experimental data and theoretical studies.

A fruitful collaboration involving positron research of semiconductors was promoted by Chris during ICPA–10 in Beijing in 1994, between the HKU positron group and that of Gerhard Brauer at FZ Dresden–Rossendorf. In recent years this has primarily involved Chris's former student Francis Ling, now an associate professor at HKU. Gerhard comments that 'it has always been a great pleasure to meet Chris at Hong Kong for a scientific discussion, or privately with him and members of his family. I shall miss him as a colleague and friend.'

Professor Fuchun Zhang, Head of Physics at HKU, writes that, together with Prof. Steve Fung, Chris educated several dozen graduate students, and that his supervision was characterized by his kindness and willingness to help those most in need. The fondness with which Chris was regarded by all who knew him was embodied in the substantial contributions from students, colleagues, friends and family to establish the C.D. Beling Memorial Prize in Physics, which will be awarded annually to three full-time Physics students at HKU.

Chris was the son of retired Paignton vicar David Beling and his wife Anne. Chris's brother Jeremy told mourners at St Paul's Church, Paignton: 'When I called for help, he gave not a second thought in helping me. How I wish he had not been on the beach that day.' His brother paid tribute to a man devoted to his religion with a spirit of adventure and an insatiable appetite for understanding. 'His religion and church life gave him strength and purpose in life. Chris had a brilliant mind but a frail body. It is such a tragedy that his body gave in before his life work was completed.' Chris leaves a widow Evelyn, daughter Cathy and son John.

Everyone who had dealings with Chris was impressed by his intellect and by his charm. He will be greatly missed by the scientific community.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Sources of positrons and antiprotons that are currently used for the formation of antihydrogen with low kinetic energies are reviewed, mostly in the context of the ALPHA collaboration and its predecessor ATHENA. The experiments were undertaken at the Antiproton Decelerator facility, which is located at CERN. Operations performed on the clouds of antiparticles to facilitate their mixing to produce antihydrogen are described. These include accumulation, cooling and manipulation. The formation of antihydrogen and some of the characteristics of the anti-atoms that are created are discussed. Prospects for trapping antihydrogen in a magnetic minimum trap, as envisaged by the ALPHA collaboration, are reviewed.

The NEutron induced POsitron source MUniCh NEPOMUC provides a high intensity positron beam with 9·10⁸ moderated positrons per second with a primary beam energy of 1keV. After remoderation, the positron beam is magnetically guided to five experimental setups: a coincident Doppler-broadening spectrometer (CDBS), a positron annihilation induced Auger-electron spectrometer (PAES), a pulsed low-energy positron system (PLEPS) as well as an interface for providing a pulsed beam with further improved brightness. An apparatus for the production of the negatively charged positronium ion Ps⁻ is currently in operation at the open multi-purpose beam port, where additional experiments can be realized. Within this contribution, an overview of the positron beam facility NEPOMUC with its instrumentation at the research reactor FRMII is given.

Intense positron sources require the pair production process for the positron generation. In case a pulsed positron source shall be constructed, a superconducting LINAC-based accelerator allows generating the required final time structure for the electron beam. This simplifies the positron beam construction. The first such setup, the EPOS system (ELBE Positron Source) at the Forschungszentrum Dresden-Rossendorf (FZD), is described.

Positron annihilation Lifetime (PAL) spectroscopy has been used to study the effect of PEG concentrations on the free volume properties of PHB. The data revealed that the ortho-positronium (o-Ps) lifetime τ_Ps increases with 20% increase in concentration, decrease as the concentration increases to 40%, then rapid increase at 50% concentration of PEG. The o-Ps intensity, I₃, shows a linear dependence as the concentration increases with a discontinuity at 20% concentration of PEG. Furthermore, the results presented and discussed in this work show that the PHB and PEG are miscible up to 40% of PEG but greater than 40%, the blend is immiscible. In addition, the mechanical properties of PHB are well improved by the addition of PEG with a low concentration up to 20%, while at higher concentration the blend becomes waxy.

Gamma irradiation posses a serious role for casting the membranes. Acryl amide /poly (methacrylic acid) membrane was synthesized under γ-radiation effect. The structure of the membrane was characterized by FTIR, thermo-gravimetric analysis and the scanning electron microscope. The properties of the membranes were also investigated in terms of proton conductivity and positron annihilation lifetime (PAL) parameters. On the basis of the values of the long-lived components in the lifetime spectra, the size of the free volume and their intensity were calculated. The positron lifetime study on these irradiated casted membranes shows that the cross-linking and degradation within the membrane matrix affect the free volume content and hence the microstructure.

Positron beam studies on the silicidation in Pd/Si, Ni/Si and Co implanted Si thin film systems are presented. In the case of Pd/Si, the evidence for the occurrence of divacancies across the Pd₂Si/Si interface consequent to Pd₂Si formation is shown. In Ni/Si system, the identification of a multitude of silicide phases of Ni₂Si, NiSi and NiSi₂ is obtained from depth-resolved S-parameter, which is corroborated with GIXRD and AES results. In the case of the Co implanted Si, the recrystallization of amorphized silicon and the formation of CoSi₂ precipitates in Si are brought out clearly.

Transport properties of positron swarms drifting and diffusing in neutral gases under the influence of crossed electric and magnetic fields are investigated using a multi-term theory for solving the Boltzmann equation and Monte Carlo simulation technique. In the presence of magnetic fields the number of transport properties is increased compared to the situation when the positron swarm is acted on solely by an electric field. Since the longitudinal and transverse components of the drift velocity show different sensitivities with respect to the strength of the magnetic field, it is found that the negative differential conductivity effect in a crossed field configuration can be controlled through the variation of the magnetic field strengths. Various diffusion tensor elements also exhibit different sensitivities with respect to the magnetic field and also with respect to the positronium (Ps) formation process.

