Positron Lifetime Research Articles

A single shot gamma-induced positron spectroscopy based on laser wakefield accelerator

Single shot gamma-induced positron annihilation spectroscopy (GiPS) based on a laser wakefield accelerator is proposed. In this spectroscopy, a large number of gamma rays generated by laser-wakefield accelerated electrons are injected into a sample in a very short time (<2 ps), and the positron lifetime is detected by measuring the time profile of the annihilation radiation (511 keV gamma ray) from the sample. The processes including gamma-ray production, shielding, annihilation generation, and annihilation detection were simulated by using the Monte Carlo method. The results show that up to 1010 gamma rays with a duration of 1.4 ps can be produced per shot with a 45 TW laser system. A Cherenkov radiator PbF2 was used to detect the annihilation radiation, and its optimal thickness of 15 mm was obtained. A whole process simulation with optimized parameters was conducted. A low statistical fluctuation lifetime spectrum can be produced, which confirms the feasibility of single shot GiPS with a typical 45 TW laser system.

Fine tuning of pore architecture and morphology of stiffened Zeolitic Imidazolate Frameworks synthesized using fast current driven method and mixed ligand strategy

Zeolitic Imidazolate Frameworks (ZIFs) show great promise in molecular separation as well as recognition owing to their crystalline morphology consisting of a well designed pore architecture. However, the sieving performance of ZIFs remains limited due to their fixed pore sizes and framework flexibility. In the present study, fast current driven synthesis (FCDS) has been used to prepare the stiffened phase of mixed ligands (2-methylimidazole and benzimidazole) based ZIFs having tunable pore sizes and morphologies. X-ray diffraction has been used to determine the crystal structure of the frameworks, which were observed to be crystalline through a wide range of ligand ratio (benzimidazole fraction: ∼0.206–0.972). The random distribution of both the ligands in the frameworks has been established through complementary methods viz. Raman spectroscopy, Fourier transform infrared and scanning electron microscopy. Morphology of the frameworks was observed to vary from micrometer size individual crystals to spherical aggregates of inter-grown nanocrystals to individual nanocrystals. Pore sizes determined using positronium lifetime measurements were observed to be consistent with the literature for pure phase of ZIF-7 and stiffened ZIF-8. The pore sizes were observed to vary with the benzimidazole ligand fraction confirming that mixed ligand strategy can be used to efficiently tune the pore sizes of the frameworks. Positronium diffusion investigation confirms that pore interconnectivity is inferior as compared to stiffened ZIF-8 but superior than ZIF-7 in these mixed ligand frameworks. • Twelve stiffened ZIF-7 x -8 ( x = 0–1) have been synthesized using Fast Current Driven Synthesis method. • All the synthesized frameworks are fully crystalline and porous. • Morphology of framework can be tuned with bIm ligand fraction. • Mixed ligand strategy is highly suitable for tuning the pore architecture of ZIFs. • Pore interconnectivity is also altered in these frameworks as confirmed by positronium diffusion measurements.

Defect Characterization Using Positron Annihilation Spectroscopy on Laser-Ablated Surfaces

In recent years, short, pulsed laser ablation has been gaining popularity for machining small-scale test geometries from bulk samples and for efficient serial sectioning. These laser-based techniques are being added to the toolbox in material science, which makes it necessary to understand the changes in the material that occur from the laser–material interaction. Positron annihilation spectroscopy is a unique, nondestructive technique to investigate small defects in materials difficult to investigate by other tools. In this work, Doppler broadening and positron lifetime annihilation spectroscopy are utilized to help quantify the damage in materials treated with short, pulsed lasers. Using a femtosecond laser on single crystal silicon, this manuscript shows that clusters of vacancy-like defects and small voids increase systematically with laser power. The damage induced by the laser can also reach to micrometer depths.

Gradient Microstructure Induced by Surface Mechanical Attrition Treatment in Grade 2 Titanium Studied Using Positron Annihilation Spectroscopy and Complementary Methods.

