A BaF_{2} positron-lifetime spectrometer equipped with a coincidence system that represents a compromise between the traditional fast-fast and fast-slow arrangements is described. The main difference of the present configuration from both the fast-slow and fast-fast ones consists in use of a sum of the energy signals from the start and stop detectors to select the coincidence events. Quality of the spectrometer response function (≈ 150 ps FWHM for ^{22}Na) is very close to that observed for our fast-slow configuration with the equivalent detectors but throughput of the present apparatus is increased almost by a factor of two. Moreover, the electronic scheme becomes less complicated, which has also a positive impact on its cost.
The size of nanopores in thermally treated samples of silica aerogels was determined by ortho-positronium lifetime measurements at room temperature. The annealing of samples in the temperature region of 300-800°C does not make substantial changes in pore size felt by positronium. The annealing at higher temperatures from an interval of 800-1050°C causes shrinkage of the samples reducing the pore size to 1-15 nm. The macropores vanish at temperature treatment at 1000°C and the positron annihilation is not affected by oxygen from air.
The intensities of ortho-positronium components related to the pores and to small voids in the bulk were determined for a series of Vycor glasses. The ratio of "pore fraction" to the total o-Ps yield depends on the specific surface area of the porous medium. The diffusion coefficient for o-Ps was found equal to 3.7×10^{-5} cm^{2}/s.
Age-momentum correlation (AMOC) measurements provide time-resolved information on the momentum distribution of annihilating electron-positron pairs through correlated determination of Doppler broadening (∆E) of the 511 keV annihilation radiation line and positron age. In the β^{+}γ∆E AMOC technique implemented at the Stuttgart pelletron accelerator the start signals for the age measurements are generated when the relativistic positrons pass through a fast plastic scintillator, which has a detection efficiency close to unity. Beam-based β^{+}γ∆E AMOC measurements thus combine high β^{+}γ∆E triple-coincidence rates with low background. From a practical point of view this constitutes a major progress compared to conventional (source-based) γγ∆E AMOC measurements. The AMOC technique permits direct access to a wide variety of time-dependent processes, among them the reaction kinetics in positronium chemistry. As an example, we present a β^{+}γ∆E AMOC investigation of the kinetics of a spin-conversion reaction of positronium in methanol induced by the presence of a nitrosyl free-radical paramagnetic solute (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl , HTEMPO). The AMOC measurements allowed the time-resolved observation of the spin-conversion process, which had been investigated before by uncorrelated positron lifetime and Doppler-broadening measurements. It is found that within the concentration range investigated (≤ 0.1 mol/l) the spin-conversion reaction rate at room temperature depends linearly on the HTEMPO concentration with a reaction-rate constant k_{conv} = (22.5 ± 0.5) × 10^{9} l mol^{-1} s^{-1}.
Angular distributions of the positron annihilation quanta were measured for polycrystalline samples deformed by uniaxial tension up to different deformation degrees. The S parameter as a function of the W parameter was determined. The data obtained for samples elongated up to different elongation degrees indicate that in the proportionality and limited proportionality regions the changes in the physical properties of samples are governed mainly by generation of vacancies and formation and kinetics of transformations of vacancy clusters occurring first of all on the grains of monocrystallites. In the region of plastic deformations the dominant defects are dislocations and vacancies and their aggregates generated due to the formation and movement of the dislocation of the primary and secondary slip. The positron annihilation data are corroborated by the results of the resistometric and tensometric measurements in the proportionality and limited proportionality regions.
Positrons lifetimes were measured in medical substances of insulin (human and animal), differing as far as the degree of purity and time of their activity in the organism are concerned. In all of the cases the spectrum of positron lifetime was distributed into three components, with the long-life component ranging from 1.8 to 2.08 ns and the intensity taking on values from 18 to 24%. Making use of Tao-Eldrup model, the average radius of the free volume, in which o-Ps annihilated, and the degree of filling in the volume were determined. It was found that the value of the long-life component for human insulin is higher than that of animal insulin. Moreover, the value of this component clearly depends on the manner of purification of the insulin. It was also noticed that there occurs a correlation between the value of this component and the time after which it begins to be active in the organism, as well as the total time of its activity.
Angular distribution of the positron annihilation quanta was measured for monocrystalline gold samples, oriented in (110) and (111) directions. The samples were deformed by elongation for different deformation degrees. The S and W parameters as a function of deformation degree of the sample were determined. It was found that the dynamics of the dislocations and vacancy generation during the sliding of some crystallographic planes, depends on the crystallographic direction.
The reconstruction of the precipitation structure after solution treatment has been investigated by positron annihilation spectroscopy. In this study the behaviour of samples taken from an aircraft "Airbus 300", being in operation for 18 years, is compared to a reference material supplied by Alcoa. The results of position annihilation spectroscopy show that there are no significant differences in both materials. This finding is in agreement with the results obtained from other methods within the scope of the general research program which points to a materials behaviour as predicted for a safe operation of this type of aircraft.
The effect of positron polarization on annihilation characteristics for ferromagnetic alloy samples, Fe_{64}Νi_{36}, Fe_{63}Νi_{37}, Fe_{25}Νi_{75} and Fe_{50}Νi_{50}, has been studied. It was observed that the global two-quantum annihilation rate depends on mutual orientation of positron beam direction and the sample magnetization. The observed changes of the rate are opposite to the ones which could be expected from the effect of mutual orientation of positron and electron spins on positron backscattering. They can be explained as a result of changing of the relation between 3γ- and 2γ-annihilation probabilities.
