Variable Energy Positron Beam Research Articles

A slow positron beam study of the oxidation behaviour of 310S stainless steel and alloy 800HT exposed to supercritical water

This paper reports a variable-energy slow positron beam characterisation of the surface oxide films in 310 S stainless steel and 800HT nickel-based alloy exposed to 550 °C supercritical water for 1000–4392 h. The evolution of the oxide layer was depth-profiled using positron beam energies from 0.5 to 20 keV, which enabled a probe of up to 2 mm of the near-surface region. SEM/EDS was used as a complementary characterisation tool to support and comprehend the positron annihilation spectroscopy data. A strong dependency on the positron trapping rate and the effective positron diffusion length on the microstructure of the studied materials enabled us to identify up to three non-bulk layers, which were preliminarily attributed to different oxide layers and Fe/Cr depletion regions. By calculating the effective positron diffusion length in the given multi-layer systems, we obtained the thicknesses of individual layers, which are in good agreement with the EDS analysis.

Characterisation of defects in porous silicon as an anode material using positron annihilation Doppler Broadening Spectroscopy

Here we present Positron Annihilation Doppler Broadening Spectroscopy (PADBS) as a powerful method to analyse the origin and development of defect processes in porous silicon structures as a result of alloying with lithium for the use in battery anode applications. Several prepared anodes were lithiated (discharged against Li+/Li) and de-lithiated (charged) with different capacities followed by a distinct treatment procedure and an analysis using the Delft Variable Energy Positron Beam. The results presented here show that we can distinguish two different processes attributed to (1) structural changes in silicon as a result of the alloying process, and (2) the formation of defects that initiate degradation of the material. The limit at which the porous material can be used for at least the first two cycles without the occurrence of damage can thus be accurately determined by using the PADBS technique.

Distribution of Hydrogen and Defects in the Zr/Nb Nanoscale Multilayer Coatings after Proton Irradiation.

Radiation damage is one of the significant factors limiting the operating time of many structural materials working under extreme conditions. One of the promising directions in the development of materials that are resistant to radiation damage and have improved physical and mechanical properties is the creation of nanoscale multilayer coatings (NMCs). The paper is devoted to the experimental comprehension of changes in the defect structure and mechanical properties of nanoscale multilayer coatings (NMCs) with alternating layers of Zr and Nb under irradiation. Series of Zr/Nb NMCs with different thicknesses of individual layers were fabricated by magnetron sputtering and subjected to H+ irradiation. The evolution of structure and phase states, as well as the defect state under proton irradiation, was studied using the methods of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction analysis (XRD), glow discharge optical emission spectroscopy (GDOES), and positron annihilation spectroscopy (PAS). The layer-by-layer analysis of structural defects was carried out by Doppler broadening spectroscopy (DBS) using a variable-energy positron beam. To estimate the binding energy and the energy paths for the hydrogen diffusion in Zr/Nb NMCs, calculations from the first principles were used. When the thickness of individual layers is less than 25 nm, irradiation causes destruction of the interfaces, but there is no significant increase in the defect level, the S parameter (open volume defects amount) before and after irradiation is practically unchanged. After irradiation of NMC Zr/Nb with a thickness of layers 50 and 100 nm, the initial microstructure is retained, and the S parameter is significantly reduced. The GDOES data reveal the irregular H accumulation at the interface caused by significant differences in H diffusion barriers in the bulk of Zr and Nb multilayers as well as near the interface’s region.

Development of a pulsed, variable-energy positron beam for atomic scale defect studies.

Positron annihilation spectroscopy provides a sensitive means of non-destructive characterization of materials, capable of probing single atom vacancies in solids with 10-7 sensitivity. We detail here the development of a magnetically guided, variable energy, pulsed positron beam designed to conduct depth-dependent defect studies in metals, semiconductors, and dielectrics, which will be the first of its kind in the United States. The design of the target stage provides capabilities for measurements during in situ annealing up to 800 °C and incorporates a new approach to minimize the background due to energetic backscattered positrons. The developed beam at Bowling Green State University provides a powerful tool for characterization of thin films, devices, and ion irradiated materials.