Microstructural changes in grade 2 titanium generated by surface mechanical attrition treatment (SMAT) were studied using positron annihilation lifetime spectroscopy and complementary methods. A significant increase in the mean positron lifetime indicated many lattice defects introduced by SMAT. Two positron lifetime components were resolved in the positron lifetime spectra measured. The longer lifetime revealed the presence of vacancy clusters containing about 3 or 4 vacancies, while the shorter one was attributed to the annihilation of positrons trapped at dislocations. The changes of the positron lifetime indicated a decreasing dislocation density and the presence of a deeper layer with a higher concentration of vacancy clusters at the distance from the treated surface for which the microhardness approached the value for the strain-free matrix. Electrochemical impedance spectroscopy showed the positive effect of SMAT on the corrosion resistance of the titanium studied in a saline environment also after removal of the original oxide layer that was formed during the SMAT.

Penetration of deuterium into neutron-irradiated tungsten under plasma exposure

Hydrogen isotope trapping at lattice defects in neutron-irradiated tungsten (W), a leading candidate as plasma facing material, is an important problem determining tritium (T) inventory in a vacuum vessel of a future fusion reactor. In this study, W samples were irradiated with neutrons in the Belgium Reactor 2 at 563 K to 0.06 or 0.016 displacement per atom (dpa). After characterizing defects by positron lifetime measurements, deuterium (D) penetration under exposure to D plasma was examined at 563–773 K. Positron lifetime showed the presence of dislocations, monovacancies and relatively large vacancy clusters. These defects trapped D atoms with different values of binding energy. Dependence of D retention on plasma exposure temperature and damage level indicated that the concentrations of weak traps with smaller binding energy increased more significantly with damage level than those of strong traps.

Positronium life-time as a new approach for cardiac masses imaging

Abstract Background/Introduction Positron Emission Tomography (PET), also a standard in cardiovascular imaging, enables semi-quantitative myocardial perfusion evaluation and assessment of atherosclerosis. PET is based on positron-electro annihilation inside a patient's body and in about 40% events it proceeds via formation of positronium. Positronium is an exotic atom created from an electron and a positron in a patient during standard PET imaging. Positronium is not stable and its life-time depends on molecular environment. Recently, it was shown that with the new generation of high sensitivity long-axial field of view PET systems is possible to perform simultaneous imaging of the metabolism rate of administered pharmaceutical and imaging of positronium life-time. Positronium life-time informs about tissue nanostructure and allows distinguishing pathology. Purpose Purpose of the presented study is the evaluation of positronium life-time as a biomarker for cardiac masses, namely cardiac myxoma (CM) and normal tissues, obtained from symptomatic patients. Methods CM and adipose tissue samples were obtained during a surgery from six symptomatic patients with ECHO confirmed cardiac masses in a left atrium. Immediately after sampling, sample were analyzed with the dedicated positronium-life time measurement system. Samples were irradiated with positrons emitted from the 22Na radionuclide. The system enabled to measure the time-difference spectrum between the formation and annihilation of positronium atom within the studied samples. The life-time spectrum was used for the extraction of a mean positronium life-time. The sample underwent histopathological and micro-computer-tomography (micro-CT) examinations, which confirmed a diagnosis and revealed microstructure of a cardiac mass. Results It was established that positronium life-time in CM tissues is significantly (at the level of 10 standard deviations) smaller than in adipose tissue. At the same time, the determined positronium life-time in CM samples from different patients was the same within two standard deviations. Micro-CT imaging showed that CM samples had different calcification spots, but did not revealed existing microvasculature. Conclusion Obtained results confirm that positronium life-time differentiates between cardiac pathologies (CM) and normal tissues. And may be used as a biomarker in cardiac masses assessment available during PET examination. Funding Acknowledgement Type of funding sources: Foundation. Main funding source(s): Foundation for Polish Science through the TEAM POIR.04.04.00-00-4204/17 Programme, the National Science Centre.

