Positron Lifetime Measurements Research Articles

Silver gallium telluride (AgGaTe2) single crystal: Synthesis, accelerated crucible rotation-Bridgman growth and characterization

A silver gallium telluride single crystal of diameter 12mm and length 80mm was successfully grown by the vertical Bridgman method using accelerated crucible rotation technique. To confirm the unit cell parameters of the grown silver gallium telluride (AgGaTe2) crystal, single crystal X-ray diffraction studies were carried out. AgGaTe2 has been studied using differential scanning calorimetry and is found to have supercooling of 13°C. AgGaTe2 shows Fourier transform infrared transmission in the spectral range of 6000–500cm−1. The composition of Ag, Ga and Te in the grown crystal was measured with energy dispersive X-ray analysis spectroscopy. The optical band gap for AgGaTe2 is 1.8eV. In the positron lifetime measurements, the average lifetime 267ps corresponds to vacancy clusters in AgGaTe2 crystal. Hall measurements confirm p-type nature of AgGaTe2.

Positron Lifetime Measurements of Vacancy Defects in Complex Oxides

Native defects in complex oxides play a crucial role in determining their optical, electrical, and magnetic properties and it is di cult to identify and characterize them. Positron lifetime spectroscopy is a powerful technique to study vacancy defects; however its application to complex oxides has been limited. In this work we apply positron lifetime spectroscopy to study open volume defects in rare earth doped yttrium aluminum garnet (YAG) complex oxides grown in argon atmosphere. In YAG single crystals, positron lifetime measurements identi ed isolated aluminum vacancies and complexes of aluminum vacancy and neighbor oxygen vacancies. Thermoluminescence measurements were also performed to elucidate the interaction between trapping defects and luminescence centers. By combining positron lifetime and thermoluminescence, both the defect type and its e ect on the optical properties of YAG crystals were revealed.

The Influence of a Magnetic Field on the Formation of Corrosion Defects in Selected Metals and Steels, Analysed Using Positron Annihilation Method

The method of positron lifetime measurement was used to analyse the in uence of a magnetic eld on the kinetics of corrosion defect formation in near-surface layers of iron, titanium as well as S20 and S0H18N9 steel grades. The listed metals, which belong to ferroand paramagnetic materials, have di erent sensitivity to corrosion. It was found that not only the presence of a magnetic eld, but also its direction in uence the dimensions and the concentration of defects formed during corrosion.

Hydrogen embrittlement of austenitic stainless steels revealed by deformation microstructures and strain-induced creation of vacancies

Hydrogen embrittlement of austenitic stainless steels has been examined with respect to deformation microstructures and lattice defects created during plastic deformation. Two types of austenitic stainless steels, SUS 304 and SUS 316L, uniformly hydrogen-precharged to 30 mass ppm in a high-pressure hydrogen environment, are subjected to tensile straining at room temperature. A substantial reduction of tensile ductility appears in hydrogen-charged SUS 304 and the onset of fracture is likely due to plastic instability. Fractographic features show involvement of plasticity throughout the crack path, implying the degradation of the austenitic phase. Electron backscatter diffraction analyses revealed prominent strain localization enhanced by hydrogen in SUS 304. Deformation microstructures of hydrogen-charged SUS 304 were characterized by the formation of high densities of fine stacking faults and ε-martensite, while tangled dislocations prevailed in SUS 316L. Positron lifetime measurements have revealed for the first time hydrogen-enhanced creation of strain-induced vacancies rather than dislocations in the austenitic phase and more clustering of vacancies in SUS 304 than in SUS 316L. Embrittlement and its mechanism are ascribed to the decrease in stacking fault energies resulting in strain localization and hydrogen-enhanced creation of strain-induced vacancies, leading to premature fracture in a similar way to that proposed for ferritic steels.