Positron annihilation studies on N+ implantation induced vacancy type defects and its recovery in SI: 6H- SiC

Variable energy positron beam with Doppler broadening spectroscopy (DBS) was used to investigate the depth-resolved defects formed by two fluencies of 130 keV N+ implanted in Semi Insulating (SI) 6H-SiC. The implanted ions induced damages near the surface and to a depth of 300 nm as simulated by SRIM analysis. Defects recovery, accumulation, and its behavior with two annealing temperatures were also considered. Defect types and its effect on annealing were analyzed from the changes in electron momentum and modeled as layers using variable energy positron fitting (VEPFIT) methods. Inclusion of electric field strength in the fitting procedure supported layer characteristics and the state of charge carriers. Di-vacancies, vacancy complexes observed in the damaged layers were agglomerated at the surface and tend to cure on annealing.

First-Principles Calculations and Experimental Study of H+-Irradiated Zr/Nb Nanoscale Multilayer System

Nanoscale multilayer coating (NMC) based on Zr/Nb layers (100/100 nm) before and after H+ irradiation was investigated by combining experimental techniques with first-principles calculations. Detailed studies of structural and phase state and defect structure of Zr/Nb NMC were performed using methods of transmission electron microscopy, X-ray structural analysis, glow discharge optical emission spectrometry, and the Doppler broadening spectroscopy using variable energy positron beam. The first-principles calculations of binding energies for hydrogen in metal Zr/Nb layers was carried out by the pseudopotential method within the density functional theory framework. First-principles calculations and experimental data indicate the presence of macro- and microstrains predominantly in the zirconium layers of Zr/Nb NMC. The main feature of the studied Zr/Nb NMC is the predominant hydrogen localization in Zr layers near the interfaces. The annihilation of positrons is shown to occur mainly in the Zr layers in the vicinity of the interface.

A Slow Positron Beam Study of Helium Clustering in Oxide Dispersion Strengthened Steels

The present work provides an innovative approach to the experimental study of the radiation tolerance of oxide dispersion strengthened (ODS) steel comparing to its non-ODS variant. Inhomogeneous radiation damage, introduced by helium irradiation, was probed in a depth profile manner by a variable energy positron beam. Although numerous studies reporting the use of slow positron beam (SPB) experiments on ion-implanted materials have been published in the literature, only a few of them address the continuous distribution of microstructural damage introduced by ion beam. In this paper, we use SPB to probe a wide range of radiation damage in a single sample of each material. By evaluating positron diffusion in different parts of the Bragg peak, we were able to track the evolution of radiation-induced damage from a high density of small defects such as helium-vacancy clusters to large bubbles grown by coalescence. Combining with the transmission electron microscopy results and slow positron Doppler broadening spectroscopy, we offer new insight into different stages of microstructural bubble evolution in steels exposed to harsh radiation conditions. The obtained results show that the higher density of vacancy-type defects in the oxide dispersion strengthened steel, which act as recombination centres, suppress the formation of large defect agglomerations. While the TEM provides an indication of this effect in high displacement damage and helium production rates, positron annihilation spectroscopy elucidates the origin of this behaviour, improving the understanding of the fundamental mechanism of helium embrittlement.