Improvements to Positron Emission Tomography (PET) technology depend to a large extent on obtaining a better understanding of positron transport processes in soft condensed matter. The most economically computational, physically tenable method of carrying out such an investigation is the fluid approach, in which the Boltzmann kinetic equation is replaced by a hierarchy of low order, velocity moment equations, in which collision terms are approximated by extending 'momentum transfer theory' to allow for coherent scattering arising from the structure of the medium. We show how these fluid equations furnish structure-modifed versions of well known semi-empirical relations linking transport coefficients, and give numerical calculations for simple models.

Emission of secondary electrons induced by the passage of low energy positrons through thin carbon foils was studied by the Monte Carlo method. The positron and electron elastic cross sections were calculated by partial wave analysis. The inelastic positron-valence-electron was described by the energy loss function obtained from dielectric theory. The positron-core-electron interaction was modelled by the Gryzinski's excitation function. Positron transport inside the carbon foil was simulated in detail. Secondary electrons created by positrons and high energy secondary electrons through inelastic interactions were tracked through the foil. The positron transmission coefficient and secondary electron yielded in forward and backward geometry are calculated and dependences on positron energy and carbon foil thickness are discussed.

The ionization of the 3p and 3s orbitals of argon by 200 eV incident positrons is studied with the DWBA approximation. Our TDCS results for the 3p orbital ionization are found to be in good agreement with recent CDW-EIS data.

The negative positronium ion Ps⁻ is a bound system consisting of two electrons and a positron. Its three constituents are point-like and stable leptonic particles with equivalent mass, which are only subjected to the electroweak and the gravitational force. Hence Ps⁻ represents an ideal object to study the quantum mechanics of three-body systems.

We present a status report on a new a measurement setup, which was used to perform several high precision lifetime measurements at the NEPOMUC facility. The combined results of systematic test measurements are discussed and an outlook to future measurements as well as preliminary results of the most recent beam time will be given.

We present an update on the positron-facility development at Argonne National Laboratory. We will discuss advantages of using low-energy electron accelerator, present our latest results on slow positron production simulations, and plans for further development of the facility. We have installed a new converter/moderator assembly that is appropriate for our electron energy that allows increasing the yield about an order of magnitude. We have simulated the relative yields of thermalized positrons as a function of incident positron energy on the moderator. We use these data to calculate positron yields that we compare with our experimental data as well as with available literature data. We will discuss the new design of the next generation positron front end utilization of reflection moderator geometry. We also will discuss planned accelerator upgrades and their impact on APosS.

Positron annihilation with a slow positron beam was applied to the characterization of composite reverse osmosis membranes. The results, obtained at different positron incident energies, indicated that the membranes are asymmetric with respect to the pore structure, consisting of a thin top layer with little porosity and an underlying thick porous layer. A relationship between the longest positron lifetime near the membrane surface and the salt rejection rate was discussed in terms of the free-volume hole size for the thin top layer.

New digital spectrometer for measurement of the Doppler shift of annihilation photons was developed and tested in this work. Digital spectrometer uses a fast 12-bit digitizer for direct sampling of signals from HPGe detectors. Analysis of sampled waveforms is performed off-line in software. Performance of the new digital setup was compared with its traditional analogue counterpart. Superior energy resolution was achieved in the digital setup. Moreover, the digital setup allows for a better control of the shape of detector signals. This enables to eliminate undesired signals damaged by pile-up effects or by ballistic deficit.

This short review presents results of new positron and positronium (Ps) experiments in condensed matter and atomic physics, as an illustration of the satisfying variety of scientific endeavours involving positron beams which can be pursued with relatively simple apparatus in a university laboratory environment. The first of these two studies – on ice films - is an example of how positrons and Ps can provide new insights into an important system which has been widely interrogated by other techniques. The second is an example of how simple positron beam systems can still provide interesting information – here on a current interesting fundamental problem in positron atomic physics.

The development stages of a system for variable-energy spin-polarised positron beam spectroscopy are described. Methods for achieving maximum practical beam polarisation include a Na-22 source configuration with a low-Z backing to reduce positron backscattering, and positioning a 250μm beryllium foil in front of the source to absorb the slower and decelerate the faster beta positrons, the latter suffering significantly less depolarisation during moderation than the former. To switch the direction of spin polarisation of electrons in the target sample the direction of the positron guiding field was reversed, and strong rare earth magnets were placed behind the sample. Systematic problems associated with low beam intensity and electronic drifts are discussed. Results for single-crystal iron compare well with those expected from theory, but suggest a beam/target polarisation of ∼ 5% of that expected. Prospects for future beam development and application are outlined.

In this paper, the evolution and depth profiles of vacancy-type defects were investigated by slow positron beam (SPB) analysis. Films of helium-containing titanium were deposited on Si substrates by direct current magnetron sputtering under the helium/argon (He/Ar) gases ambiance. Different helium concentrations were introduced into the sample films by altering the He/Ar inflow ratios (Q_He/Q_Ar). It was found that the defect-characteristic parameter S from positron-electron annihilation peaks rise with the increase of the ratio of Q_He/Q_Ar. Homogenous He-related defect profiles and their evolution deep into around ~400nm were found by SPB measurements, together with the analysis of Rutherford backscattering (RBS) and elastic recoil detection (ERD).

