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The defect structure of annealed cast, electron beam melted and ultrafine-grained titanium Ti-6Al-4V alloys before and after hydrogenation was studied. It has been established that before hydrogenation the predominant types of defects in electron beam melted and ultrafine-grained titanium alloy are dislocations and low-angle boundaries, respectively. The cast alloy after annealing is defect-free material. Hydrogenation from the gas phase to 1:00 ± 0:15 wt% leads to an increase of the concentration of the predominant type of defects. Moreover, vacancy complexes also presented in electron beam melted and ultrafine-grained Ti-6Al-4V alloys interact with hydrogen and form hydrogen-vacancy complexes. [ABSTRACT FROM AUTHOR]

The role of vacancies in hardening of Fe-Al intermetallic alloys were studied in the present work for a wide range of Al concentrations from 20 to 50 at%. The alloys quenched from 1000 °C as well as those annealed subsequently at 520 °C for 1 h were subject to study. Slow-positron beam experiments combined with Vicker's microhardness tests were utilised. Hardness of Fe-Al alloys exhibited a somewhat complex dependence on Al content which could not be fully explained purely by consideration of intermetallic phases formed. This happens due to additional hardening effect caused by quenched-in vacancies. The concentrations of vacancies were estimated from positron back-diffusion data and found to rise for Al content above 25 at%. Correlation of vacancy concentrations with hardness data for the quenched and annealed alloys has revealed that hardening of alloys with a low Al content (< 30 at%) is originated predominantly by anti-phase boundaries while hardening induced by quenched-in vacancies dominates for alloys with a higher Al content (30-50 at%). [ABSTRACT FROM AUTHOR]

The properties of positrons delocalized in the HfNbTaTiZr complex concentrated alloy are investigated theoretically and related to available experimental data. Positron lifetimes calculated for random atomic arrangements appear to be short compared to the experiment carried out on a vacancy-free sample. Contrarily, a configuration exhibiting a local atomic order provides a lifetime that is very close to the experimental value. These observations indicate that the local atomic order is occurring in the studied alloy, in agreement with other measurements. The analysis of calculated results shows that the positron lifetime and affinity are affected to some extent by the alloy substitutional disorder, lattice relaxations, and charge transfer among atomic species, which should be taken into account in precise calculations. [ABSTRACT FROM AUTHOR]

Open spaces in amorphous Al2O3 films fabricated by atomic layer deposition and in AlONx deposited by a reactive sputtering technique were probed by using monoenergetic positron beams. In these films, open spaces with three different sizes were found to coexist. The mean size and the concentration of open spaces were decreased by annealing at 800 °C in N2 atmosphere, which was associated with rearrangements of the amorphous network in the films and their crystallization. Nitrogen incorporation into Al2O3 suppressed the shrinkage of network structures due to the annealing, which was attributed to the formation of a stable network through the introduction of Al-N bonding. For Al2O3 deposited on GaN, the temperature treatment above 800 °C led to the introduction of vacancytype defects in the GaN substrate. This fact suggests that the change in the network structure of Al2O3 enhances chemical instability at the Al2O3/GaN interface and the out-diffusion of atoms from the substrate. [ABSTRACT FROM AUTHOR]

A method of reconstructing the actual depth profile of the mean positron lifetime obtained from the sequential chemical etching procedure has been proposed. In this procedure, the measured positron mean lifetime values as a function of depth are used to calculate the actual mean positron lifetime values at various depths. Two examples show the use of this method. The first one reconstructs the actual mean positron lifetime distribution in molybdenum subjected to friction. The second one reconstructs distribution in a silicon wafer irradiated with 167 MeV Xe26+ ions. These examples show this method's usefulness in detecting properties in subsurface zones generated by various surface treatments. [ABSTRACT FROM AUTHOR]

Rocks, minerals, and powdered catalysts, not considered "typical" positron annihilation spectroscopy materials were included in these studies, to find the optimal way to characterize atypical materials by the most appropriate set of parameters in a well-defined environment. The positron annihilation spectroscopies: the Doppler broadening and positron lifetime were compared with other supplementary techniques to validate, interpret, and understand the results in different materials better. It was found that although rocks are heterogeneous mineral aggregates, some positron annihilation spectroscopy values were specific for various types of rocks, potential oil deposits, and their conditions; and quite different for opals due to Ps formation there; moreover in order to characterize properly samples with small mass and density (catalytic powders), their measuring geometry, matrix effect, and set of operational parameters needed to be well-defined. [ABSTRACT FROM AUTHOR]