Positron Annihilation Lifetime Spectroscopic Analysis of Plastic Deformation of High-Density Polyethylene

In this paper, for the first time, positron annihilation lifetime spectroscopy, after an appropriate adaptation, was used to analyze the plastic deformation process of high-density polyethylene as a representative semicrystalline polymer. It was shown that the average size of the free volume pores of the amorphous phase in the studied strain range decreased in comparison with the undeformed polyethylene, even after the initiation of the cavitation phenomenon due to highly anisotropic, ellipsoidal shape of cavities with the aspect ratio amounting to ∼45. The mean positron lifetime was practically constant in the analyzed range of strains even after activation of the micromechanisms of plastic deformation of crystals due to the mutually compensating effects activated in the crystalline phase. A clear change of the dispersion of positron lifetime as a function of local strain was observed. This effect was correlated with the reduction of the crystallites length by the relative displacement (slips) of adjacent crystalline blocks within individual lamellae.

Positron annihilation investigation of thermal cycling induced martensitic transformation in NiTi shape memory alloy

Thermal cycling of a Ni-excess NiTi alloy was conducted between 50 °C and liquid nitrogen temperature to induce martensitic transformations and to reverse them after. The starting point was an annealed and slowly cooled state, the end point a sample thermally cycled 1500 times. Positron annihilation lifetime spectra and Coincidence Doppler Broadening profiles were obtained in various states and at various temperatures. It was found that the initial state was low in defects with positron lifetimes close to that of bulk NiTi. Cycling lead to a continuous build-up of a defect structure up to 200−500 cycles after which saturation was reached. Two types of defects created during cycling were identified, namely pure dislocations and vacancies attached to dislocations.

Evolution and thermal stability of the subsurface zone in copper formed in dry and lubricated sliding tests studied by positron annihilation and EBSD techniques

The evolution of the subsurface zone formed during the dry sliding in OFHC copper samples was traced using the positron annihilation technique. As the sliding distance increases the total range of the subsurface zone increases and a tribolayer thickness of about 20 μm appears. This layer contains fine grains with high angle grain boundaries, which indicates continuous dynamic recrystallization proceeds there. Below is a layer of larger grains, but at their boundaries, there are placed fine grains formed by the discontinuous dynamic recrystallization process. Both layers are well recognized by the depth dependencies of the mean positron lifetime but also are observed directly in electron backscattering diffraction images. A temperature close to the melting point of approx. 900 °C is needed to recover and rebuild the microstructure and heat almost all defects of the tribolayer, while for layers located much deeper, a much lower temperature is sufficient. No tribolayer was observed in the copper sample exposed to the sliding test with the oil lubricant with MoS2 particles, even at a very long sliding distance. This is reflected in a significant reduction in the wear rate of such a sample compared to a sample exposed to the dry sliding test.

Influence of severe plastic deformation on superconducting properties of Re and In

The positron lifetime annihilation spectroscopy, magnetic and electrical resistivity data obtained for the pure polycrystalline rhenium and indium samples were compared with corresponding results obtained for samples after severe plastic deformation (rolling at room temperature). In the case of Re, the results clearly show that the superconducting properties of this material can be greatly enhanced by introducing large number of lattice defects into Re matrix. In particular, the rolled Re sample is type-II superconductor with the critical temperature Tc= 2.6 K and the upper critical field μ0Hc2≤0.034 T. Moreover, the resistivity data reveal that in the highly defected parts of the Re metal, the superconductivity emerges at 3.2–3.3 K and the magnetic field of μ0H=0.09 T is not high enough to fully restore the normal state in those parts of the studied material. In contrast to Re, severe plastic deformation performed at room temperature does noticebly not affect the superconducting properties of the In material. The positron lifetime annihilation spectra reveal that the concentration of the lattice defects in the rolled and the annealed samples is close to each other. We conclude that, the observed changes in superconducting properties of rolled Re (this work) and Ta (earlier work) cannot be caused by introducing additional quantities of impurities during rolling procedure and it must be connected with the presence of lattice defects in the rolled foils.

