Positron Trapping Research Articles

Positron Annihilation Lifetime Spectroscopy Insight on Free Volume Conversion of Nanostructured MgAl2O4 Ceramics.

Herein we demonstrate the specifics of using the positron annihilation lifetime spectroscopy (PALS) method for the study of free volume changes in functional ceramic materials. Choosing technological modification of nanostructured MgAl2O4 spinel as an example, we show that for ceramics with well-developed porosity positron annihilation is revealed through two channels: positron trapping channel and ortho-positronium decay. Positron trapping in free-volume defects is described by the second component of spectra and ortho-positronium decay process by single or multiple components, depending on how well porosity is developed and on the experimental configuration. When using proposed positron annihilation lifetime spectroscopy approaches, three components are the most suitable fit in the case of MgAl2O4 ceramics. In the analysis of the second component, it is shown that technological modification (increasing sintering temperature) leads to volume shrinking and decreases the number of defect-related voids. This process is also accompanied by the decrease of the size of nanopores (described by the third component), while the overall number of nanopores is not affected. The approach to the analysis of positron annihilation lifetime spectra presented here can be applied to a wide range of functional nanomaterials with pronounced porosity.

Non-neutral plasma manipulation techniques in development of a high-capacity positron trap.

Preliminary experiments have been performed toward the development of a multi-cell Penning-Malmberg trap for the storage of large numbers of positrons (≥1010 e+). We introduce the master-cell test trap and the diagnostic tools for first experiments with electrons. The usage of a phosphor screen to measure the z-integrated plasma distribution and the number of confined particles is demonstrated, as well as the trap alignment to the magnetic field (B = 3.1 T) using the m = 1 diocotron mode. The plasma parameters and expansion are described along with the autoresonant excitation of the diocotron mode using rotating dipole fields and frequency chirped sinusoidal drive signals. We analyze the reproducibility of the excitation and use these findings to settle on the path for the next generation multi-cell test device.

Identification of lead vacancy defects in lead halide perovskites

Perovskite photovoltaics advance rapidly, but questions remain regarding point defects: while experiments have detected the presence of electrically active defects no experimentally confirmed microscopic identifications have been reported. Here we identify lead monovacancy (VPb) defects in MAPbI3 (MA = CH3NH3+) using positron annihilation lifetime spectroscopy with the aid of density functional theory. Experiments on thin film and single crystal samples all exhibited dominant positron trapping to lead vacancy defects, and a minimum defect density of ~3 × 1015 cm−3 was determined. There was also evidence of trapping at the vacancy complex ({{{{{rm{V}}}}}}_{{{{{rm{Pb}}}}}}{{{{{rm{V}}}}}}_{{{{{rm{I}}}}}})^{-} in a minority of samples, but no trapping to MA-ion vacancies was observed. Our experimental results support the predictions of other first-principles studies that deep level, hole trapping, {{{{{{rm{V}}}}}}}_{{{{{{rm{Pb}}}}}}}^{2-}, point defects are one of the most stable defects in MAPbI3. This direct detection and identification of a deep level native defect in a halide perovskite, at technologically relevant concentrations, will enable further investigation of defect driven mechanisms.

Phase Transformations upon Ageing in Ti15Mo Alloy Subjected to Two Different Deformation Methods

Ti15Mo alloy was subjected to two techniques of intensive plastic deformation, namely high pressure torsion and rotary swaging at room temperature. The imposed strain resulted in the formation of an ultrafine-grained structure in both deformed conditions. Detailed inspection of the microstructure revealed the presence of grains with a size of around 100 nm in both conditions. The microstructure after rotary swaging also contained elongated grains with a length up to 1 µm. Isothermal ageing at 400 °C and 500 °C up to 16 h was applied to both conditions to investigate the kinetics of precipitation of the α phase and the recovery of lattice defects. Positron annihilation spectroscopy indicated that the recovery of lattice defects in the β matrix had already occurred at 400 °C and, in terms of positron trapping, was partly compensated by the precipitation of incoherent α particles. At 500 °C the recovery was fully offset by the formation of incoherent α/β interfaces. Contrary to common coarse-grained material, in which the α phase precipitates in the form of lamellae, precipitation of small and equiaxed α particles occurred in the deformed condition. A refined two-phase equiaxed microstructure with α particles and β grain sizes below 1 μm is achievable by simple rotary swaging followed by ageing.