Scandium nitride (ScN) is a semiconductor with a rocksalt-structure that has attracted attention for its potential applications in thermoelectric energy conversion devices, as a semiconducting component in epitaxial metal/semiconductor superlattices. Two ScN films of 118 nm and 950 nm thicknesses were deposited at the same conditions on MgO (001) substrate by reactive magnetron sputtering. Poly-orientation of films was observed with first an epitaxial growth on MgO and then a change in the orientation growth due to the decrease of the adatom mobility during the film growth. Positron lifetime measurements showed a high concentration of nitrogen vacancies in both films with a slightly higher concentration for the thicker ScN film. Presence of nitrogen vacancies explains the values of direct band gaps of 2:53±0:01 eV, and 2:56±0:01 eV which has been measured on ScN films of 118 nm and 950 nm thicknesses, respectively. [ABSTRACT FROM AUTHOR]

Silicon-based nanochanneled converters for the production of cold positronium (Ps) have been introduced in 2010 and since then provided a reliable source of cold Ps to a variety of experiments. With the goal of advancing the optimization of such converters, we have formulated a classical model to describe the production and cooling of Ps in nanochanneled converters. The simulation of this process poses several challenges due to the complexity of the geometry in which it takes place. We will here briefly give an overview of our model and discuss several simplifications of the simulation process which, without altering significantly the simulation results, reduce the computational costs enough to allow for systematic scans of the nanochanneled converter construction parameter space. [ABSTRACT FROM AUTHOR]

The present paper reports on the recent activity of the data analysis software development for total-reflection high-energy positron diffraction, a novel experimental technique for surface structure determination. Experiments using TRHEPD are being conducted intensively at the Slow Positron Facility, Institute of Materials Structure Science, High Energy Accelerator Research Organization, revealing surface structure of interest. The data analysis software provides a solution to the inverse problem in which the atomic positions of a surface structure are determined from the experimental diffraction data (rocking curve). The forward problem is solved by the numerical solution of the partial differential equation in the quantum scattering problem. A technical demonstration with a test problem was carried out to confirm the software functioned as expected. Since the analysis method has a general mathematical foundation, it is also applicable to other experiments, such as X-ray or electron diffraction experiments. [ABSTRACT FROM AUTHOR]

In this work, we propose a new positron annihilation lifetime spectrum measuring method for pulsed positrons. The spatially arranged positron burst detection system is based on a state-of-the-art calcium-doped LYSO crystal and silicon photomultiplier array, with readout by a multichannel fast waveform digitizing DRS4 chip; this system simultaneously provides high detection efficiency and decent timing resolution. The positron burst generation signal serves as the starting trigger and the timing signal of all the detector channels is the stopping signal, so that the time spread distribution is the lifetime spectrum. The current system has 2048-channel modularized crystals coupled to 2048-channel 3 × 3 mm² silicon photomultipliers. All analog signals from the detectors are digitized separately before implementing digital constant fraction discrimination by a field-programmable gate array. The performance of the single-channel detector was investigated and optimized by improving the silicon photomultiplier readout electronics and the light collection of the scintillator. Preliminary results showed that the best coincidence timing resolution by two identical, 5 mm long, single LYSO detectors was 84 ps and that the measurement was double the 511 keV coincidence of a 22Na source. [ABSTRACT FROM AUTHOR]

A new code PLRF (version 19) was developed for decomposition of positron lifetime spectra. The PLRF code is based on the least squares fitting of positron lifetime spectra and utilizes a minimization routine MINUIT from the ROOT package developed at CERN. Multiple positron lifetime spectra can be fitted simultaneously using the PLRF code. Each fitting parameter can be selected to be either common (i.e., the same value for all spectra) or individual (i.e., different values for each spectrum). Positronium (Ps) contribution is considered as a double exponential component consisting of short lived para-positronium (p-Ps) and long lived ortho-positronium (o-Ps) component. Ratio of o-Ps and p-Ps contribution can be constrained. Several models were implemented into the PLRF code: independent exponential components, simple trapping model, and diffusion trapping model. Physically relevant parameters of the model are varied to get best agreement with experimental positron lifetime spectra. As a consequence the user obtains physically meaningful parameters from fitting, e.g., positron trapping rates in the case of the simple trapping model, grain or cell size and dislocation density in the case of the diffusion trapping model, etc. [ABSTRACT FROM AUTHOR]