Analysis of positron profiling data using e[formula omitted]DSc computer code

The Green’s function method was applied to solve the one-dimensional positron diffusion equation for a system consisting of up to four layers that contain defects with different trapping rates. These allow us to obtain the analytical relationships valid for the evaluation of data obtained from variable energy positron measurements. They have been implemented in user-friendly free computer code available to users. Fitting strategies are presented to extract the relevant physical parameters. The code was used to determine positron diffusion length in samples of polycrystalline pure, well-annealed iron, depleted uranium, and titanium. Program summaryProgram Title: e+DSC-1CPC Library link to program files: https://doi.org/10.17632/jxpj25kjvr.1Licensing provisions: MIT licenseProgramming language: Microsoft Visual Basic 2015External routines/libraries: Accord.NETNature of problem: The program enables the analysis of data obtained from a variable energy positron beam. The shape parameter of the annihilation line as a function of the incident positron energy is evaluated using a positron diffusion trapping model in which the positron trapping rate function is expressed as a four-step function.Solution method: The one-dimensional diffusion equation was solved by the Green’s function method. This allows the use of an exact form for the positron implantation profile, which is used in the program as the Makhov’s function. The parameters of this function are obtained from the MC simulation with GEANT4 and stored in the program.

Gradient Microstructure Induced by Surface Mechanical Attrition Treatment (SMAT) in Magnesium Studied Using Positron Annihilation Spectroscopy and Complementary Methods.

Surface mechanical attrition treatment (SMAT) was used to generate a gradient microstructure in commercial grade magnesium. Positron annihilation lifetime spectroscopy and variable energy positron beam measurements, as well as microhardness tests, electron backscatter diffraction, X-ray diffraction, and electrochemical corrosion tests, were used to investigate the created subsurface microstructure and its properties. It was found that SMAT causes an increase in dislocation density and grain refinement which results in increased hardness of the subsurface zone. The mean positron lifetime values indicate trapping of positrons in vacancies associated with dislocations and dislocation jogs. The increase of the SMAT duration and the vibration amplitude influences the depth profile of the mean positron lifetime, which reflects the defect concentration profile. Electrochemical measurements revealed that the structure induced by SMAT increases the susceptibility of magnesium to anodic oxidation, leading to the enhanced formation of hydroxide coverage at the surface and, as a consequence, to the decrease in corrosion current. No significant effect of the treatment on the residual stress was found.

Evaluation methodology of diffusion coefficient of guest substances associated with angstrom-scale open spaces in materials by slow positron beam.

The knowledge of elemental migration associated with angstrom-scale open spaces is a prerequisite for sophisticated material design with respect to nanodispersion as well as a better understanding of molecular migration in a geological environment. Here, we show the methodology for evaluating the diffusion coefficient of guest substances focusing on angstrom-scale open spaces, where the depth-variation of open spaces caused by elemental diffusion in the micrometer-scale surface region is examined by the variable-energy slow positron beam. The present methodology determines a low diffusion coefficient of ∼10-14 cm2 s-1 for potassium at 533 K in SiO2 glass within the framework of Fick's principle providing the evidence that ∼2.5 Å open spaces intrinsically present in the glass matrix govern the diffusion process.

Effect of Proton Irradiation on the Defect Evolution of Zr/Nb Nanoscale Multilayers

Nanoscale multilayer coatings (NMCs) with different crystal structures are considered as capable of self-healing after radiation damage due to the recombination of vacancies and interstitials. This work is focused on a defect distribution study of NMCs based on Zr/Nb layers (25/25 nm and 100/100 nm) after proton irradiation. Coatings with a total thickness of 1.05 ± 0.05 µm were irradiated by 900-keV protons using a pelletron-type electrostatic accelerator with an ion current of 2 µA for durations of 60 min to 120 min. The influence of the irradiation effect was studied by X-ray diffraction analysis (XRD), glow discharge optical emission spectrometry (GD–OES), and Doppler broadening spectroscopy using a variable energy positron beam. The results obtained by these methods are compatible and indicate that defect concentration of Zr/Nb NMCs remains unchanged or slightly decreases with increasing irradiation time.

A multi-stop time-of-flight spectrometer for the measurement of positron annihilation-induced electrons in coincidence with the Doppler-shifted annihilation gamma photon.