Variable-energy positron annihilation spectroscopy (VEPAS) has been used to probe the structure of gold nanoparticles suspended in a silica aerogel matrix by measuring the annihilation parameters S and W, and the ortho-positronium (o-Ps) fraction parameter F, as a function of incident positron energy. Correction methods have been developed to improve the sensitivity of F by removing background contributions to the number of recorded o-Ps annihilation events. Charging effects have also been observed and investigated.

This paper presents the results of a theoretical study of positron surface and bulk states and annihilation characteristics of surface trapped positrons at the oxidized Cu(100) single crystal and at both As- and Ga-rich reconstructed GaAs(100) surfaces. The variations in atomic structure and chemical composition of the topmost layers of the surfaces associated with oxidation and reconstructions and the charge redistribution at the surfaces are found to affect localization and spatial extent of the positron surface-state wave functions. The computed positron binding energy, work function, and annihilation characteristics reveal their sensitivity to charge transfer effects, atomic structure and chemical composition of the topmost layers of the surfaces. Theoretical positron annihilation probabilities with relevant core electrons computed for the oxidized Cu(100) surface and the As- and Ga-rich reconstructed GaAs(100) surfaces are compared with experimental ones estimated from the positron annihilation induced Auger peak intensities measured from these surfaces.

Positronium (Ps) formation measurements in homogeneous porous silica (Xerogel of 85 mg cm⁻³ and swollen MCM-41 of 390 mg cm⁻³) were performed at different temperatures (8K–293K) by means of a variable energy positron beam equipped with a Ge detector. The results indicate that Xerogel and swollen MCM-41 samples have a high Ps production, which is independent on the temperature. An estimation of the ortho-Ps mean diffusion length was obtained by measuring samples capped with an Al film (~110 nm). An efficient formation of cooled Ps atoms is a requisite for efficient production of antihydrogen, with the aim of a direct measurement of the Earth gravitational acceleration of antimatter, a primary scientific goal of AEgIS (Antimatter Experiment: gravity, Interferometry, Spectroscopy, CERN). Porous materials with open pores at the surface are necessary to produce a high yield of Ps atoms as well as to cool Ps through collisions with the inner walls of the pores before emerging in the free space outside the target. The results indicate that Xerogel and swollen MCM-41 are good candidates for an efficient formation of cold Ps atoms within the framework of the AEgIS project.

A feasibility study of a non–conventional Penning-Malmberg trap is presented. The study is based on simulations performed by the WARP code. [1,2] Comparisons with previous simulations done with the Charged Particle Optics (CPO) programs [3] are presented under homogeneous magnetic field conditions; however, real B fields do not have a significant effect.. The analysis has been carried out on micro-traps of radii ranging from 10 to 100 micrometers and length of 126 mm. Different configurations of the applied electric fields have been considered and an axial uniform magnetic field of 7 T was used in most of the simulations. For the final device, the microtraps should be combined into a multiple parallel tube array. Results from the simulations with the WARP and CPO codes indicate that decreasing the microtrap radius allows an increase in the total number of positrons per macrotrap. This is due to the increased number of micro-tubes when reducing the beam radius.

Characterisation of clays is generally achieved by traditional methods, such as X-ray diffraction (XRD) and transmission electron microscopy (TEM). However, clays are often difficult to characterise due to lack of long-range order, thus these tools are not always reliable. Because interlayer spacing in clays can be adjusted to house molecules, there is growing interest to use these materials for drug delivery. Positron annihilation lifetime spectroscopy (PALS) was examined as an alternative tool to characterise a series of well-known clays. XRD of two layered double hydroxides; MgAl-LDH and MgGd-LDH, natural hectorite, fluoromica and laponite, and their PALS spectra were compared. XRD data was used to calculate the interlayer d- spacing in these materials and results show a decrease in interlayer spacing as the heavy metal ions are substituted for those of large ionic radii. Similar results were obtained for PALS data. This preliminary study suggests PALS has potential as a routine tool for characterising clay particles. Further work will examine the sensitivity and reliability of PALS to percent of metal doping and hydration in clay microstructure.

The nano-porosity of metakaolin-based geopolymers and the effect of heat-treatment on porosity have been studied with conventional and beam-based positron annihilation lifetime spectroscopy (PALS). Conventional PALS found significant nano-porosity in the geopolymers, as indicated by the presence in the PALS spectrum of two long lifetime components, τ₃ = 1.58 ns and τ₄ = 47 ns, associated with pore diameters of approximately 0.5 and 3 nm respectively. The lifetime of the shorter component was found to decrease monotonically with successive heat treatments of 300°C and 600°C. Beam-based PALS, conducted at 5 keV, also indicated two long lifetime components, τ₃ = 4.84 ns and τ₄ = 54.6 ns. These are significantly longer than those observed by conventional PALS and the monotonic decrease of τ₃ with successive heat treatments was not observed. As the beam-based PALS probed only the near-surface region, with an average implantation depth of about 350 nm, these results suggest that the near-surface structure may vary significantly from that of the bulk. This could be an inherent property of the samples or an artefact caused by surface effects or sample outgassing.

