Positron Beam Doppler Research Articles

Positron annihilation spectroscopy guided by two-component density functional theory calculations distinguishes irradiation-induced vacancy type in 4H-SiC

Based on two-component density functional theory integrated with the projector augmented-wave basis and incorporating both calculated and experimental data from Positron Annihilation Spectroscopy (PAS), this study introduces a novel method for identifying and analyzing specific types of vacancies when multiple types of vacancies are coexisting. This method was then tested on 4H-SiC irradiated by 300 keV C4+ ion beams. By calculating charge density to analyze positron annihilation lifetime spectroscopy and calculating wave functions to analyze slow positron-beam Doppler broadening spectroscopy, for the first time, silicon monovacancies (VSi) and carbon monovacancies (VC) in irradiated 4H-SiC were quantitatively detected separately, allowing them to be distinguished with high accuracy. In addition, a decreasing trend in the relative percentage of VC with increasing irradiation dose, consistent with that expected when irradiating with carbon ions, was also observed, illustrating both the effectiveness and potential of this method for broader applications in material defect analysis. This study not only addresses the challenges of identifying multiple coexisting vacancy types using PAS but also extends the applicability and depth of PAS in fields such as nuclear energy, aerospace, and semiconductors.

Effect of Mo alloying on vacancy-defect evolution and irradiation damage in titanium alloy

The evolution of defects and irradiation damage in Ti and Ti-Mo alloy under hydrogen ion irradiation were investigated by slow positron-beam doppler broadening spectroscopy (DBS) and coincidence doppler broadening spectroscopy (CDB), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD results indicate that the existence of the β phase enhances the solid solubility of hydrogen in Ti-Mo alloy. The metastable γ-TiH hydrides are formed in pure titanium after hydrogen ion irradiation, while no hydride is found in Ti-Mo alloy under the same irradiation conditions. Moreover, DBS and CDB results show that hydrogen vacancy-complexes are formed in Ti and Ti-Mo alloys after hydrogen ion irradiation. Calculated doppler curves trend qualitatively agrees well with the experimental results. The TEM results show lower defect concentration and smaller defect sizes in Ti-Mo alloy with (α + β) than pure titanium. The addition of molybdenum significantly refines the grains and increases the density of the interface, which plays an important role in the management of irradiation-induced defects.

Study on UV-laser fatigue damage of sol-gel antireflective coating using XPS and slow positron beam Doppler broadening spectrum

The fatigue damage behavior of antireflective coating over KDP crystal under vacuum environment was studied with varied UV-laser irradiation, and the dynamic process from initial damage to complete failure of this coating was obtained. Scanning electron microscope (SEM) and slow positron beam (SPB) technique was employed to study the macro- and micro-defects in the process of evolution. The formation of non-bridging oxygen (NBO) and the variation of O/Si ratio along the direction of coating depth were obtained during the damage process based on X-ray photoelectron spectroscopy (XPS). The idea that oxygen with low binding energy absorbs UV light and produces oxygen defects to induce damage was put forward, and the damage mechanism of sol-gel antireflective coating under UV-laser irradiation was given.

Improving the exploration of vacancy evolution in P92 alloy under Fe ion irradiation using positron annihilation

P92 alloy, a candidate structural material for supercritical water reactors, was irradiated with 1 MeV Fe ions at 1, 3, and 7 dpa at room temperature, 300, 450, and 550°C. Slow positron beam Doppler broadening spectroscopy was used to characterize the evolution of the open-volume defects. Irradiation of the Fe9Cr alloy at 300°C induced an unexcepted phenomenon in which the S parameter decreased with an increasing irradiation dose; the phenomenon was ascribed to the pinning effect of Cr on dislocation loops, which promoted the recovery of vacancies. Whereas the reduction in the S parameter was generally inhibited in the P92 alloy. These distinctions are related to the evolution of vacancies. We show that the migration of vacancies hindered by C atoms is mainly related to carbon–vacancy complexes in the P92 alloy at approximately 300°C. As the irradiation temperature increased, the complexes gradually dissociated, and the vacancies were captured and annihilated with interstitial atoms. A better understanding of the evolution of vacancies is helpful for investigating irradiation swelling.