Positron lifetime spectroscopy applied to pure Tellurium

High-purity crystalline Tellurium has been investigated using positron lifetime spectroscopy technique in order to determine basic information missing in the current experimental knowledge of the positron annihilation spectroscopy field. Three different pairs of samples have been studied in the as-received state and, in order to eliminate the vacancy-type defects, after consecutive isothermal treatments at 300 °C. Lifetime corresponding to the annihilation in the Tellurium bulk has been determined as 282(1) ps. Previous theoretical calculations present in the bibliography that used different methods and parameterization provided a wide range of values for the annihilation lifetime of the positron in the bulk of Tellurium. The obtained result has been used to identify the most accurate results among them.

Defect driven tunable optical properties of Li+ and Zr4+ co-doped Eu3+: MgF2 compounds

A novel approach of defect engineering in Eu3+ doped MgF2 compounds has been carried out in order to tune the optical properties of Eu3+ ion. Upon doping various co-dopant ions such as Li+ and Zr4+ ions, it has been observed for the first time in the photoluminescence (PL) study that such co-dopant ion would control the characteristics transition lines of Eu3+ ions and results in tunable color characteristics of the phosphor materials. While Li+ co-doping results in a higher intense magnetic dipole (MD) line (5D0 → 7F1), doping with Zr4+ results in an intense electric dipole (ED) line (5D0 → 7F2). Further, Li+ co-doping also results in an unusual highly intense 5D0 → 7F4 line. From PL decay profile it has been observed that although for both cases of the co-dopant ions two different Eu3+ components are found to exist, their contribution is completely different. From positron annihilation lifetime study it was observed that various Mg-vacancies and their cluster forms are responsible for the tunable emission characteristics. First principle based theoretical study has also been carried out to understand the configuration of various cluster vacancies as suggested by their respective positron lifetime values. The short-lived Eu3+ component, which gives rise to highly intense 5D0 → 7F4 line, originates from a Mg-site at the surface of the particle and close to a vacancy cluster VC1. While the long lived Eu3+ component is responsible for highly intense ED line and close to Mg mono- or di-vacancies in the bulk of the particle. Therefore, defects (both dopant impurity ions and lattice vacancies) play a significant role in tuning the emission characteristics of Eu3+ doped MgF2 compounds.

Positron Annihilation Spectroscopy as a Diagnostic Tool for the Study of LiCoO2 Cathode of Lithium-Ion Batteries

Positron annihilation spectroscopy using lifetime and Doppler broadening allows the characterization of the lithiation state in LiCoO2 thin film used in cathode of lithium-ion batteries. The lifetime results reflect positron spillover because of the presence of graphite in between the oxide grains in real cathode Li-ion batteries. This spillover produces an effect in the measured positron parameters which are sensitive to delocalized electrons from lithium atoms as in Compton scattering results. The first component of the positron lifetime corresponds to a bulk-like state and can be used to characterize the state of charge of the cathode while the second component represents a surface state at the grain-graphite interface.

Positron annihilation studies of defect structure of (TiZrHfNbV)N nitride coatings under Xe14+ 200 MeV ion irradiation

A study of defect structure in (TiZrHfNbV)N coatings irradiated with 200 MeV high energy Xe14+ ions with different fluences by positron annihilation spectroscopy (PAS) was carried out. Positron lifetime studies showed that the vacancy-defects are the dominant defects type in investigated specimens. Although (TiZrHfNbV)N coatings were irradiated with high energy Xe+ ions with different fluences: 5 × 1011, 5 × 1012 and 5 × 1013 ions/cm2, no amorphous or intermetallic phase are detected.

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.