An intense slow positron beam has been established at the 1-MW PULSTAR nuclear reactor. The beam is operational generating mono-energetic positrons with an energy of 1-keV. The maximum measured intensity slightly exceeds 10⁹ e+/s. The beam is operated routinely with an intensity of approximately 5×10⁸ e+/s. The positrons are generated through gamma-ray pair production interactions in two back-to-back banks of tungsten converter/moderators. The gamma-rays are produced in the PULSTAR core and by thermal neutron capture in a cadmium shroud that surrounds the tungsten. The primary utilization of the PULSTAR positron beam is the characterization of nanoscale structure in materials. Consequently, the beam has been equipped with two state-of-the-art PALS spectrometers. The first spectrometer is dedicated to measurements in materials such as metals and semiconductors. This spectrometer is based on pulsing and bunching of the primary beam and is currently operating with a timing resolution of approximately 390 picoseconds. The second spectrometer is dedicated to measurements in materials where positronium formation is promoted. The timing resolution of this spectrometer is designed to be ~ 0.5 nanosecond with an on-sample spot size of 1–2 mm. For both spectrometers, the energy of the positrons can be varied to allow depth profiling with on-sample intensity exceeding 10⁶ e+/s.

Free volume changes in Zr₅₀Cu₄₀Al₁₀ bulk glassy alloys irradiated by 200 and 2.5 MeV Xe ions, 180 keV He ions, and 2 MeV electrons were investigated at room temperature using positron annihilation lifetime and Doppler broadening techniques. In addition, a slow positron beam was used to probe the change in free volume in the 180 keV He ion-irradiated sample. X-ray diffraction revealed that no crystallization took place in any of the irradiated samples. The Doppler broadening spectra from the annihilated gamma rays remained essentially constant in all ion-irradiation cases; however, an extremely minor change of positron mean lifetime was detected in each case. For electron- and He ion-irradiated samples the positron lifetime increased, and the opposite was seen in heavy-ion irradiated samples. The Doppler broadening S parameter increased with He-ion radiation dose, and the depth profile correlated well to the damage profile.

The Slow Positron Facility at the Institute of Material Structure Science (IMSS) of High Energy Accelerator Research Organization (KEK) is a user dedicated facility with an energy tunable (0.1 – 35 keV) slow positron beam produced by a dedicated 55MeV linac. The present beam line branches have been used for the positronium time-of-flight (Ps-TOF) measurements, the transmission positron microscope (TPM) and the photo-detachment of Ps negative ions (Ps⁻). During the year 2010, a reflection high-energy positron diffraction (RHEPD) measurement station is going to be installed. The slow positron generator (converter/ moderator) system will be modified to get a higher slow positron intensity, and a new user-friendly beam line power-supply control and vacuum monitoring system is being developed. Another plan for this year is the transfer of a ²²Na-based slow positron beam from RIKEN. This machine will be used for the continuous slow positron beam applications and for the orientation training of those who are interested in beginning researches with a slow positron beam.

Positron annihilation spectroscopy coupled with a newly built slow positron beam at National University of Singapore has been used to study the free volume, pore, and depth profile (0 – 10 μm) in cellulose acetate polymeric membrane at the bottom and top sides of membranes for ionic separation in water purification applications. The S and R parameters from Doppler broadening energy of annihilation radiation representing free volumes (0.1–1 nm size) and pores (>1 nm–μm) as a function of depth have been analyzed into multilayers, i.e. skin dense, transition, and porous layers, respectively. The top side of membrane has large free volumes and pores and the bottom side has a skin dense layer, which plays a key role in membrane performance. Positron annihilation lifetime results provide additional information about free-volume size and distribution at the atomic and molecular scale in polymeric membrane systems. Doppler broadening energy and lifetime spectroscopies coupled with a variable mono-energy slow positron beam are sensitive and novel techniques for characterization of polymeric membrane in separation applications.

Recent developments in application of the convergent close-coupling approach to antimatter-matter scattering are outlined. These include positron collisions with alkalis and helium, in the ground or metastable states, as well as extension of the method to heavy projectiles, such as antiprotons.

Metal (Fe and Ni) samples ion-irradiated at 100 – 773 K were characterized by Doppler broadening measurements with slow positron beams to investigate depth profiles of irradiation-induced defects. Obtained results were compared with defect profiles calculated by Monte-Carlo simulation. Defects profiles obtained by positron measurements were always deeper than those by the simulation. The difference between measured and simulated profiles was observed even in the case of the Fe sample irradiated at 100 K where vacancies are immobile. The origin of such differences was discussed with respect to vacancy diffusion, depth calculation, surface contamination and ion channeling.

A lot of light aluminium alloys achieve their favourable mechanical properties, especially their high strength, due to precipitation of alloying elements. This class of age hardenable Al alloys includes technologically important systems such as e.g. Al-Mg-Si or Al-Cu. During ageing different precipitates are formed according to a specific precipitation sequence, which is always directed onto the corresponding intermetallic equilibrium phase. Probing the defect state of individual precipitates requires high spatial resolution as well as high chemical sensitivity. Both can be achieved using the finely focused positron beam provided by the Bonn Positron Microprobe (BPM) [1] in combination with the High Momentum Analysis (HMA) [2]. Employing the BPM, structures in the micron range can be probed by means of the spectroscopy of the Doppler broadening of annihilation radiation (DBAR). On the basis of these prerequisites single precipitates of intermetallic phases in Al-Mg-Si and Al-Cu, i.e. Mg₂Si and Al₂Cu, were probed. A detailed interpretation of these measurements necessarily relies on theoretical calculations of the DBAR of possible annihilation sites. These were performed employing the DOPPLER program. However, previous to the DBAR calculation the structures, which partly contain vacancies, were relaxed using the ab-initio code SIESTA, i.e. the atomic positions in presence of a vacancy were recalculated.