Investigation on structural and optical properties of ZnSe thin films prepared by selenization

In this paper, ZnSe films were prepared on quartz substrates by selenizing as-sputtered zinc films at different temperatures for 6 h. The crystal structure, morphology, microscopic defects and optical properties of the ZnSe films were characterized by XRD, SEM, Raman spectroscopy, UV–visible spectrophotometer and slow positron beam Doppler broadening spectroscopy, respectively. The results showed that ZnSe thin films had the best structural and optical properties when the selenization temperature was 700 °C . All ZnSe thin films have a hexagonal wurtzite structure. The results of XRD、SEM and Raman spectra all showed that the crystallization and molar ratio of the films were significantly optimized with the increase of temperature. The transmission spectra of the films showed an obvious absorption edge of ZnSe and increased transmittance with the increase of temperature. The variation of defect concentration and types in the ZnSe thin films were released visually by slow positron beam Doppler broadening energy spectra.

The investigation of distribution on size and concentration of helium bubbles in Y-bearing ODS steel using by SAXS and GIXRD

The nucleation and growth of the helium bubble in the structure material usually were subjected to temperature in a thermodynamic environment. In this study, Y-bearing oxide dispersed strengthened (ODS) ferritic/martensitic steels were used as the research material. The Y-bearing ODS steel samples with a thickness of 30 microns were irradiated successively by helium ions with three energies of 300 keV, 400 keV and 550 keV at room temperature and 450°C respectively. Firstly, the evolution of defects caused by H e + irradiation is analyzed by using slow positron beam Doppler broadening spectroscopy (DBS) in positron annihilation spectroscopy technology (PAS). The formation and growth of helium bubbles are deduced from the changes in the concentration of vacancy-type defects. The increase in the size of the helium bubbles at high temperatures leads to a certain distortion of the crystal lattice, which is also verified by analyzing the grazing incidence X-ray diffraction (GIXRD) patterns. The characterization of radius and concentration of helium bubbles in samples irradiated at room temperature and 450°C have been studied by Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM). The results showed that the intensity (I) ~ q-vector (q) curves of samples vary with irradiation temperature over ranges of 0.13 ≤ q ≤ 4.6 nm −1 , which is attributed to the growth of helium bubbles at different irradiation temperature. It is found that the average size of helium bubbles under room temperature irradiation is about 1.78 nm, and the size distribution is narrow, while at the irradiation temperature of 450°C, the size of helium bubbles increases to 2.9 nm, and the size distribution is wider. This result is also verified from the TEM analysis. Besides, the shape of helium bubbles was assumed roughly and tends to spherical with higher temperatures.

Effect of film thickness on structural and optical properties of ZnS:Cu films prepared by vulcanization

Abstract ZnS:Cu thin films were prepared by vulcanizing the sputtered Zn:Cu thin films at 500°С in sulfur vapor. The crystal structure, surface morphology, optical properties and microscopic defects of the films were characterized by XRD, SEM, UV–visible spectrophotometer, Raman spectroscopy and slow positron beam Doppler broadening spectroscopy, respectively. The XRD results showed that the ZnS:Cu thin films without any detectable impurities were a cubic structure, further confirmed by Raman spectra. The grain size of ZnS:Cu films was observed to increase from ~150 nm to ~300 nm with film thickness. Besides, apparent changes in the compactness and stoichiometry of the ZnS:Cu films were found with increasing the film thickness. These ZnS:Cu films exhibited a low transmittance in the visible range, with the band-gap energies of 3.52–3.62eV. Also, the effect of film thickness on microscopic defects of ZnS:Cu films were discussed in detail.

Study of defects in potassium-doped tungsten alloy by positron annihilation technique

