Positron Annihilation Spectroscopy Methods Research Articles

Reduction of background radiation effects for positron lifetime measurements in the slow positron beamline at the Kyoto University Research Reactor.

Positron annihilation lifetime spectroscopy (PALS), which is recognized as one of the major analytical methods of positron annihilation spectroscopy, can directly detect information related to the size of vacancy-type defects from lifetime values. PALS measurements performed under high background radiation have been previously reported. It is well known that coincidence techniques such as age-momentum correlation (AMOC) measurements are effective for the background reduction, but count rates decline significantly. In this study, a preliminary experiment was performed to reduce the influence of the background radiation without the coincidence technique in the pulsing system of the Kyoto University research Reactor (KUR) slow positron beamline. This experiment involved the introduction of a gate circuit for the background radiation discrimination using a dynode signal from a single scintillation detector (photomultiplier). After introducing the gate circuit, the time resolution and the lifetime value of Kapton were 308 ps and 388 ± 3 ps, respectively, with count rates of ∼400 counts/s at a KUR 5 MW operation. In the AMOC measurement, the time resolution and the lifetime value of Kapton were 297 ps and 380 ± 7 ps, respectively, with count rates of ∼40 counts/s at a KUR 5 MW operation. When the single detector with the gate circuit was used, the count rate was ∼1 order of magnitude higher than those of the AMOC measurements, while the time resolutions of the two methods were comparable.

Microstructure and Positron Lifetimes of Aluminide Coatings on Inconel 713

The microstructure of the palladium modified and non-modified aluminide coatings was examined by the EDS and the positron annihilation spectroscopy methods. Both coatings have a double layer structure: β-NiAl phase or β-(Ni,Pd)Al phase on the top and the interdiffusion zones with the chromium and molybdenum rich phases in the β-NiAl or or β-(Ni,Pd)Al phase below. Palladium, that forms the β-(Ni,Pd)Al phase and substitutes for nickel atoms causes the increase of the positron lifetime value due to the increase in the number of open volume defects in the lattice which are jogs or vacancies on dislocation lines.

Accumulation of Defects in Austenitic Stainless Steels with Phosphorus and Titanium Additions upon Electron Irradiation at 573 К Investigated Using Positron Annihilation Spectroscopy

Using the positron annihilation spectroscopy method, the accumulation of vacancy-type defects in steels Kh16N15М3 and Kh16N15М3T1 alloyed with phosphorus has been studied at early stages of irradiation at a temperature of 573 К. The obtained data have showed that vacancies in the steels interact with phosphorus atoms at this temperature with the formation of immobile and low-mobile vacancy—impurity complexes, which results in the accumulation of vacancy-type defects. In the steel Kh16N15М3Т1 under irradiation, the formation of nanosized particles of intermetallic Ni3Ti precipitates occurs, which intensify recombination of point defects and reduce their accumulation as compared to the steel Kh16Н15М3. As this occurs, the presence of phosphorus in the steel Х16Н15М3Т1 likewise leads to the intensification of the accumulation of vacancy defects.

Characterisation of atomic-scale defects in ODS ferritic alloys by positron annihilation

ABSTRACTThe vacancy-type defects and their local chemical environment in different ODS alloys produced in the USA (14YWT), China (K5) and Russia (ODS EP-450) are studied. The Angular Correlation of Annihilation Radiation (ACAR), which is one of the positron annihilation spectroscopy method, was used. It was shown that in all alloys, except 14YWT, the dominant type of positron traps are vacancy-like defects, localised in matrix or associated with dislocations and/or interfaces of the incoherent particles. In the case of 14YWT alloy, which contains Y–Ti–O nanoclusters of a high density, the positrons confine and annihilate at O-vacancy pairs or complexes within nanoclusters. It is testified by enhanced electron density in annihilation sites and neighbourhood of Ti and Y atoms. These results, obtained by the ACAR method, indicate that the vacancies play an essential role in the formation of nanoclusters in ODS 14YWT alloy as it was theoretical predicted by first-principle calculations.