Positron lifetime spectroscopy of defect structures in Cd1–x Zn x Te mixed crystals grown by vertical Bridgman–Stockbarger method

Positron annihilation lifetime spectroscopy was used to examine grown-in defects in Cd1–x Zn x Te mixed crystals as a function of Zn content (x = 0, 0.07, 0.11, 0.49, 0.9, 0.95, 1) and measuring temperature. All samples were prepared using the high-pressure modified vertical Bridgman–Stockbarger method. The crystal structure and material phase were characterized by X-ray diffraction. The positron lifetime spectra reveal the presence of both open volumes and shallow traps regardless of the sample composition. In particular, both average and bulk lifetimes are found to be much higher in ternary alloys (CdZnTe) than those in binary systems (CdTe and ZnTe). This originates from distinct differences in average electron densities and the nature of open-volume defects between binary and ternary samples. Competition in positron trapping with increasing Zn content is observed between defects characteristic for both structural systems. Moreover, a clear correlation is shown between defects and the lattice thermal conductivity of studied samples. The applicability of the positron trapping model to CdTe-based materials is discussed.

Microstructure of bismuth centers in silicon before and after irradiation with 15 MeV protons

A decrease of two-gamma annihilation rate of a positron in a strong spin–orbit field of the annihilation site of bismuth impurity center 209Bi (J = 9/2) in silicon with natural isotope composition was revealed (J is the nuclear spin). This decrease was observed along with increasing occupancy of Bi donor states (binding energy E{Bi} ≈ 69 meV). Atoms of 29Si (J = 1/2) isotope are involved in spin interactions of positron with Bi impurity centers. The growth of occupancy of Bi donor states inhibits two-gamma annihilation rate. The estimated cross-section of positron trapping by the Bi impurity center is σ + ≈ (1.23–1.5) × 10−13 cm2. Together with this surprisingly large value, the integral rate of two-gamma annihilation in a hypothetical polyelectron system of the Bi impurity center is by a factor of just Δ ∼ 2.18 higher compared to the magnitude ≈2.09 × 109 s−1 known for elemental isolated polyelectron, (e−e+e−). Possible formation of the positron-containing exciton-like states, (e+)D 0 X (D = Bi, P) is also discussed. Irradiation of material with 15 MeV protons results in decreasing the factor Δ by ∼11% due to forming the radiation complex in which Bi atom is in an open volume ambient it. Such complex is suggested to have D 3d symmetry and be the deep donor. Low-temperature measurements of both the positron annihilation rate and Hall effect have been applied for studying the isochronal annealing of these point radiation defects which were found to be thermally stable up to ∼370 °C; they can be annealed at ∼430 °C – 470 °C. According to available data of ab initio cluster calculations, the complex of Bi atom with a simulated vacancy has D 3d symmetry with the energy gain ∼0.92 eV, thus indicating qualitative agreement between experimental and theoretical data.

Wave propagation with degenerate electron–positron in magnetically quantised ion beam plasma

In an ion beam plasma system, the effect of magnetically quantised degenerate trapped electron and positron, on small-amplitude ion-acoustic solitary waves are studied with the help of Korteweg–de Vries (KdV) equation. Here the magnetised positive ions and beam ions are considered as non-degenerate. The effect of magnetic quantisation, degenerate temperature, normalised positron concentration, normalised ion beam concentration along with other relevant physical plasma parameters of the origin of astrophysical plasma environment, on solitary wave propagation is studied especially in the nonlinear regime. On the other hand, in the linear regime, the dependency of frequency and wavenumber on the aforementioned plasma parameters are discussed. It is found that both compressive and rarefactive type solitary waves can exist in such a plasma environment. Three different modes of propagation, the fast beam and slow beam modes and the inherent ion-acoustic mode coexist in such plasma systems and the role of different degenerate plasma parameters on these wave modes are also discussed in detail. The normalised positron concentration and velocity, as well as the normalised ion beam concentration along with different degenerate plasma parameters, has an astounding control on the amount of compression and rarefaction, a typical plasma wave mode is supposed to undergo. Within the chosen degenerate and non-degenerate plasma parameters, for a compressive (rarefactive) fast beam mode, the slow mode appears as rarefactive (compressive). For some typical combinations of degenerate parameters, the fast mode can propagate with a hypersonic phase velocity which shows distinctive characteristics with positron density.

Combined study of internal nanovoids in Cu0.1Ni0.1Co1.6Mn1.2O4-based thick-film layers formed near grain boundaries

Microstructural features of Cu0.1Ni0.8Co0.2Mn1.9O4-based thick films screen-printed on substrate with formed Ag–Pd contacts were investigated using X-ray diffraction, scanning electron microscopy methods. It is established that diffusion processes of elements from contacts area occurs into internal structure of thick films located near grain boundaries. Formed internal nanovoids in thick-film structures were studied by positron annihilation lifetime spectroscopy. The observed positron trapping sites can be defined within two-state positron trapping model at fitting on two components. It is established that positron trapping processes in thick films formed in three and four layers and bulk Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics are similar testifies the same structural type and successful transformation of bulk ceramics into thick films. In some cases, the size of free-volume nanovoids in thick films can rise due to redistribute space of their internal structure at addition glasses and other compounds. Positrons are trapped more strongly in bulk ceramics due to more ramified their grain/pores structure. Studied thick films are characterized by high temperature sensitivity and their properties can be stabilized after thermally induced treatment.