Tungsten alloy is known as a promising plasma-facing material (PFM) in IETR because of high strength, high-temperature stability, low sputtering erosion, low tritium retention, etc. However, tungsten has some disadvantages, such as high ductile-brittle transition temperature, low temperature brittleness, and radiation embrittlement. For the severe environment of PFM, various techniques have been adopted to improve W-based materials, among which the potassium doping is an effective bubble strengthening method, it can bring in nano-sized K bubbles, and enhance the toughness and strength, thermal shock performance, irradiation resistance of the materials. The K bubbles, which can pin grain boundaries (GBs) and dislocations, are the most characteristic defects in W-K alloy and have been widely reported. However, little attention is paid to other defects such as vacancies, GBs and dislocations. In fact, high-density dislocations exist in W-K alloy and vacancies play a considerable role in forming the K bubbles. Thus, positron annihilation technique (including the positron annihilation lifetime spectrum and slow positron beam Doppler broadening spectrum), which is a useful technique for detecting defects in solids, can be used to study these defects in W-K alloy samples. The positron lifetime of potassium bulk is about 376 ps and the positron lifetime of tungsten bulk is about 110 ps. But by simulating positron lifetime of defects in tungsten, it is found that potassium atoms in tungsten lattice do not exhibit the characteristic positron lifetime. Therefore, potassium is not considered in analyzing positron annihilation lifetime spectra of W-K alloy samples with different potassium content (46, 82, 122, 144 ppm). Three-state capture model is established in this paper, the dislocation density and vacancy cluster concentration of these samples are obtained. From the results, the dislocation densities in all samples are very high, but vacancy cluster concentrations are relatively low, and the vacancy cluster concentration in the sample with 82 ppm potassium content is the lowest in all samples. The behavior of potassium atoms in the sintering process is also discussed. Then the slow positron beam Doppler broadening spectra of W-K alloy samples and pure tungsten samples are measured and the obtained data are fitted by VEPFIT. It is noted that the defects in W-K alloy samples are much more than those in pure tungsten sample, and are distributed homogeneously with depth. The positron diffusion length information simultaneously obtained is compared with these values computed by dislocation density and vacancy cluster concentration, confirming the positrons trapped by potassium bubbles and grain boundaries are existent.

Vacancy defects and their influence on mechanical properties in as-dealloyed and Fe+ ion irradiated nanoporous Au

Continuous depth sensing nano-indentation hardness measurements coupled with low-energy positron beam Doppler broadening line shape measurements were performed to investigate the influence of vacancy defects on mechanical properties in as-dealloyed and Fe+ ion irradiated nanoporous Au. A linear relationship has been obtained from the actual data between the nano-indentation hardness and square root of the concentration of defects. It suggests that the vacancy-type defects that evolved in the ligaments play a significant role in the modification of mechanical properties in both as-dealloyed and Fe+ ion irradiated nanoporous Au. The results have been also compared with those obtained on bulk Au.

Effect of low temperature vulcanization time on the structure and optical properties of ZnS thin films

The ZnS films were prepared by vulcanizing the sputtered Zn films at 440°C for different vulcanization time. The crystal structure, surface morphology, microscopic defects and optical properties of the samples were measured by XRD, SEM, slow positron beam Doppler broadening spectroscopy and UV–visible spectrophotometer, respectively. The results show that the vulcanization time has significant effect on the crystallinity and optical properties of the samples. As the vulcanization time increases, the crystallinity of the samples will be improved. But excessive vulcanization time makes the sample crystallinity weak. The transmittance of all samples in the visible range is above 60%. The crystallinity of the sample after 6 h vulcanization is the best, and the band gap value is also up to 3.49 eV. The samples form hexagonal ZnS, whose surface free energy is the lower. The effect of grain specific surface area and S vapor atmosphere on grain growth is also discussed. The slow-positron beam Doppler broadening test results intuitively reveal the changes of microscopic defects in the ZnS film, which are consistent with the results of XRD and SEM analysis.

Study of vacancy defects and their thermal stability in MeV Fe ion irradiated RAFM steel using positron beam Doppler broadening spectroscopy

ABSTRACTIndian reduced activation ferritic-martensitic steel was irradiated with 1.1 MeV Fe ions to various doses from 1 to100 dpa at room temperature. The depth profiling of irradiation-induced vacancy-type defects and the defect-recovery under post-irradiation annealing was studied using variable low-energy positron beam Doppler broadening spectroscopy. The influence of irradiation-induced defects on the microstructural properties was studied by glancing incidence x-ray diffraction (GIXRD) and nanoindentation technique. Positron annihilation study showed the signatures of reduced vacancy concentration at the peak damage region due to injected interstitial effect from 30 to 100 dpa and the widening of vacancy-interstitial recombination-rich region towards the end of ion range with the increase in dose. The GIXRD results were analysed by modified Williamson–Hall plot method, and the variation of coherent domain size and micro-strain with irradiation dose was studied. Irradiation-induced hardening was observed in the nanoindentation study. The features observed in the GIXRD and nanoindentation study are correlated with the depth-resolved defect distribution observed in the positron annihilation study.