We describe preliminary results from studies of the formation of silicon nano-crystals (Si-ncs) embedded in stoichiometric, thermally grown SiO₂ using Variable Energy Positron Annihilation Spectroscopy (VEPAS). We show that the VEPAS technique is able to monitor the introduction of structural damage. In SiO₂ through the high dose Si⁺ ion implantation required to introduce excess silicon as a precursor to Si-nc formation. VEPAS is also able to characterize the rate of the removal of this damage with high temperature annealing, showing strong correlation with photoluminescence. Finally, VEPAS is shown to be able to selectively probe the interface between Si-ncs and the host oxide. Introduction of hydrogen at these interfaces suppresses the trapping of positrons at the interfaces.

The adsorption of hydrogen on transition-metal surface has been studied by both experimental and theoretical methods. Hydrogen adsorbed Ni (111) surface had measured by using slow positron beam apparatus. The positron annihilation γ-ray energy spectra emitted from hydrogen-adsorbed Ni (111) surfaces are analyzed. We have discussed the stability of the disorder phase of adsorbed hydrogens on Ni (111) surfaces. Positron annihilation results showed that the disordered phase is unstable until about 300K.

We describe a new Monte-Carlo-simulation which models output signals of scintillator-photomultiplier tube (PMT) pairs . For the whole simulation from the initial photon-cascade within the scintillator to the final pulse as it is digitized at the output of the PMT only values from the scintillator's and PMT's technical manuals are used, no parameters need to be adapted. We find an excellent agreement between sampled and simulated signals. This allows to determine the influence of single parameters independent from the others.

With a special focus on positron lifetime as well as perturbed angular correlation spectrometers, which have high demands on the γ-photon incidence time determination's accuracy, the influence of the tts-parameter on the detector's overall time resolution was studied in detail.

Various oxide dispersion strengthened (ODS) steels (PM2000, MA956, ODM751 and ODS Eurofer) have been investigated using positron annihilation lifetime spectroscopy. Preliminary characterization of the vacancy type defects and the yttria nanoparticles are reported in this paper. The interpretation of the experimental data considers also the results of magnetic Barkhausen noise (BN) measurements as well as SEM and TEM investigations. Significant differences due to the presence of the yttria oxides (namely Y2O3) were observed in the studied materials in comparison to conventional ferritic/martensitic steels (e.g. Eurofer). Higher positron mean lifetime in ODS steels is, however, not only due to the presence of dispersoids, but also a result of vacancy agglomeration (clusters of 6–8 vacancies) which have been confirmed in recrystallised ODM751 and MA956 materials. On the other hand, positron trapping at dislocations was observed in the as-extruded ODS Eurofer in contrast to the other, recrystallised, materials where the high temperature treatment had led to the static recovery of these defects. It is suggested that some defects which are present (dislocations, vacancy clusters and dispersoids) and affect positron trapping in the materials are also important pinning sites for the magnetic domains. This is reflected by a shift of the signal peak in the BN spectra. The present complementary study provides more comprehensive information about materials microstructure and can support the interpretation of the physical/mechanical testing results (hardness, fracture mechanics etc.) obtained on these materials.

To obtain a highly spin polarized positron beam, ⁶⁸Ge isotope have been produced using a nuclear reaction of ⁶⁹Ga(p,2n)⁶⁸Ge. As target materials, we examined a metal form ⁶⁹Ga stable isotope and a GaN substrate. By 20 MeV proton irradiation, the production of ⁶⁸Ge source was confirmed in both targets. The production rates of ⁶⁸Ge were 0.16 and 0.53 MBq/μA/h for the metal Ga and GaN target, respectively. The spin polarizations of positrons emitted from ⁶⁸Ge was estimated to be approximately 50 to 70%.

The defect structure at the organic heterojunctions is studied using slow positron beam. The structural and electronic properties of heterojunctions are of technological and fundamental importance for understanding and optimization of electronic processes in organic devices. Interface trap centres play a significant role in the electrical conduction through the junctions. Depth dependent Doppler broadened annihilation measurements have been carried out in p- and n-type organic semiconductor thin films (30–80 nm) both single as well as multilayers grown on quartz substrate. The objective of the present study is to investigate the defect structure and to understand the behavior of positrons at the charged organic interfaces. Our result shows the sensitivity of positrons to the interfacial disorders that may be a convoluted effect of the presence of defects as well as the influence of the charge dipole in multilayers.

AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) experiment in progress at CERN has planned to produce antihydrogen () by using the resonant charge exchange process between orthopositronium (o-Ps) and antiproton (). Its primary goal is the realization of a cold beam for the first measurement of the Earth's gravitational acceleration on antimatter. A demanding condition for large o-Ps - cross section is the cooling of o-Ps at temperatures lower than at least 160 K. We will present recent results of o-Ps formation and cooling at cryogenic temperatures in a novel silicon positron/Ps converter in which oxidized nano-channels perpendicular to the surface were produced. The possibility to tune the nano-channels dimension to optimize o-Ps kinetic energy emission and to avoid o-Ps quantum confinement is discussed.

Characterization of point defects in a fully stabilized ZrO₂ + 9 mol.% Y₂O₃ single crystal with cubic structure was performed in this work. It was found that the crystal contains a high density of vacancy-like defects characterized by a lifetime of 175 ps. First principle theoretical calculations showed that this lifetime is comparable with lifetime of positrons trapped in zirconium vacancies associated with hydrogen. In particular, in the vicinity of the zirconium vacancy hydrogen forms an O-H bond with one of the nearest neighbour oxygen atoms. The calculated bond length is close to 1 Å. Using nuclear reaction analysis it was found that the hydrogen concentration in the crystal is 0.3 at.%. This amount of hydrogen is sufficient to form zirconium vacancy – hydrogen complexes capable of saturated positron trapping.