A positronium-based systematic study of the physico-chemical properties of zeolite pores

A systematic study of the influence of surface acidity of pores, water content, crystal size and structural type of zeolites on positronium behavior was carried out over a wide range of Si/Al ratios using the positron annihilation lifetime spectroscopy technique. A clear decrease in the ortho-positronium pick-off annihilation lifetime, as well as the total ortho-positronium formation fraction, was detected at lower Si/Al ratios due to the positronium oxidizing effect and positron trapping of the Brønsted acid sites. Upon water adsorption, the suppression of the positronium diffusion by the water molecules adsorbed in the tubular pores and their trapping action of positrons and electrons was found to significantly reduce both the ortho-positronium lifetime and intensity. These results clarified the relation between positronium behavior and the chemical reactivity of the zeolite pores.

Study of Interaction Mechanism between Positrons and Ag Clusters in Dilute Al-Ag Alloys at Low Temperature.

The microstructural evolution of dilute Al–Ag alloys in its early aging stage and at low temperatures ranging from 15 K to 300 K was studied by the combined use of Positron annihilation lifetime spectroscopy (PALS), high resolution transmission electron microscopy (HRTEM), and positron annihilation Coincidence Doppler broadening (CDB) techniques. It is shown that at low temperatures below 200 K, an Ag–vacancy complex is formed in the quenched alloy, and above 200 K, it decomposes into Ag clusters and monovacancies. Experimental and calculation results indicate that Ag clusters in Al–Ag alloys can act as shallow trapping sites, and the positron trapping rate is considerably enhanced by a decreasing measurement temperature.

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.

Split Ga vacancies in n-type and semi-insulating β-Ga2O3 single crystals

We report a positron annihilation study using state-of-the-art experimental and theoretical methods in n-type and semi-insulating β-Ga2O3. We utilize the recently discovered unusually strong Doppler broadening signal anisotropy of β-Ga2O3 in orientation-dependent Doppler broadening measurements, complemented by temperature-dependent positron lifetime experiments and first principles calculations of positron–electron annihilation signals. We find that split Ga vacancies dominate the positron trapping in β-Ga2O3 single crystals irrespective of the type of dopant or conductivity, implying concentrations of at least 1×1018 cm−3.

Late-time Observations of Calcium-rich Transient SN 2019ehk Reveal a Pure Radioactive Decay Power Source

Abstract We present multiband Hubble Space Telescope imaging of the calcium-rich supernova (SN) SN 2019ehk at 276-389 days after explosion. These observations represent the latest B-band to near-IR photometric measurements of a calcium-rich transient to date and allow for the first opportunity to analyze the late-time bolometric evolution of an object in this observational SN class. We find that the late-time bolometric light curve of SN 2019ehk can be described predominantly through the radioactive decay of 56Co for which we derive a mass of M(56Co) = (2.8 ± 0.1) × 10−2 M ⊙. Furthermore, the rate of decline in bolometric luminosity requires the leakage of γ-rays on timescale t γ = 53.9 ± 1.30 days, but we find no statistical evidence for incomplete positron trapping in the SN ejecta. While our observations cannot constrain the exact masses of other radioactive isotopes synthesized in SN 2019ehk, we estimate a mass ratio limit of M(57Co)/M(56Co) ≤ 0.030. This limit is consistent with the explosive nucleosynthesis produced in the merger of low-mass white dwarfs, which is one of the favored progenitor scenarios in early-time studies of SN 2019ehk.

Mapping Defects During Phase Transformation in High Cr Content NiCr Solid-Solution Through Positron Trapping

Mapping Defects During Phase Transformation in High Cr Content NiCr Solid-Solution Through Positron Trapping

Multiphoton light emission in barium stannate perovskites driven by oxygen vacancies, Eu3+ and La3+: accessing the role of defects and local structures.

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO3 exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO3 in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu3+ ions stabilize at both Ba2+ and Sn4+ sites consistent with the highly negative formation energy of around -6.26 eV. Eu3+ doping leads to an intense 5D0→7F1 orange emission and a feeble 5D0→7F2 red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of EuBa and EuSn sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn2+ in the PL of BaSnO3 or Eu2+ in the PL of BaSnO3:Eu3+. Interestingly, when co-doped, Eu3+ stabilizes at Sn4+ sites whereas La3+ stabilizes at Ba2+ sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La3+ ions lead to lowering of symmetry around Eu3+ due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO3 in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO3 and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO3 is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.