Since the decomposition of positron lifetime spectra requires solving an ill-posed and inverse problem, the accurate knowledge of the spectrometer's instrument response function is crucial for extracting the true underlying physical information of the phenomenon under investigation. In general, the instrument response function is modelled by a superposition of Gaussian functions, since an analytical solution for the convolution with an exponential distribution function exists and, hence, the characteristic lifetimes and its corresponding contributions can be obtained by non-linear least-squares fitting. In contrast, the iterative least-squares re-convolution approach determines the best fit of the recorded lifetime spectrum by re-convoluting a sum of N expected exponential decays with the numerical data of the experimentally obtained instrument response function. For a laboratory setup, two methods exist to estimate the shape of the instrument response function from experiment: (1) the direct method using a 60Co isotope and an indirect method by graphically deconvoluting the monoexponential lifetime spectrum obtained from 207Bi. For both variants, the energies of the incident gamma-rays are considerably different to the energies accompanying the creation (1274 keV) and annihilation (511 keV) of a positron using 22Na: 60Co (1173 keV, 1333 keV), 207Bi (570 keV, 1064 keV). Here we present a detailed study on the basis of plastic scintillators regarding the spectra decomposability by using the re-convolution technique with experimentally obtained instrument response functions. We can clearly show that beyond incident gamma-ray energy differences, the Compton scattering effects and pile-up events represent the limiting factors in this approach. [ABSTRACT FROM AUTHOR]

Low-energy positron beams are applied in a number of experiments in solid state physics and materials science as well as in atomic and particle physics. These experiments comprise of surface studies, depth dependent defect spectroscopy, and fundamental research with leptonic systems. The neutron induced positron source in Munich (NEPOMUC) is operated as a user facility providing a positron beam with an intensity of about 109 moderated positrons per second. In this paper, a review of the status of the positron beam facility with its instruments is given and several experiments are exemplarily highlighted. Recent and future developments for further improving the performance of positron beam experiments are discussed. [ABSTRACT FROM AUTHOR]

Relativistic positron beams were generated by laser wakefield electrons bombarding on solid target. Very stable positron beams were generated in our experiments. The total yield of positrons is about 4.4×108/shot. The energy spectra of positrons and electrons obey quasi-Maxwell distribution. Compared with the direct method, the indirect method produces positrons (38.5 MeV) and electrons (50.5 MeV) with much higher slope temperature. [ABSTRACT FROM AUTHOR]

Temperature dependence of triplet positronium (ortho-Ps) lifetime in N,N,N-Trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) was obtained with positron energies of 2 keV and 12 keV using the vertical slow positron beamline installed at AIST. Positrons could be injected from the liquid surface of TMPA-TFSI and then the surface structure could be detected even at 100 °C. The ortho-Ps lifetime became shorter at higher temperature in near surface and bulk. It indicated that the Ps bubble structure in room temperature ionic liquids was different from that in usual molecular liquids. Ps bubble size obtained from ortho-Ps lifetime can give information of the size of soft region consisting of van der Waals force surrounded by the stiff structure consisting of the Coulomb force. [ABSTRACT FROM AUTHOR]

We report here positron annihilation-induced Auger electron spectroscopy measurements of an as received TiO2(110) surface. We observe peaks at ~ 250 eV and ~ 500 eV due to the Auger decay of annihilation-induced 1s holes in carbon (C KVV), and oxygen (O KVV) respectively. In addition to these KVV Auger peaks we observe a broad, low energy peak extending up to 15 eV. Since the incident positron beam energy of 1.5 eV is well below the energy threshold required to directly knock out electrons with kinetic energies up to 15 eV, this peak must be Auger-related. Based on energy conservation requirements and its considerable spectral weight, we argue that this low energy peak is principally due to the Auger decay of both annihilation induced 2s core holes in oxygen and annihilation induced valence holes in carbon. [ABSTRACT FROM AUTHOR]

We present initial results that demonstrate the ability of Doppler broadening spectroscopy (DBS) to extract elemental information from the topmost atomic layer of a sample surface. Doppler broadening spectra from graphene grown on a polycrystalline Cu substrate was compared to the Doppler broadened spectra obtained after the partial removal of the bilayer graphene from the Cu surface. The Doppler broadening of the annihilation photopeak showed a clear difference between the as-inserted and the partially sputtered sample. We posit that this difference was due to the contribution of the gamma photons originating from the annihilation of positrons with the substrate (Cu) electrons to the total Doppler broadened spectrum. The Doppler broadened spectrum of the partially sputtered sample was well fit with a line shape produced by combining the Doppler broadened gamma spectrum of the as-inserted sample with the theoretically calculated annihilation gamma photo peak from bulk Cu. The fraction of the calculated Doppler broadened spectrum from bulk Cu in the fit spectrum is consistent with the increase in intensity of the Cu M2,3VV Auger peak observed in the positron annihilation induced Auger electron spectra (PAES) of the partially sputtered sample. This suggests that analysis of the high momentum region of the gamma photopeak due to photons emitted following the annihilation of surface trapped positrons can provide a quantitative estimate of the elemental composition of the top surface. Our results provide evidence for the feasibility of using DBS as a new spectroscopic technique for the elemental characterization of the topmost atomic layer of surfaces. [ABSTRACT FROM AUTHOR]