Monolayer dispersion of CoO on Al2O3 probed by positronium atom

CoO/Al2O3 catalysts were prepared by wet impregnation method with CoO contents ranging from 0wt% to 24wt%. X-ray diffraction and X-ray photoelectron spectroscopy measurements suggest formation of CoO after calcined in N2. Quantitative X-ray diffraction analysis indicates monolayer dispersion capacity of CoO in CoO/Al2O3 catalysts to be about 3wt%. Positron annihilation lifetime and coincidence Doppler broadening measurements were performed to study the dispersion state of CoO on Al2O3. The positron lifetime measurements reveal two long lifetime components τ3 and τ4, which correspond to ortho-positronium annihilation lifetime in microvoids and large pores, respectively. It was found that the positronium atom is very sensitive to the dispersion state of CoO on Al2O3. The presence of CoO significantly decreases both the lifetime and the intensity of τ4. Detailed analysis of the coincidence Doppler broadening measurements suggests that with the CoO content lower than the monolayer dispersion, spin conversion reaction of positronium is induced by CoO. When the cobalt content is higher than the monolayer dispersion capacity, inhibition of positronium formation becomes the dominate effect.

Effect of constraining the source lifetime parameter during least-square-fit analysis on positron lifetime measurements

Effect of constraining the source lifetime parameter during least-square-fit analysis on positron lifetime measurements

Investigation of Commercially Pure Titanium Structure during Accumulation and Release of Hydrogen by Means of Positron Lifetime and Electrical Resistivity Measurements

In this work the goal was to study the behavior of titanium-hydrogen system with hydrogen concentration of 1 wt. % at high temperature vacuum annealing by means of positron lifetime (PL) spectroscopy and electrical resistivity measurements. The range of hydrogen concentrations under investigation requires the γ phase formation. The study is related with the X-ray diffraction (XRD) and thermal desorption spectra (TDS) analysis. The registered positron lifetime spectra were analyzed by multiexponential decomposition into two components. Correlation of changes of the parameters of the PL spectrum and the electrical resistivity is revealed in this paper.

Depth profiling with the Chalmers pulsed positron beam

Positron lifetime measurements are used as a tool for obtaining a deeper understanding of positron behaviour in solids and calibrating the Chalmers Pulsed Positron Beam. Experiments are executed for positron energies up to 13 keV corresponding to the maximum penetration depth in Au of roughly 400 nm and measurement results are compared to PENELOPE simulations. Reference towards Makhov modelling is made. The future goal of the study is to investigate the depth dependence of ion-induced radiation damage where positrons will be used to characterise sizes and intensities of vacancy-type lattice defects.

Positronium Production in Engineered Porous Silica

Positronium (Ps) has been the subject of several experimental and theoretical investigations due to its many scientific applications. In this work high positronium yield was found in engineered porous silica. The studied materials were pellets of swollen MCM-41 and of commercial Davicat 1700, obtained by different compression pressures, with mesopores characterized by different structural and chemical features. The measurements were performed with a variable energy positron beam at room temperature. An estimation of the Ps mean diffusion length was obtained by measuring capped samples. A selected swollen MCM-41 sample (0.39 g/cm3) was measured also at cryogenic temperature (8 K). In this material both the Ps yield and the Ps diffusion length are found to be independent of temperature. The pore surface of the swollen MCM-41 samples is very interesting in comparison to commercial silica as it possesses hydrophobic patches to avoid ice formation at low temperature. Positron lifetime measurements show a high ...

Gadolinium substitution induced defect restructuring in multiferroic BiFeO3: case study by positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) comprising of the measurements of positron lifetime and coincidence Doppler broadening spectra has been carried out to understand and monitor the evolution of the vacancy-type defects arising from the ionic deficiencies at lattice points of the multiferroic perovskite bismuth ferrite (BiFeO3) doped with 1, 5 and 10 at% gadolinium (Gd3+) ions. Negatively charged defects in the form of Bi3+ monovacancies were present in the undoped nanocrystallites, which strongly trapped positrons. During the successive doping by Gd3+ ions, the positron trapping efficiency decreased while the doped ions combined with the vacancies to form complexes, which became neutral. A fraction of the positrons got annihilated at the crystallite surfaces too, being evident from the very large positron lifetimes obtained and confirming the nano-size-specific characteristics of the samples. Further, the intercrystallite regions provided favourable sites for orthopositronium formation, although in minute concentrations. The dopant ion-complex formation was also depicted clearly by the defect characteristic S–W plot. Also, the large change of electrical resistivity with Gd concentration has been explained nicely by invoking the defect information from the PAS study. The study has demonstrated the usefulness of an excellent method of defect identification in such a novel material system, which is vital information for exploiting them for further technological applications.