Structural evolution of zinc selenite nanocomposite system with molybdenum trioxide as modifier studied by positron annihilation and allied experimental methods

The successful synthesis of the metal oxide nanocomposite xMoO3–(1-x)(0.5SeO2–0.5ZnO) for x = 0.0, 0.1, 0.2, …, 0.9 by mixing and melt-quenching of the stoichiometric ratios of the metal oxides is reported in this work. X-ray diffraction patterns and transmission electron microscopy images confirmed the formation of the nanocomposites and their evolution to the amorphous glassy phase and newer nanocrystallites as x increased from 0–0.3, 0.3–0.7 and 0.7–0.9 respectively. The findings from Brunauer–Emmett–Teller isotherms also supported these observations, as one observed a significant enhancement in the pore volume and total pore surface area when the nanocomposites evolved from x = 0.3 to 0.9. The optical bandgap energy was another physical quantity that showed a steady decrease with increase in x and is attributed to the generation of additional electronic defect levels towards the conduction band within the bandgap. An important step that was taken up in this study was to characterize the defects in the composites by positron annihilation studies. Positron lifetimes and intensities pointed out the presence of vacancy-type defects within the nanocomposites, porous defects within the glassy amorphous phase and interfacial defects between the latter and the newer nanocrystallites formed. Coincidence Doppler broadened spectra revealed the decrease of positron trapping in cationic vacancies as x increased, showing that the annihilations within vacancy type defects decreased and in interfacial defects and nanocrystallite surfaces increased, confirming the formation of the newer nanocrystallites and their uniform dispersion in the glassy amorphous matrices.

Investigating defect evolution during thermal treatment in Ni–Cr alloy using positron annihilation spectroscopy

Defects evolved during thermal treatment in high Cr content Ni–Cr alloy have been studied using Positron lifetime and coincidence Doppler broadening spectroscopy complemented with microscopic technique. The single-phase gamma-quenched specimen obtained by solutionization at 1200 °C has been thermally aged at 650 °C and 800 °C. Thermal treatment resulted in the phase transformation of the matrix through precipitation of Cr. It is seen that solutionization treatment followed by quenching resulted in the stabilization of thermal vacancies generated at 1200 °C which effectively trap the positrons. The results infer that these vacancies may form vacancy-Cr complexes which act as nucleation site for Cr aggregation. Positron lifetime and momentum distribution of annihilation electrons indicated that in the aged alloys, positrons are trapped in vacancy-like defects (vacancy clusters/vacancy-solute complex) and the interface between Cr precipitate and matrix, the latter becoming more predominant with subsequent ageing. At the highest ageing studied, vacancy-like defects are evolved within the Cr precipitates. The results indicated that positron trapping sites (vacancies and precipitate interface) are not specific to either Ni or Cr atoms which surround the trapping site in nearly equal proportion. Therefore, unambiguous identification of the defect type at each stage of thermal treatment becomes difficult. Considering positron trapping at the surface of precipitates, we have calculated positron specific trapping coefficient to be ~ 1.33 × 10−6 cm3 s−1, which is of the order estimated for large voids and estimated the number density of precipitates.

Electron irradiation induced molecular changes in PMMA: A positron spectroscopy study

To examine the positron annihilation parameters with regard to microstructural changes, the positron annihilation lifetime spectroscopy and measurements of the Doppler broadening of annihilation radiation (DBAR) of electron‐irradiated poly(methyl methacrylate) were carried out at room temperature. Trapping rate equations were applied in terms of the positron bulk lifetime as well as the positron lifetime fitting results to investigate the molecular changes due to electron irradiation effects. It was found that the mean positron lifetime shows the same trend as the o‐Ps intensity. In disordered regions, the positronium trapping rate suggests that trapping of o‐Ps increases with increasing electron irradiation fluence until electron fluence reached 5.9 × 1013 electrons/cm2, after which it begins to decrease. The line shape S‐parameter was found to have similar behavior as the o‐Ps intensity, and suggests that poly(methyl methacrylate) undergoes chain scission, followed by cross‐linking, and eventually degradation, due to the electron radiation effects. The results of positron annihilation spectroscopy were verified by X‐ray results, mechanical, and electrical measurements.

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.

Investigation of buffer gas trapping of positrons

A study of positron capture in a two-(pressure) stage buffer gas accumulation apparatus is presented for a variety of species, including some molecules which are known to be either efficient for positron trapping, or are frequently used to cool the particles when held in these devices. Absolute accumulation efficiencies are reported for all species. A detailed optimisation procedure, which has identified the main processes responsible for positron capture and loss in the trap, has been deployed to explore accumulation efficiency as the gas pressure and the electrostatic well depth in the trap are systematically varied. Accumulation exploiting energy loss via molecular vibrational transitions has been observed for the first time for a number of gases, though at much lower efficiency than achieved using electronic excitation processes.