Thermal evolution of irradiation defects in ferritic/martensitic steel during isochronal annealing

The aim of investigating the annealing effect induced by helium/hydrogen ion irradiation on the micro-defects of ferritic/martensitic (FM) steel is to gain a basic understanding on the development of fusion reactor components. In this study, 250 keV He2+ and 130 keV H+ were used for irradiation. Then, micro-defects were annealed isochronally between 423 K and 673 K, and were monitored by the slow positron beam Doppler broadening technique. The results revealed that the He ions implanted into the steels combined with vacancies to form HenVm clusters, and were likely to grow and form overpressured HenVm clusters by thermal activation. The S-W line of the irradiated specimen deviated the line segment, which indicates the formation of overpressured HenVm clusters in the He-irradiated steel. Moreover, the ΔS-E curves indicate residual vacancies that were induced by He-irradiation at 723 K and later migrated toward the bulk of the sample at 423 K. Subsequently, the vacancies tended to migrate to the sample surface as the annealing temperature increased. The S parameter of the He+H-irradiated sample was much larger than that of all other samples, owing to the synergy effect of H and He. After annealing at 423 K, the H-V clusters decomposed and the hydrogen atoms in the He+H-irradiated sample escaped from the surface leaving a large number of vacancies.

Evolution of defects in nanoporous gold with annealing temperature using low-energy positron beam experiments

Defects in nanoporous gold (np-Au) samples, prepared by selective dissolution of silver from a binary Ag75Au25 alloy, with annealing temperature have been investigated using the positron beam Doppler line shape S-parameter. It is shown that there is an initial formation of vacancy clusters arising due to the aggregation of thermally migrating vacancies up to 473 K, followed by the breakup of these clusters at higher temperatures, coincident with the rapid growth of ligaments. In addition to the defect-specific S-parameter, the low-energy positron beam experiments have also been used to evaluate the variation of the effective positron diffusion length in np-Au with annealing temperature. The variation of effective positron diffusion length with the ligament diameter, d, scales as which points to the diffusion of positrons along the ligament length, that is also substantiated through measurement of the positronium fraction.

Positron annihilation Doppler broadening spectroscopy study on Fe-ion irradiated NHS steel

In order to study the evolution of irradiation-induced vacancy-type defects at different irradiation fluences and temperatures, a new type of ferritic/martensitic (F/M) steel named NHS (Novel High Silicon) was irradiated by 3.25MeV Fe-ion at room temperature and 723K to fluences of 4.3×1015 and 1.7×1016ions/cm2. After irradiation, vacancy-type defects were investigated with variable-energy positron beam Doppler broadening spectra. Energetic Fe-ions produced a large number of vacancy-type defects in the NHS steel, but one single main type of vacancy-type defect was observed in both unirradiated and irradiated samples. The concentration of vacancy-type defects decreased with increasing temperature. With the increase of irradiation fluence, the concentration of vacancy-type defects increased in the sample irradiated at RT, whereas for the sample irradiated at 723K, it decreased. The enhanced recombination between vacancies and excess interstitial Fe atoms from deeper layers, and high diffusion rate of self-interstitial atoms further improved by diffusion via grain boundary and dislocations at high temperature, are thought to be the main reasons for the reversed trend of vacancy-type defects between the samples irradiated at RT and 723K.

Positron beam Doppler broadening spectra and nano-hardness study on helium and hydrogen irradiated RAFM steel

The reduced activation martensitic steel was irradiated with 250 keV He2+ to 3000 appm and 130 keV H+ to 15,000 appm alone and in sequence at 450 degrees C. The S-W curve obtained by the positron beam Doppler broadening spectra reveals that (He, H)-V complexes were induced by irradiation. The S parameters show that the H irradiation can decrease defects by forming H-V complexes. The He irradiation can form He-V complexes and the sequentially implanted H can accelerate the formation of (He, H)-V complexes. Hardening induced by He+H sequential irradiation is greater than that by He or H ion irradiation alone, suggesting synergistic effects of He and H on defects and hardening. (C) 2014 Elsevier Ltd. All rights reserved.

Dose effects in He implanted Eurofer97 steel

Reduced Activation Ferritic/Martensitic steels are being extensively studied because of their foreseen application in fusion reactors. To reproduce neutron irradiation conditions, Eurofer97 samples were implanted at room temperature with helium ions at energies of 500 keV and 2 MeV and doses of 1×1015-1017 He /cm2. The implantation induced defects were characterized by positron beam Doppler Broadening (DB). The samples were annealed in the range 300 – 1500 K, in 100 K steps. As the temperature increases, the annealing of vacancies and vacancy clusters is noticed and followed by the coalescence of HenVm clusters. At temperatures around 1200 K HeV pairs dissociate and bubbles are formed. Above 1300 K the helium release from bubbles is observed. The S-W graphs reveals that the samples have similar positron traps up to 1200 K. At 1200 K helium bubbles are noticed and the S, W pair shows a clearly distinct behaviour from the S, W values of vacancy type defects. As the temperature increases and helium is release, the S, W pairs shift towards the S, W's of vacancies.