In this study, we describe an advanced multi-functional, variable-energy positron beam system capable of measuring the energies of multiple "positron-induced" electrons in coincidence with the Doppler-shifted gamma photon resulting from the annihilation of the correlated positron. The measurements were carried out using the unique characteristics of the digital time-of-flight spectrometer and the gamma spectrometer available with the advanced positron beam system. These measurements have resulted in (i) the first digital time-of-flight spectrum of positron annihilation-induced Auger electrons generated using coincident signals from a high-purity Ge detector and a micro-channel plate, (ii) a two-dimensional array of the energy of Doppler-broadened annihilation gamma and the time-of-flight of positron-annihilation induced Auger electrons/secondary electrons measured in coincidence with the annihilation gamma photon, and (iii) the time-of-flight spectra of multiple secondary electrons ejected from a bilayer graphene surface as a result of the impact and/or annihilation of positrons. The novelty of the gamma-electron coincidence spectroscopy has been demonstrated by extracting the Doppler-broadened spectrum of gamma photons emitted due to the annihilation of positrons exclusively with 1s electrons of carbon. The width of the extracted Doppler-broadened gamma spectrum has been found to be consistent with the expected broadening of the annihilation gamma spectrum due to the momentum of the 1s electrons in carbon.

Automation of variable low energy positron beam experiments using LabVIEW based program

Implementation of a state of the art data acquisition and analysis program for variable low energy positron beam using LabVIEW based automation software is described. The program does the following functions: (a) controlling the remote programmable high voltage–power supply to bias the sample negatively (i.e., to vary the positron beam energy), (b) control of Windows based PCI MCA card, (c) calculating the S, W, R and ortho-positronium self-annihilation fraction (fPs) parameters, errors, and other relevant parameters, (d) saving the raw spectra, results and log data file and (e) dynamic display of raw spectrum and S, W, fPs versus energy for easy visualization of progress of the experiment. The USB based interfacing standard of this design provides easy connectivity to the personal computers.

Surface and Subsurface Defects Studies of Dental Alloys Exposed to Sandblasting

The defects created in commercial dental alloys during blasting with alumina particles propelled in compressed air under pressure 0.1 and 0.4 MPa have been studied using positron annihilation spectroscopy, scanning electron microscopy and X-ray diffraction. It was observed that higher pressure causes the increase in roughness and damaged zone range. The type of defects was determined as vacancies on dislocations. The defect concentration decreases with the depth and depends on alloys’ type and applied pressure. The Rutherford backscattering spectroscopy and variable energy positron beam studies indicate shallow alumina deposition in material and show that small pressure of 0.1 MPa is not enough to remove metal surface oxides completely in 60 s in all studied dental alloys.

Application of positron beam for the long range effect studies in proton implanted iron

The paper reports studies of pure, polycrystalline iron samples exposed to proton implantation with energies of 73 keV and 173 keV and doses of 3 × 1015, 1016 and 3 × 1016 cm−2. Additionally, two samples were exposed to multiple implantation with different energies and fluencies. Variable energy positron beam (VEP) was applied to the detection of irradiation-induced defects and their distribution. In turn, elastic recoil detection (ERD) method was used to estimate hydrogen concentration and its range. Analysis of VEP results was performed with VEPFIT code. The strong reduction of positron diffusion lengths in the implanted layers point out the presence of defects with high concentration. The thicknesses of these layers are in good agreement with hydrogen range evaluated by ERD. However, the presence of defects with lower concentration behind implanted zone was also detected when the free fitting was performed. The long range effect cannot be excluded.