This paper shows results of conventional positron lifetime and slow positron beam studies of the long range effect. These techniques are not commonly used for this topic. The experimental investigations of pure metals exposed to ion irradiation are reported and discussed on the basis of the literature data. The positron lifetimes measured in the etching experiment give information about kind of defects, their distribution, and existence of the long range effect. The above can be achieved using the positron beam for the case of low energy ion implantation. [ABSTRACT FROM AUTHOR]

We report on the positron chemical potential, positron band effective mass, positron affinity, and positron bulk lifetime in cadmium telluride under compression. The calculations are carried out using a pseudo-potential approach. The ionic potential is treated using the point-core approximation. At zero pressure, our results agree reasonably well with those reported in the literature. Upon compression, the positron characteristics vary monotonically indicating that the positron annihilates differently in CdTe when pressure is applied. The present study should provide essential information in the positron annihilation in CdTe under pressure. [ABSTRACT FROM AUTHOR]

In the present work, Fe9Cr model alloy was implanted with 100 keV He and 60 keV H ions at room temperature. Positron annihilation Doppler broadening spectroscopy was employed to detect (H, He)-V complexes with open volume in the irradiated specimens. The results show that the positron is sensitive to the relative position of the gas atoms decorating the open volume; i.e., to the configuration of the (H, He)-V complexes. The formation of H-vacancy clusters after H implantation and a large number of vacancies can be retained after H-V decomposes at room temperature. For the pre-implanted He situation (He + H), He-vacancy clusters are formed in advance and the post-implanted H introduces a large number of vacancies and decomposable H-vacancy clusters. The He-vacancy clusters forming in advance may recombine with vacancies and H atoms to form stable He-vacancy clusters and a small amount of He-H-V complex. [ABSTRACT FROM AUTHOR]

Probing of small-scale samples such as thin ion-implanted layers or TEM discs using conventional radioisotope sources is a rather complicated task due to the range of energies of the emitted positron. Besides, experimental spectra obtained from samples containing internal transmutation-based positron source are naturally disturbed by positron annihilation outside of the sample. To estimate these contributions to the spectra, the Geant4 simulation toolkit was used. In addition to various absorption profiles of positrons from realistic encapsulated sources, the use of TEM disc containing 44Ti/44Sc as an external positron source is discussed and suggested. [ABSTRACT FROM AUTHOR]

The GBAR experiment aims to measure the gravitational acceleration of antihydrogen H. It will use H+ ions formed by the interaction of antiprotons with a dense positronium cloud, which will require about 1010 positrons to produce one H+. We present the first results on the positron accumulation, reaching 3.8±0.4×108 e+ collected in 560 s. [ABSTRACT FROM AUTHOR]

The neutron-induced positron source Munich (NEPOMUC) provides a monochromatic low-energy positron beam with an intensity of > 1 × 109e+ s-1 and a full width at half maximum (FWHM) diameter of about 10 mm. To create a small beam focus or sharp positron pulses of 100 ps FWHM the beam brightness needs to be enhanced by remoderation. This is achieved by focusing the primary beam magnetically onto a tungsten single crystal W(100) in reflection geometry. Afterwards the beam exhibits an intensity of > 5 × 107e+s-1 and a diameter of < 2 mm FWHM. To further optimize the beam quality of NEPOMUC we redesigned the remoderation unit. The new setup allows a precise positioning of the remoderator crystal within the focus of the magnetic lens. Additionally, a replacement of the crystal within several minutes and without breaking the beamline vacuum is possible that offers the opportunity for systematic tests of different remoderator materials. [ABSTRACT FROM AUTHOR]

We present calculations of differential, integrated elastic, total momentum transfer cross-sections and spin polarization parameters S for scattering of positron from Eu and Bi atoms in the energy range 2.0 to 500.0 eV using relativistic approach. The target projectile interaction is represented both by real and complex parameter- -free optical potentials in the solution of the Dirac equation for the scattered positrons. The theoretical results are obtained from relativistic approach based on solving the Dirac equation using Hartree-Fock and Dirac-Fock wave functions to calculate cross-sections at all the energies considered. [ABSTRACT FROM AUTHOR]

Nuclear facilities as nuclear power stations, nuclear research reactors, particle accelerators and linear accelerator in medical institution using concrete in building construction. The different type materials of the aggregate as component of concrete were analyzed to provide radiation protection. The energy deposited the transmission factor and the mass attenuation coefficients in ordinary and barite concretes have been calculated with the photon transport Monte Carlo software. The numerical simulations results show that using barite as an aggregate in the concrete is one of the solutions for gamma ray shielding. Thereat, it is shown non-destructive method for determining the gamma radiation absorption characteristics of concrete. [ABSTRACT FROM AUTHOR]