Quenched-in vacancies in Fe-Al alloys with Al content ranging from 24 to 49 at.% were investigated employing two complementary techniques of positron annihilation: slow positron implantation spectroscopy and positron lifetime measurements. It was found that quenched alloys exhibit a very high concentration of vacancies. Although the free positron component cannot be resolved in positron lifetime spectrum in majority of samples, the concentration of quenched-in vacancies can be still determined from the positron diffusion length measured by a variable energy positron beam. The lowest concentration of vacancies was found in a stoichiometric (SM) Fe₃Al alloy. The concentration of defects increases with increasing degree of non-stoichiometry with respect to Fe₃Al, i.e. in alloys with under-SM and over-SM Al concentration. However, the increase in concentration of quenched-in defects is more pronounced in Al-rich alloys, i.e. alloys containing more than 25 at.% of Al.

A series of nuclear sensors were designed to assess the chemistry within the nanopores of a porous material. The nuclear sensors of varying size, charge, and hydrophobicity were exposed to hollow silica shells (HSS) at varying pH. Uptake and release kinetics were studied over a 24 h period at room temperature. Preliminary study indicate positively charged nuclear sensors were selectively and rapidly (within 10 min) absorbed by the HSS at pH 7 to 9. PALS showed there were two types of pores (1.7 and 0.7 nm) present. The data suggest the nuclear sensors sit within the larger pore of the HSS. Both PALS and nuclear sensors are required to obtain an accurate insight into the nanoporosity of the hollow silica shells.

The production of an energy-tunable positronium beam by photodetachment of positronium negative ions, which was proposed many years ago, has become feasible by the recent development of efficient formation of the ions and successful photodetachment. Here we shall describe our current efforts directed towards the production of a positronium beam using this technique.

We have measured the total cross sections for positron scattering from He using an electrostatic high-brightness slow positron beam apparatus. Our results are in reasonable agreement with the data by Jaduszliwer et al in the energy region below 15 eV, which have been obtained under the influence of the lower axial magnetic field than other groups.

We report on the current status of a project to develop a dedicated superconducting accelerator for slow positron production at AIST. Two 500 MHz, 5 cell cavities will form the basis of the new accelerator. Initial set-up and preliminary design activities are reported.

A positron probe microanalyzer (PPMA) is a positron lifetime measurement system with a positron microbeam. The PPMA is used for obtaining average positron lifetime distribution images by scanning the positron injection position on a sample. Methods of data acquisition and analysis for obtaining average positron lifetimes by the PPMA are reported.

The Nafion -117 (H⁺ form) was converted to Ag⁺ form by ion exchange. The silver nanoparticles were formed in the membrane by reduction of Ag⁺ (of Ag⁺ form membrane) using an ionic reducing agent, NaBH₄, and a neutral reducing agent, formamide. The free volume hole sizes in the nanoparticle doped and undoped Nafion were measured by positron annihilation lifetime spectroscopy (PALS). The surface modifications in the membrane by incorporation of nanoparticles were investigated using slow positron beam. The changes in the bulk free volume of Nafion with presence of nanoparticles were found to be nominal. In contrast, the surface microstructure appears to have undergone significant change.

In this paper we give a review of two recent developments in positron transport, calculation of transport coefficients for a relatively complete set of collision cross sections for water vapour and for application of they Monte Carlo technique to model gas filled subexcitation positron traps such as Penning Malmberg Surko (Surko) trap. Calculated transport coefficients, very much like those for argon and other molecular gases show several new kinetic phenomena. The most important is the negative differential conductivity (NDC) for the bulk drift velocity when the flux drift velocity shows no sign of NDC. These results in water vapour are similar to the results in argon or hydrogen. The same technique that has been used for positron (and previously electron) transport may be applied to model development of particles in a Surko trap. We have provided calculation of the ensemble of positrons in the trap from an initial beam like distribution to the fully thermalised distribution. This model, however, does not include plasma effects (interaction between charged particles) and may be applied for lower positron densities.

Positron impact ionization of helium is studied with fully resolved momentum vectors of all continuum particles. An imaging multi-particle momentum spectrometer (reaction microscope) detecting all final state particles over the full solid angle was applied. This apparatus was connected to the NEPOMUC facility delivering intense positron beams tuneable over a large energy range with good beam quality. At 80 eV impact energy about 5000 triple coincidence events were collected. Cross sections as function of the longitudinal particle momentum show strong differences compared to respective electron impact ionization data most likely originating from the reversed post collision interaction in both cases. Calculations using the 3 Coulomb wavefunction method show clear discrepancies from the experimental results.

Positron annihilation lifetime spectroscopy (PALS) has been used to study the free volume in dry synthetic polymer nanoparticles of various sizes. A series of poly(styrene/divinyl benzene) particles with diameters in the range of 100 to 500 nm were synthesized and then carefully chemically treated using the sulfonation process, to increase their porosity. The particles were characterised by Scanning Electron Microscopy (SEM), light scattering and PALS. Light scattering gave larger size for the treated particles, reflecting the hydration effect and therefore the increase in porosity. PALS spectra of untreated and treated particles gave four and three life-time components, respectively. Analysis by PAScual version 1.3.0 program indicated there was a reduction in the intensity and the type of the micropores in the treated particles. The data suggest PALS is a sensitive tool for detecting changes in microporosity in particles. The conflicting results obtained for light scattering compared to PALS for chemically treated particles is difficult to resolve and suggests sample preparation of polymeric materials for PALS is the critical factor.