Zn Vacancy Formation Energy and Diffusion Coefficient of CVT ZnO Crystals in the Sub-Surface Micron Region

By using positron annihilation spectroscopy methods, we have experimentally demonstrated the creation of isolated zinc vacancy concentrations >1020 cm−3 in chemical vapor transport (CVT)-grown ZnO bulk single crystals. X-ray diffraction ω-rocking curve (XRC) shows the good quality of ZnO single crystal with (110) orientation. The depth analysis of Auger electron spectroscopy indicates the atomic concentrations of Zn and O are almost stoichiometric and constant throughout the measurement. Boltzmann statistics are applied to calculate the zinc vacancy formation energies (Ef) of ~1.3–1.52 eV in the sub-surface micron region. We have also applied Fick’s 2nd law to calculate the zinc diffusion coefficient to be ~1.07 × 10−14 cm2/s at 1100 °C. The zinc vacancies began annealing out at 300 °C and, by heating in the air, were completely annealed out at 700 °C.

Nanocomplexes of Magnesium Phenylsiloxanes - Moleculer Structere and Properties

Polyphenylsiloxanes containing magnesium (polymagnesiumphenyl siloxanes) with Si:Mg ratios of 1:1.5 and 1:2 have been synthesized by the exchange reaction in a mixture of benzene (toluene)–dimethyl sulfoxide with yields of 58 and 72%, respectively. The composition and structure of the synthesized polymers have been studied by the methods of positron annihilation spectroscopy (PAS), X-ray diffraction analysis (XRD), 29Si nuclear magnetic resonance (NMR), and infrared spectroscopy (IR). The cross-section surface area (S) (by the Miller–Boyer method) and coherent scattering regions (CSR) volumes have been calculated based on the XRD data. The PAS data allowed calculating the specific volumes of Ps and e+ “traps”. The T3 and T2 signal areas in 29Si NMR spectra enabled us to determine the structure of the unit cell of polymagnesiumphenyl siloxanes (PMgPS). As was demonstrated by the electron microscopy, PMgPSs were in the form of spherical particles of a radius of 50–120 nm. The polymers physical–chemical characteristics were determined from nitrogen adsorption. Based on the specific adsorption area, the radii of spherical particles in a densely packed layer, whose size depends proportionally on the specific polarization potential and the specific area, have been calculated. The calculated size values for spherical particles are similar to those found by the method of electron microscopy. An assumption on a fractal structure of spherical particles has been made on the basis of the calculated data, and data of scanning electron microscopy.

Microstructure and positron lifetimes of zirconium modified aluminide coatings

The microstructure of the zirconium modified and non-modified aluminide coatings was examined by the EDS, XRD, TEM and the positron annihilation spectroscopy methods. Both coatings have a double layer structure: β-NiAl phase on the top and γ′-Ni3Al below. Small zirconium nanoparticles were found along grain boundaries in the β-NiAl phase. The positron lifetime in both coatings is the same. The formation of zirconium precipitates neither affects, the number of defects nor the volume diffusion. Zirconium nanoparticles that precipitate along grain boundaries stand against the outward diffusion of Al ions through the coating to the coating/oxygen interface. This “blocking effect” may be responsible for the reduction of the alumina scale growth rate and may delay pore formation on the coating/oxygen interface.

Study of the structure of polyphenylsiloxanes containing the metal-ions by physical–chemical methods

The study of the structure of polylphenylsiloxanes containing magnesium, cobalt, nickel, and zinc has been performed by the methods of PAS, XRD, and XPS. Using the XRD data, the areas of cross-section of polymer molecules (according to the Miller–Boyer method) and coherent scattering (CS) volumes have been calculated. The data of positron annihilation spectroscopy (PAS) allowed calculation of the specific volumes of positron (Ve+) and positronium (Vps) “traps”. A direct proportional dependence of the volume of the “trap”, in which single positron annihilation takes place, on the positron lifetime and an inverse dependence on the density of metal-siloxanes have was demonstrated. The number of positron annihilations per elementary unit volume has been calculated. Here, for polycobaltphenyl-siloxane and polyphenyl-siloxane the number of oxane fragments, in which annihilation takes place, is equal to 3, whereas for polynickel- and polyzincphenyl-siloxane it is equal to 4. The number of elementary units (e.u.) in the CS volume for magnesium, cobalt, and polyphenyl-siloxane is equal to 10, while for nickel and zinc – to 12, which must be related to higher density in case of nickel-siloxane and to larger CS volume for zinc-siloxane.The Т3 and Т2 values in the 29Si NMR spectrum for polyphenyl-siloxane coincide with the respective data for polynickel- and polycobaltphenyl-siloxanes and differ significantly from those for polyzincphenyl-siloxane. The increase of the number of Т1-and Т2-fragments for zinc is related to the increase of the percentage of octahedral polyhedra of these ions, while for nickel and cobalt – to a greater degree, to the tetrahedral structure of these ions.The study of polyzincphenyl-siloxane by the method of X-ray photoelectron spectroscopy has been performed, and the presence of two types of zinc atoms has been demonstrated: the content of zinc atoms with 2p3/2 was 64.3%; the content of those with 2p1/2 was 35.7%. The latter corresponds to the content of the tetrahedral surrounding of Т1 and Т2 silicon atoms in the 29Si NMR spectrum.