Positron and thermal desorption studies on He ion implanted nuclear graphite

The positron beam Doppler Broadening (DB) and Thermal Helium Desorption Spectroscopy (THDS) techniques are applied to study the behavior of radiation induced point defects in IG-110 nuclear graphite. The defects are introduced by irradiation at room temperature with 200keV He+ at doses ranging from 1015 to 1017 He/cm2. In the thermal desorption spectroscopy, the release of He+ is observed between 500K and 800K. With increasing He+ implantation dose, the fraction of He+ desorbed decreases from 27% to 3%. In the DB-curves showing the S parameter values versus positron implantation depth, the derived vacancy type defect distribution are in accordance with those obtained by SRIM calculations. Subsequent annealing of the implanted samples in steps of 100K for 5 minutes up to 1200K shows a distinct decrease of the S parameter value to its reference value between 500K and 700K. This temperature interval corresponds to the literature values for single vacancy migration energy of 1-2eV.

Vacancy type defect formation in irradiated α-iron investigated by positron beam Doppler broadening technique

Vacancy type defects formations have been investigated in virgin and irradiated a-iron samples using slow positron beam Doppler broadening technique. Mono-vacancies and vacancy clusters are observed in 1.5 MeV 4He ions irradiated Fe samples at varying fluences from 1×1013 to 1×1017 cm−2. In the 1.2 MeV Yttrium ions implantation at low fluence 1×1014 cm−2 vacancy clusters with higher concentration and larger size are formed. In this sample, vacancy defects are detected deeper than predicted by SRIM calculation due to channelling.

Slow positron beam and nanoindentation study of irradiation-related defects in reactor vessel steels

In order to understand the nature of the hardening after radiation in reactor vessel steels, China A508-3 steels were implanted by proton with an energy of 240keV up to 2.5×1016, 5.5×1016, 1.1×1017, and 2.5×1017ionscm−2, respectively. Vacancy type defects were detected by energy-variable positron beam Doppler broadening technique and then nanoindentation measurements were performed to investigate proton-induced hardening effects. The results showed that S-parameter increased as a function of positron incident energy after irradiation, and the increasing rate of the S-parameter near the surface was larger than that in the bulk due to radiation damage. The size of vacancy type defects increased with dose. Irradiation induced hardening was shown that the average hardness increased with dose. Moreover a direct correlation between positron annihilation parameter and hardness was found based on Kasada method.

Evolution of defects in silicon carbide implanted with helium ions

Effects of accumulation of radiation damage in silicon carbide are important concerns for the use of silicon carbide in advanced nuclear energy systems. In the present work lattice damage in silicon carbide crystal (4H type) implanted with 100keV 4He+ ions was investigated with Rutherford backscattering spectrometry in channeling geometry (RBS/c) and positron beam Doppler broadening spectrometry (PBDB). Helium implantation was performed at the specimen temperature of 510K to avoid amorphization of the SiC crystal. Fluences of helium ions were selected to be in the range from 1×1016 to 3×1016ionscm−2, around the dose threshold for the formation of observable helium bubbles under transmission electron microscopes (TEM).The RBS/c measurements show distinctly different annealing behavior of displaced Si atoms at doses below or above the threshold for helium bubble formation. The RBS/c yield in the peak damage region of the specimen implanted to 3×1016He-ionscm−2 shows an increase on the subsequently thermal annealing above 873K, which is readily ascribed to the extra displacement of Si atoms due to helium bubble growth. The RBS/c yield in the specimen implanted to a lower ion fluence of 1.5×1016He-ionscm−2 decreases monotonously on annealing from ambient temperatures up to 1273K. The PBDB measurements supply evidence of clustering of vacancies at temperatures from 510 to 1173K, and dissociation of vacancy clusters above 1273K. The similarity of annealing behavior in PBDB profiles for helium implantation to 1×1016 and 3×1016ionscm−2 is ascribed to the saturation of trapping of positrons in vacancy type defects in the damaged layers in the specimens helium-implanted to the two dose levels.