The NEPOMUC positron beam facility hosts four successfully operated positron instruments which cover the fields of solid state, surface and fundamental physics. Two further setups are under construction and a variety of positron experiments were accomplished at the open beam port. The success of almost all experiments till now has been dependent on the quality of the positron beam provided by NEPOMUC. Therefore, the source, the beamline, and some key components as e.g. the remoderator and the beam switch are the subject of continuous optimizations and improvements. A crucial part of our efforts is a meaningful characterization of the positron beam. The most important measure of the quality of a positron beam is the phase space volume occupied by the beam particles and hence, a novel device was developed whose measurement concept is presented here.

Surface changes in ZnO single crystals electrochemically doped with hydrogen were investigated in this work using slow positron implantation spectroscopy (SPIS) combined with atomic force microscopy (AFM) and optical microscopy. It was found that hexagonally shaped pyramids were formed on the surface of hydrogen-loaded crystals. The formation of these pyramids can be explained by hydrogen-induced plastic deformation realized by a slip in the [0001] direction. Such a picture is supported (i) by AFM where steps of a height comparable with the c-lattice parameter were found at the base of the pyramids, and (ii) by SPIS which revealed a defected subsurface layer, formed by the hydrogen-induced plastic deformation and exhibiting an enhanced concentration of open-volume defects in hydrogen-loaded crystals.

Co-doped rutile TiO₂ films were synthesized by ion implantation. Variable energy positron annihilation Doppler broadening spectroscopy and coincidence Doppler broadening measurements were performed for identification of the vacancies. A newly formed type of vacancy can be concluded by the S-W plot and the CDB results indicated that the oxygen vacancy (V_O) complex Ti-Co-V_O and/or Ti-V_O are formed with Co ions implantation and the vacancy concentration is increased with increase of dopant dose.

We have investigated the evolution of damage during fatigue loading in Al–Cu 2024-T3 alloys using the positron annihilation Doppler Broadening (DB) technique. This technique enables us to monitor in a non destructive way, at the atomic and vacancy level, the formation of deformation defects and their interaction with solute atoms at selected stages of fatigue testing. The changes in the S and W Doppler Broadening parameters are linked to the changes in fatigue behavior at lower stress levels. The material was tested under constant amplitude fatigue loading at four different stress levels and DB tests were conducted at selected stages of fatigue lifetime. The results are compared to those obtained during static tensile tests.

The positron annihilation lifetimes of He-ion-irradiated Fe samples were measured by a pulsed positron beam technique. The relative thickness of the surface oxidation layer was measured by Auger microprobe measurements. Unirradiated samples with different surface oxidation layer thicknesses had very similar second lifetime component intensities, indicating that the surface oxidation layer thickness does not affect positron lifetime spectra. In lifetime measurements using 10-keV positrons, the second lifetime component decreased monotonically with increasing irradiation dose. The mean lifetime in the 10-keV measurements was longer than that in 15-keV measurements (which correspond to the peak region of vacancy production); this is thought to be due to vacancy clusters absorbing He atoms.

The chromate-based epoxy primer film was prepared on glass and metal surfaces using various graded doctor blades. The quality and consistency of the films was assessed using scanning electron microscopy and the porosity within the film characterized by positron annihilation lifetime spectroscopy. The positron lifetime (τ) distribution for the epoxy polymer matrix was resolved using the CONTIN program. The free volume was found from the ortho-positronium component. The optimum thickness for the films was established for future structure/activity studies.

A sustainable nano-manufacturing future relies on optimisation of the design and synthetic approach, detailed understanding of structure/properties relationships and the ability to measure a products impact in the environment. This article outlines how bench-top PALS and nuclear techniques can be used in the routine analysis of a wide range of nanomaterials. Traditionally used in the semiconductor industry, PALS has proven to be useful not only in measuring porosity in polymeric materials but also in the monitoring of milling processes used to produce natural fibre powders. Nuclear sensors (radiotracers), designed to probe charge, size and hydrophilicity of nanomaterials, are used to evaluate the connectivity (availability) of these pores for interaction with media. Together they provide valuable information on structure/properties relationship of nanomaterials and insight into how the design of a material can be optimised. Furthermore, the highly sensitive nuclear sensors can be adapted for monitoring the impact of nanomaterials in vivo and the environment.

Using a high resolution (~60 meV FWHM) positron beam from a Surko buffer-gas trap, we have measured grand total cross sections as well as total elastic, positronium formation and electronic excitation cross sections in the rare gases. Strong threshold (Wigner) cusps are observed in the elastic scattering channel in all the rare gases at the opening of the positronium formation channel. We present low energy elastic scattering measurements for neon which show excellent agreement with recent theory and potentially establish this system as a 'benchmark'.

In this study we simulate positron transport properties for various configurations of the gases and electric fields used in the Australian Positron Beamline Facility positron trap, which is based on the Surko buffer-gas trap. In an attempt to further improve the time and energy resolution of the trap and thus the associated scattering experiments, we apply a Monte-Carlo simulation procedure to a variety of possible configurations of the dumping stage of the trap.

The emission of positronium negative ions from a Na coated W(100) surface has been studied. The emission efficiency was found to increase with time following Na deposition to a maximum of 1.4% after 6 × 10⁴ s. The efficiency then gradually decreased but was still higher than 0.5% after 3 days.