Tunable monoenergy positron annihilation spectroscopy of polyethylene glycol thin films**Project supported by the National Natural Science Foundation of China (Grant Nos. 11575205, 11475197, 11675188, and 11475193).

Doppler broadening and coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol (PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy. The obtained positron annihilation parameters are interpreted from two aspects: surface effect and differences in micro-structure or chemical environment of positron annihilation. The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to the bulk region in the film forming process can be well realized by changing its molecular weight. Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods is a powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.

Positron annihilation spectroscopy on a beam of positrons the LEPTA facility

The results and possibilities of the samples surfaces research by the Doppler method of positron annihilation spectroscopy (PAS) for a monochromatic beam of positrons at the LEPTA facility are presented in this paper. Method with high-resolution sensitivity to defects like vacancies and dislocations allows scanning of the surface and near-surface sample layers to a depth of several micrometers by the method of Doppler broadening of annihilation lines. The opportunities for the development of a PAS method based on the measurement of the positron lifetime in the sample irradiated by ordered flow of positrons from the injector of accelerator complex LEPTA at JINR are discussed.

Positron-annihilation studies of the influence of nanodimensional intermetallic precipitates on the evolution of radiation defects in the Fe–Ni–Al alloy

The influence of precipitates of the Ni3Al intermetallic compound on the accumulation of vacancy defects in the aged Fe–Ni–Al alloy upon electron irradiation has been studied by the method of positron annihilation spectroscopy. The samples of the alloy with different initial microstructures (quenched, aged under different conditions) were irradiated at temperatures of 300 and 423 K to a damaging dose of 5 × 10–4 displacements per atom (dpa), after which they were isochronously annealed in the temperature range of 300–850 K. The results obtained have shown that the presence of particles of the intermetallic precipitates leads to the retardation of the accumulation of vacancy defects. The rate of accumulation substantially depends on the irradiation temperature. Furthermore, the effect of precipitates depends on the size, density, and type of particles. An analysis of the experimental data has shown that this effect is caused by the presence of elastic stresses at the precipitate–matrix boundaries.

On the application of positron annihilation spectroscopy for studying the photodestruction of biological membranes

Possible therapeutic applications of methods of positron annihilation spectroscopy (PAS) with low-energy positron beams are discussed in this review. These methods are used to study the defect structure of material layers of micro- and submicrometer size (surface layers in electronic devices, cell walls and liquid-crystal units in biomedical applications, etc.). We consider a model of analysis with the positron annihilation-lifetime spectroscopy (PALS) method for the states of irradiated membranes of bacterial cells in a cycle of photodynamic therapy, which lies in the destruction or irreversible oxidative damage of pathogenic cells by the photo-inactivated oxygen of a photosensitizer incorporated into the target cell immediately before light exposure.

Positronics of radiation-induced effects in chalcogenide glassy semiconductors

Using As2S3 and AsS2 glasses as an example, the principal possibility of using positron annihilation spectroscopy methods for studying the evolution of the free volume of hollow nanoobjects in chalcogenide glassy semiconductors exposed to radiation is shown. The results obtained by measurements of the positron annihilation lifetime and Doppler broadening of the annihilation line in reverse chronological order are in full agreement with the optical spectroscopy data in the region of the fundamental absorption edge, being adequately described within coordination defect-formation and physical-aging models.