The influence of interstitial hydrogen concentration and structural defects caused by quenching of palladium, begun at different temperatures up to the temperature of liquid nitrogen or by stretching by external forces, on spectra of lifetimes of positrons in this metal was investigated. It was found that the spectra of positron lifetimes in the examined systems are best described by their distribution into two components. On the basis of the two-state model of annihilation, the conclusion was drawn that the initial temperature from which quenching of palladium is commenced and also the relative elongation, which is caused by external forces, markedly influence the concentrations and sizes of defects in the sample.
A pulsing system for a slow-positron beam was applied to study the region near the surface of low-density polyethylene film using positron-annihilation lifetime measurement. The lifetime and intensity of ortho-positronium near the surface were measured as a function of the incident positron energy (1.6-9.1 keV). The size of intermolecular spaces in the surface region (~ 1500 nm) was larger than that in the bulk region. This tendency was especially strong in the region around 200 nm below the surface. On the other hand, the intensity of ortho-positronium decreased at lower incident positron energies, which was attributed to a decrease in the density of the spur electrons and/or an increase in the number of reemitted positrons from the surface.
The size and shape of the free volume in molecular crystals was estimated from crystallographic data. The o-Ps lifetime values were calculated from respective models and compared with the experimental data. The best agreement for channel-like voids has been obtained for the value of DR parameter equal 0.19 nm. The threshold value of radius allowing Ps trapping in the void was found to be (0.12-0.13) nm.
Positron lifetimes and Doppler broadening of the annihilation line measurements were performed to study the vacancy type defects in polycrystalline GeTe and (GeTe)_{(1-x)}(AgBiTe_2)_x (x=0,0.03,0.05,0.1,0.15,0.2, and 1) solid solutions. The values of lifetimes obtained are explained as due to positron and positronium saturated trapping at vacancies and vacancy clusters. The interdependence between mean positron lifetime and hole concentration is discussed.
The superstructures Cu_{3}Mn and Cu_{5}Mn have been characterized for the first time by X-ray and differential scanning calorimetry measurements. In addition, positron lifetime measurements are presented in order to show the defect structure of these alloys. It turned out that the annealing behaviour is equal for the two different alloys and the two different deforming processes. An annealing temperature of 300°C is not sufficient to obtain a defect free state.
The effect of annealing on free-volume properties of poly( ethylene terephthalate) was studied using a set of samples annealed for two hours in air at eight temperatures between 80ºC and 220ºC. The fractional free-volume in the samples was estimated from ortho-positronium lifetime and its relative intensity. The variations of fractional free-volume and of the macroscopic specific volume with annealing temperature were compared. The variation of the pore volume probability density functions with the annealing temperature was found. The interdependence between S- and W-parameters was also discussed.
The measurements of angular correlation of annihilation radiation and positron lifetime were performed to study defects in commercial stainless steel and nickel subjected to deformation and electron irradiation. It follows from variation of S-parameter derived from angular correlation of annihilation radiation spectra that hydrogen charging of Fe-Cr-Ni-Mn austenite steel results in appearance of modest amount of vacancies which agglomerate into small clusters during annealing at low temperatures. Electron irradiation produces much more defects, so that subsequent hydrogenation does not add much to their concentration. The major part of defects anneals out gradually to 700 K. The residual part is preserved up to 950 K. This fact testifies to bonding state of vacancies. Nickel of 3N purity was prepared in three initial states, i.e. cold-rolled and hydrogenated before and after deformation. In an initial state all samples contain, along with simple defects, vacancy clusters. Their size and concentration are much larger in hydrogenated samples, as compared with merely cold-rolled one. Annealing above 400 K causes growth of cluster size and decrease in their concentration. At 500 K clusters are observed to disappear in cold-rolled sample, while in the sample hydrogenated after deformation they survive to 600 K.
Three stages of thermal and chemical processing of Vycor glass were carried out. Raw material, the glass with small and big pores was studied. In each stage of study various crystallinity degrees of investigated material were obtained. The relation between crystallinity degree and the intensity of o-Ps components was determined. In nonporous material a high correlation between o-Ps intensity and crystallinity degree was found. In porous material the decrease in total o-Ps intensity can be a measure of the crystallinity degree.
The interaction between hydrogen and thermally induced defects has been studied in nickel samples using the positron annihilation technique. From measurements further evidence has been found for agglomeration of vacancies and small vacancy complexes into large-size three-dimensional void-like defects in the plastically deformed and electron beam irradiated samples containing hydrogen.
This paper presents positron lifetime studies of the subsurface region of AK12 aluminium alloy and Al_2O_3-particle-reinforced AK12 aluminium alloy composite after sliding against steel in the pin-on-disc machine. The defect depth profile detected in the AK12 alloy extended up to 300μm but for the composite AK12 the range of this profile was significantly shortened to less than 90μm. The positron lifetime dependence on depth evidences a steep gradient of defect concentration near the surface. The subsurface zones have been also examined using scanning electron microscopy.
Quantitative description of the effect of γ-irradiation of PE and PMMA on positronium formation on the trapped electrons at low temperatures is suggested. The consideration shows that, at least in these substances, the number of the trapping centers is probably enough to localize all ortho-positronium, and therefore the intensity of the long-lived positronium component is dependent mostly on the probability of Ps formation but not on the number of the trapping centers.
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