Defects Responsible for Hydrogen Embrittlement in Austenitic Stainless Steel 304 by Positron Annihilation Lifetime Spectroscopy

Hydrogen-related defects in the metastable austenitic stainless steel 304 were analyzed by positron annihilation lifetime spectroscopy to determine the factors responsible for hydrogen embrittlement. Hydrogen was introduced by the cathodic electrolysis method to the ~10-µm topmost layer which was then etched by electrochemical polishing to investigate the formation of hydrogen-induced defects in the bulk. Although hydrogen embrittlement did not occur simply upon removal of the hydrogen-charged layer, a decrease in ductility was confirmed by applying 10% tensile strain and subsequent polishing. In this latter sample a positron lifetime component of ~180 ps was detected, which is longer than that of dislocations, and disappeared upon annealing at 100°C. After further straining the sample until fracture, the formation of vacancy clusters was observed. These results suggest that hydrogen diffused to deeper regions upon application of tensile stress, where vacancy-hydrogen complexes were generated and developed into vacancy agglomerates. The detection of the precursors of the vacancy clusters, which are thought to lead to the brittle fracture, represents a fundamental step forward in the understanding of the hydrogen embrittlement process in austenitic stainless steels.

Low molecular weight ‘liquid’ polymer extended compounds, impact on free volume and crosslink density studied by positron lifetime spectroscopy and stress-strain analysis according to Mooney-Rivlin

Liquid polymers are used in elastomeric compounds to modify their processing and final characteristics. The material is considered as a polymeric plasticizer and is used to substitute or supplement conventional plasticizer like paraffinic or aromatic oils, or hydrocarbon resins. Their plasticizing effect and contribution to the crosslinking in sulfur cured SSBR mixtures is studied. Aromatic oil, aliphatic resin and different liquid polymers, varied in type, molecular weight and microstructure are discussed. Different plasticizing effects are shown with corresponding curing torques and Shore hardness values. The free volume – measured by positron annihilation lifetime spectroscopy - increases or decreases according to the liquid polymer's incorporation in the main polymer.

Effect of Ag doping on crystallinity and microstrain of LaMnO3 nanoparticles: Confirmations of defect levels with positron lifetime and Doppler-broadening calculations

LaMnO3 nanoparticles with different levels of Ag doping are prepared and their impact on defect concentration and crystallite sizes are investigated. The crystallinity studies and phase identifications are done with XRD analysis. Williamson-Hall plots and size-strain plots are used to analyse the effect of doping on crystallinity and defect induced microstrain. With the help of Positron Annihilation spectroscopy, the identification of defects and mapping of the defect densities are carried out. Positron lifetime calculations and analysis on Doppler-broadened profile of the positron-electron annihilation radiation in undoped and various doped LaMnO3 are carried out and compared with W-H plots and size-strain plots.

Effects of dislocations and hydrogen concentration on hydrogen embrittlement of austenitic 316 stainless steels

The effects of dislocations and hydrogen concentration on the hydrogen embrittlement (HE) in 316 austenitic stainless steels (316SS) were systematically investigated in the present study. The results revealed a decrease in tensile strength and ductility of 316SS after hydrogen charging. Furthermore, the severity of HE was dependent on the hydrogen concentration. The results of x-ray diffraction revealed the presence of martensite in the hydrogen-charged (50 mA /cm2) specimen annealed at 1000 ℃. This was attributed to the internal stress originating from hydrogen-induced lattice expansion. The analysis of the fracture surface of all the specimens revealed an increase in the brittleness with the absorption of hydrogen. An increase in the hydrogen concentration induced a transformation of the fracture mode i.e., from ductile dimple fracture to cleavage and intergranular fractures. The evolution of hydrogen in 316SS was associated with the defects induced by deformation. The shift in the major desorption peaks of the annealed specimen to the high-temperature region indicated the formation of effective hydrogen-trapping sites during hydrogen-charging. The specimens were subjected to positron lifetime spectroscopy and thermal desorption spectroscopy. The results for the raw specimen, charged at 20 mA /cm2, revealed the gradual escape of hydrogen atoms and recovery of vacancy defects with an increase in the annealing temperature. In addition, the hydrogen-induced and cold rolling-induced dislocations was preserved.

Review of Positron Lifetime Studies of Lattice Defects Formed during Tensile Deformation in a Hydrogen Environment

Review of Positron Lifetime Studies of Lattice Defects Formed during Tensile Deformation in a Hydrogen Environment