Application of positron spectroscopy for investigation of threshold space of oil-gas rocks

Abstract This article considers the possibility of applying the positron annihilation spectroscopy method for investigating the pore space of rocks from oil-gas and methane-coal deposits. The diagnostics of the structure was performed using the method of spectrometry of angular correlation of annihilation rays (ACAR). Using the samples of porous silicon, the authors have shown the applicability of the ACAR method for determination of the average dimensions of spherical and cylindrical nanosized objects and their concentration

Investigation of materials for fusion power reactors

The possibility of application of nuclear-physical methods to observe radiation damage to structural materials of nuclear facilities is nowadays a very actual topic. The radiation damage to materials of advanced nuclear facilities, caused by extreme radiation stress, is a process, which significantly limits their operational life as well as their safety. In the centre of our interest is the study of the radiation degradation and activation of the metals and alloys for the new nuclear facilities (Generation IV fission reactors, fusion reactors ITER and DEMO). The observation of the microstructure changes in the reactor steels is based on experimental investigation using the method of positron annihilation spectroscopy (PAS). The experimental part of the work contains measurements focused on model reactor alloys and ODS steels. There were 12 model reactor steels and 3 ODS steels. We were investigating the influence of chemical composition on the production of defects in crystal lattice. With application of the LT 9 program, the spectra of specimen have been evaluated and the most convenient samples have been determined.

Russian

The article presents the data of the study of vessel steel, used in acting pressurized-water reactors-440. The studied steel patterns had vacancy deffects, also caused by neutron irradiation. The sizes of the defects were detected. The article proves that the method of positron annihilation spectroscopy is one of the most efficient methods in detecting the size of nanoobjects (vacancies, vacancies cluster), independent volume of reed marks, cavities, hollows, their concentrations and chemical compositions at the site of positron annihilation in defect materials.

On the application of methods of positron annihilation spectroscopy for studying radiation-stimulated processes in chalcogenide glassy semiconductors

Unirradiated and γ-irradiated (average energy E = 1.25 MeV and dose Φ = 2.41 MGy) chalcogenide glassy semiconductors (CGSs) As2S3 and Ge15.8As21S63.2 are studied by positron annihilation lifetime spectroscopy (PALS) and Doppler broadening of the 0.511-MeV annihilation line (DBAL). Two 22Na positron sources with activities of 0.6 and 2.0 MBq and Kapton film thicknesses of 8.0 and 25.0 μm, respectively, are used. It is shown that radiation-induced changes in the PALS parameters of the CGS types under study are within measurement errors. The DBAL method appeared more efficient and accurate for studying radiation-stimulated processes in CGSs.

Russian

This article provides the findings of an experimental study into steels utilized in the nuclear industry, which employed the positron annihilation spectroscopy method (APAD). The processing and comparison of the experimental data with literary data helped identify vacancy defects, including those caused by neutron irradiation, and determine their dimensions.

Electronic properties of the amorphous FeCuNbSiB alloy

Variations in the electronic properties of amorphous Fe-Cu-Nb-Si-B alloys during crystallization have been studied by methods of positron annihilation spectroscopy. The measurements of the positron lifetime in samples and the angular distribution of annihilation photons have demonstrated substantial variations during crystallization. There is a correlation of the results with the performed measurements of the thermopower. The experimental results allow the assumption that degenerate electron gas is not formed in the amorphous alloys under study, and localized electrons participate mainly in the conduction. It can be suggested that the main role in formation of the amorphous alloy structure is played by the covalent bond. The spectrum of momentum distribution of the amorphous alloy differs little from the pure iron spectrum. Irradiation induces vacancy-copper complexes in the amorphous alloy.

Determination of nanoobject sizes in porous systems, nanomaterials, and some defective materials using the positron annihilation spectroscopy method (survey)

It is shown that the positron annihilation spectroscopy (PAS) method is one of the most effective ways of determining the sizes of nanoobjects (vacancies, vacancy clusters, free pore spaces, cavities, and voids) and their concentrations in the place of annihilation of positrons in porous systems and in a wide range of technically important materials and nanomaterials. The possibilities of the method are demonstrated by example of porous silicon and silicon irradiated with protons.