Pressurized melt grown zinc oxide (ZnO) single crystals purchased from Cermet Inc. were irradiated by 2MeV electrons with fluence of 6x10¹⁷cm⁻². Isochronal annealing from 100°C-800°C was performed on the crystals under argon and air ambience. Variable Energy Doppler Broadening Spectroscopy (VEDBS) was carried out on both the as-grown and the irradiated samples at each annealing step. The migration, agglomeration and annealing of grown-in and irradiated-introduced defects were studied. It was observed that the grown-in vacancy-type defects concentration decreased at 300°C and 600 °C. For the irradiated sample annealed in argon, the positron trapping vacancy-type defect concentration decreased at 300°C and 600°C. Further annealing the as-grown and irradiated samples in argon increased the S parameter further. For the irradiated sample annealed in air, the vacancy-type defect concentration decreases at 300°C and 700°C.

We present experimental results for in situ study of atomic-vacancy production and its evolution occurring during irradiation by using a slow-positron beam. Thermal stability of the vacancy produced during irradiation is investigated under elevated temperature conditions. An annealed Ni specimen was irradiated with 400 keV He ions at three different temperatures of 296 (RT), 368 and 713 K. Doppler broadenings of positron-annihilation γ-rays were measured and variation of the line-shape parameter S was observed under beam-on (during irradiation) and beam-off (non-irradiation) conditions. Results indicate that variation of the S depends on the specimen temperature, showing that vacancy cluster consisting of about 15 vacancies is formed predominantly via thermal evolution of atomic-sized vacancies under irradiation at high temperatures. We found that formation of the activated vacancies occurs during irradiation, which leads to vacancy clustering.

Developments of the semiconductor industry depend on ability to decrease the size of integrated circuits. Significant efforts to introduce new materials and their fabrication processes have been made in order to achieve such trends. However, one faces difficulties in the control of materials properties caused by atomic scale disorder. In the present paper, we used monoenergetic positrons to characterize ultra shallow junction formed on Si substrates and materials buried in LSI circuit structures. It was shown that the positron annihilation technique is a powerful characterization tool for modern Si-technology related materials.

A pulsed positron lifetime system has been designed using RF pulsing scheme. The mechanical design of the ultra high vacuum(UHV) beam line has been made to incorporate the basic requirements of a magnetically guided slow positron beam as well as that of an RF pulsing system consisting of a reflection type grid based chopper, a double gap pre-buncher and a main buncher. The trajectory and the timing characteristics of positrons have been simulated using SIMION code. The amplitude of the chopper and buncher RF voltages has been optimized based on these simulations.

The effect of hydrogen on ZnO while annealing at 1100 °C with Zn vapour or Ti metal is studied. ZnO turns red only when hydrogen is present in combination with oxygen deficiency. The coloration is reversible at 1100 °C by removing hydrogen either by evacuating or by gettering hydrogen with Ti at low temperature while annealing the ZnO at >950 °C. The latter is a simple method to lower the hydrogen impurity concentration. The annealing data are consistent with an activation energy >1.5 eV to reverse the coloration. Hall data on red ZnO reveal only a shallow donor level around 45 meV, too low for the colour change. Temperature dependent X-ray diffraction reveals a lattice expansion due to the presence of hydrogen which is sufficient to explain the colour change.

A three-dimensional (3D) positron annihilation spectroscopy system (3DPASS) capable of determining 3D electron-positron (e⁻-e⁺) momentum densities from measurements of deviations from co-linearity and energies of photons from e⁻-e⁺ annihilation events was employed to examine the effects of O-atom defects in 6H SiC. Three-dimensional momentum datasets were determined for 6H SiC irradiated with 24 MeV O³⁺ ions. Angular correlation of annihilation radiation (ACAR) and coincidence Doppler-broadening of annihilation radiation (CDBAR) analyses are presented. In addition, a novel technique is illustrated for analyzing 3D momentum datasets in which the parallel momentum component, p_|| (obtained from the CDBAR measurement) is selected for annihilation events that possess a particular perpendicular momentum component, p_- observed in the 2D ACAR spectrum.

Corrosion-related defects in pure Al and AA 2037 Al alloy have been investigated by positron beam-based Doppler broadening energy spectroscopy. Defect profiles have been analyzed by measuring the S parameter as a function of incident positron energy up to 30 keV. When pure Al samples are immersed in 1M NaOH for various times, a significant increase in the S parameter near the surface is observed. This implies that the corrosion process involves the creation of defects and nanometer voids. In contrast, a significant decrease in the S parameter is observed after the corrosion of water-quenched Al alloy by the same method, which is interpreted as being a result of Cu enrichment near the metal-oxide interface layer.

Effects of growth temperature and Si doping on the formation of vacancy defects in molecular beam epitaxy GaCrN films have been studied by positron annihilation spectroscopy. No vacancy defects were detected in the GaCrN films grown at 700^oC. In the undoped GaCrN film grown at 540^oC, vacancy clusters with sizes of V₈ to V₁₂ were responsible for positron trapping. Vacancy clusters were much reduced by Si doping, but complexes related to nitrogen vacancies still survived.

A stress-corrosion-cracked Type 304 austenitic stainless steel was investigated by the Doppler broadening of annihilation radiation (DBAR) spectroscopy with a positron microbeam. Clear increase of S parameter was observed over 200-400 μm areas from the stress corrosion cracking (SCC) crack. From the comparison of the DBAR spectra obtained from the stress-corrosion-cracked sample and plastically deformed sample, the increase of S parameter around the SCC crack was attributed to plastic-deformation-induced vacancies.

Several so-gel prepared silica thin films with ordered meso-pores were studied with a variable energy positron beam. Doppler broadening measurements show that in CTAB-templated samples, film porosity is the superior factor upon pore size and affects the final S parameter. While for P123 and F127-templated films with similar porosity, as hexagonal pores extend along the direction parallel to film surface, a larger effective pore size could be sensed by positrons, which enhances open volume ratio favourable for Ps formation.