Defects and their range in pure bismuth irradiated with swift Xe ions studied by positron annihilation techniques

Investigations of defects and their spatial distribution in Bi irradiated with 167 MeV Xe26+ ions of different doses have been performed using conventional positron lifetime spectroscopy and variable energy positron beam. In an implanted layer, in which ions are traveling, interacting with atoms and stopping, only clusters which consist of more than eight vacancies were found. It was assigned from ab initio theoretical calculations of positron lifetime in vacancy clusters in Bi. The thickness of this layer corresponds to the range of implanted ions calculated from the SRIM code. However, beyond this layer, an extended layer with such defects has also been found. Its thickness is comparable to the thickness of the implanted layer and it depends on the dose. Defects induced by implantation are also present near the entrance surface, and their concentration depends on the dose of implanted ions as well. Three methods for reconstructing the actual mean positron lifetime and thus the induced depth defect distribution have been proposed, two of them are used in current research.

Positron annihilation study of defects in copper irradiated with swift Xe26+ ions

Variable energy positron beam and conventional positron lifetime spectroscopy were used to study pure copper exposed to irradiation with 167 MeV Xe26+ heavy ions with different doses of 1012, 1013, 5 × 1013, 1014 ions/cm2. The presence of vacancy-type defects induced by implantation was confirmed in Doppler spectroscopy characteristics. Decreasing of the positron diffusion length from 135 nm for unirradiated sample, to 82 nm for the lowest and to 46 nm for the highest applied dose was noted. It points out increasing defect concentration with the increase of the fluence. Additionally, the existence of one kind of defects in all irradiated samples was noticed. The defected zone covered with the implanted range and the so called “long range effect” was not confirmed.

Positron Annihilation and Complementary Studies of Stainless Steel Exposed to Sandblasting at Different Angles

Defect studies in subsurface zone in stainless steel 304 AISI samples exposed to sandblasting were performed using positron annihilation spectroscopy techniques. Samples were sandblasted with a different impact angle. Conventional experiments based on positrons emitted directly from the radioactive source allowed us to detect vacancies on the dislocation edges in all samples; however, the total depths of subsurface zones depended strictly on the impact angle, i.e., 35 µm for impact angle of 90° and about 12 µm for 30°. The complementary methods such as SEM and optical profilometry revealed also dependencies between the impact angle and roughness of the surface which are not observed in the variable energy positron beam examinations.

Low energy positron beam system for the investigation of 2D and porous materials

An advanced variable energy positron beam (~2 eV to 20 keV) has been designed, tested and utilized for coincidence Doppler broadening (CDB) measurements at the University of Texas at Arlington (UTA). A high efficiency solidified rare gas (Neon) moderator was used for the generation of a slow positron beam. The gamma rays produced as a result of the annihilation of positrons with the sample electrons are measured using a high purity Germanium (HPGe) detector in coincidence with a NaI(Tl) detector. Modifications to the system, currently underway, permits simultaneous measurements utilizing Positron annihilation induced Auger Electron Spectroscopy (PAES) and CDB. The tendency of positrons to become trapped in an image potential well at the surface will allow the new system to be used in measurements of the chemical structure of surfaces, internal or external and interfaces. The system will utilize a time of flight (TOF) technique for electron energy measurements. A 3m flight path from the sample to a micro-channel plate (MCP) in the new system will give it superior energy resolution at higher electron energies as compared to previous TOF systems utilizing shorter flight paths.

Variable energy positron beam studies of defects in heavy ion implanted palladium

Abstract The paper reports experimental studies of Palladium (Pd) exposed to 107 MeV Kr 17 + ion irradiation. Heavy ion implantation of samples with fluences of 10 12 , 10 13 , 5 × 10 13 and 10 14 was performed at IC-100 cyclotron at Flerov Laboratory of Nuclear Reactions at Joint Institute for Nuclear Research (JINR) in Dubna, Russia. Radiation damages were investigated with variable energy positron beam (VEP). Doppler broadening spectroscopy (DB) was applied and the line shape S parameter of annihilation line was extracted. The analysis of S parameter profiles gives information about the presence of open volume defects in irradiated samples. The positron diffusion length extracted from the profile decreases with the dose increasing from 112 nm for a non-irradiated sample to 26 nm for the biggest fluence. It points out the increase of defect concentration with the increase of the fluence.