Positron lifetime measurements of hydrogen passivation of cation vacancies in yttrium aluminum oxide garnets

A charge compensation mechanism is proposed for cation vacancy defects in complex oxides based on positron lifetime measurements, infrared spectroscopy, and composition analysis. Defects were characterized in samples of yttrium aluminum garnet grown in O${}_{2}$ or Ar. However, no positron trapping was detected in samples grown in H${}_{2}$. This is attributed to decoration of cation vacancies with hydrogen, thereby passivating charges of vacancies that otherwise function as positron traps. Infrared spectroscopy gave direct evidence of the presence of hydrogen. Passivation of cation vacancies with hydrogen is proposed as an important mechanism for charge compensation in the defect physics of oxides.

Characterization of helium implanted Fe–Cr alloys by means of positron annihilation methods

Doppler broadening spectroscopy and positron lifetime measurements have been used for the characterization of radiation damage and helium effects on the microstructure of the ferritic Fe–Cr alloy with 12% (wt.) chromium content. Severe displacement damage (up to 100dpa) was introduced by the implantation of helium ions with fluence 1.25×1018cm−2. The temperature during the implantation process was below 80°C. Positron lifetime spectra obtained from slow positron beam experiments confirmed the presence of large voids in the region of the displacement peak. Further investigation by Doppler broadening spectroscopy in the conventional and slow positron beam setups revealed a strong retention of helium in this region, which contributes to the broadening of the momentum peak and at the same time it reduces the positron lifetime in radiation-induced vacancy clusters. The measured Doppler broadened profiles of the implanted materials reasonably correspond with the calculated data showing the helium effect range within 5–12×10−3m0c. Both experimental techniques show empty vacancy clusters to be a dominant type of defect in the first 400nm region, while helium filling of these defects occurs further in depth. This is in a good agreement with the results of SRIM simulation.

Persistent Photoconductivity in Strontium Titanate

Persistent photoconductivity was observed in strontium titanate (SrTiO(3)) single crystals. When exposed to sub-bandgap light (2.9 eV or higher) at room temperature, the free-electron concentration increases by over 2 orders of magnitude. After the light is turned off, the enhanced conductivity persists for several days, with negligible decay. From positron lifetime measurements, the persistent photoconductivity is attributed to the excitation of an electron from a titanium vacancy defect into the conduction band, with a very low recapture rate.

The Annealing Behavior of the Subsurface Zone Induced by Friction in Bismuth Detected by Positron Lifetime Technique

The annealing behavior of the subsurface zone (SZ) in pure bismuth induced by dry sliding was studied using the positron lifetime measurement. This measurement allows us to detect the SZ and its recovery, and recrystallization processes. The comparative measurements of the sample exposed to compression revealed the thermal stability of the SZ. The compressed sample rebuilt its structure due to the recovery and recrystallization processes at the temperature of 60 °C, whereas the sample exposed to dry sliding does it at higher temperature of 260 °C, which is close to the melting point. The isothermal annealing at the temperature of 100 °C confirmed these results. The defect depth profile induced by dry sliding evolves with the annealing temperature in such a way that the concentration of defects at the worn surface gradually decreases, but at the depth between 50 and 170 μm, the generation of new defects takes place at the temperature of 75, 100 and even at 175 °C. At the temperature of 175 °C, the defects still are extended up to the depth of about 60 μm from the worn surface. The results were qualitatively confirmed by the measurements of the Vickers microhardness depth profile. Similar annealing behavior of the SZ was observed in pure magnesium.

Early stage irradiation effects in F82H model alloys

Neutron irradiation-induced vacancy defects in F82H model alloys were studied using positron lifetime measurements. A comparison between the results from neutron irradiation and our previous electron irradiation in F82H model alloys was conducted. The results showed that Cr has a consistent effect on void swelling resistance. The void swelling suppression by Cr was explained using the configuration trapping model. However, for the W, V and Ta solute effects, the results of two types of irradiation support the solute and dislocation interaction model. The effects on metal carbides (M23C6) were investigated using neutron irradiation at 363K and electron irradiation. For neutron irradiation, the solute effect may be the dominating factor compared to the effects of the M23C6. Solute depletion caused by metal carbides can explain the difference.

Microdefects and Electron Densities in Near-Equiatomic NiTi Alloys Studied by Positron Annihilation Spectroscopy

Abstract Microdefects and free electron densities in near-equiatomic NiTi alloys of five different chemical compositions have been studied by positron lifetime measurements. The results indicate that some 3d electrons are localized to form covalent bonds when Ni and Ti atoms aggregate to form NiTi alloys, leading to the decrease of the free electrons participating in metallic bonds. The densities of free electrons in the bulk and defects of near-equiatomic NiTi alloys change with the increase of Ni content and reach the maximum value with 51.0% Ni (atom fraction). It is also found that the Ni51Ti49 alloys have the fewest defects, which contributes to the Martensite transformation. Consequently, the Ni51Ti49 alloys have the best shape memory effect.

Ligament coarsening in nanoporous gold: Insights from positron annihilation study

Positron lifetime experiments, coupled with scanning electron microscopy studies, have been used to follow the coarsening of the ligaments of nanoporous Au (np-Au), prepared by electrochemical dealloying of Ag75Au25. Positron lifetime measurements in the as-prepared np-Au indicate two lifetime components, identified with annihilation at vacancies within the ligaments and at the ligament-pore interface. The variation of these lifetime parameters with annealing temperature indicates distinct changes that correlate the migration of vacancies to the growth of ligaments. The lifetime component, corresponding to the annihilation at the ligament-pore interface, shows signature of ligament surface instability.

Development of a new time and position resolving detector for the pulsed low energy positron system PLEPS

The pulsed low energy positron system PLEPS at the Munich research reactor FRM-II is a user facility for depth resolved positron lifetime measurements. Besides positron lifetime measurements 2D-AMOC (Two Dimensional Age Momentum Correlation) experiments are also possible. 2D-AMOC provides in coincidence the lifetime of the positron and the longitudinal momentum distribution of the annihilated electron. It would be of great scientific concern to measure simultaneously the entire 3D-momentum distribution of the electron annihilating with the positron and the corresponding lifetime of the positron (4D-AMOC). To perform 4D-AMOC measurements a time and position resolving detector is required in coincidence with a pixelated Germanium detector. Therefore a time and spatially resolving detector is currently developed at our institute with envisaged time resolution of 100 ps (FWHM) and a spatial resolution of about 2.6 mm (FWHM) over an area of 12 cm2. First test measurements have been carried out with a 25 mm diameter MCP (Micro Channel Plate) image intensifier and with special delay-line anode readout for the spatial information. Up to now 178 ps (FWHM) time resolution and on average 3.4 mm (FWHM) position resolution have been achieved with BaF2 as scintillator material and a 60Co source.

Study of bicontinuous phase in (TTAB+pentanol)/water/n-octane reverse micellar system using positron lifetime spectroscopy

A phase diagram of (TTAB+pentanol)/water/n-octane has been mapped by using optical method. It exhibits a reverse micellar (L2) phase extending over a wide range of concentrations of the constituents. To investigate the fine structure of the L2 phase, a series of (TTAB+pentanol)/n-octane ternary mixtures having initial concentrations of (TTAB+pentanol) (1:1) in n-octane as 35%, 50% and 65% by weight were prepared. In each of these mixtures, positron lifetime measurements were performed as a function of the concentration of water, using a standard lifetime spectrometer. At water concentrations of 11.8%, 8.5% and 8.4% by weight respectively for the above systems, the o-Ps pick-off lifetime τ3 shows an oscillatory behaviour while I3 representing the Ps formation exhibits an abrupt change. These changes in the positron annihilation parameters have been explained on the basis of onset of bicontinuity in the microemulsion phase. The positron annihilation technique thus suggests the existence of droplet-like and bicontinuous structures in the L2 phase which is otherwise considered optically to be a single phase as the system remains clear and isotropic throughout this phase. Supporting evidence has been provided by the electrical conductivity measurements performed in these systems. These results are presented in this paper.

Development of a sample chamber with humidity control for an atmospheric positron probe microanalyzer

In order to perform positron lifetime measurements on thin films under atmospheric conditions, a slow positron microbeam was extracted into air using silicon nitride thin films (30 nm and 200 nm) as a vacuum window. Even the thinner window (30 nm) was found to reliably withstand a differential pressure of 1 atm under various stress tests. By placing the sample in an enclosed chamber through which gas with a fixed, controllable relative humidity (RH) was continuously passed, the RH dependence of the ortho-positronium lifetime for bulk fused silica was examined.