Positron Lifetime Experiments Research Articles

Analysis of the experimental positron lifetime spectra by neural networks

This paper deals with the analysis of experimental positron lifetime spectra in polymer materials by using various algorithms of neural networks. A method based on the use of artificial neural networks for unfolding the mean lifetime and intensity of the spectral components of simulated positron lifetime spectra was previously suggested and tested on simulated data [Pzzsitetal, Applied Surface Science, 149 (1998), 97]. In this work, the applicability of the method to the analysis of experimental positron spectra has been verified in the case of spectra from polymer materials with three components. It has been demonstrated that the backpropagation neural network can determine the spectral parameters with a high accuracy and perform the decomposi-tion of lifetimes which differ by 10% or more. The backpropagation network has not been suitable for the identification of both the parameters and the number of spectral components. Therefore, a separate artificial neural network module has been designed to solve the classification problem. Module types based on self-organizing map and learning vector quantization algorithms have been tested. The learning vector quantization algorithm was found to have better performance and reliability. A complete artificial neural network analysis tool of positron lifetime spectra has been constructed to include a spectra classification module and parameter evaluation modules for spectra with a different number of components. In this way, both flexibility and high resolution can be achieved.

Dependence of positron lifetime parameters on the preparation techniques and structural defects for YBCO cuprates

Based on possible existence of technique effect and scattered distribution in positron results, a special investigation was made for YBa 2Cu 3O 7− δ systems in this paper. The dependence of preparation techniques and the structural characteristics on the sintering temperature and time are also given by positron lifetime, XRD and SEM experiments. The results show that the positron lifetime parameters and the microstructure show a significant dependence on sintering temperature and time. It is proved that, in the range of 920–950 °C/12–72 h, the positron results have good stability and reliability in the sintering times and temperature. The distribution characteristic of defect was evaluated and had a good stability in the preparation technique. The present studies provide an important experimental evidence for the study of Y-123 systems by positron experiment.

Theoretical Calculation of Positron Lifetimes in CoAl and CoTi

Positron states for the bulk and vacancy in intermetallic compounds CoAl and CoTi crystallizing in the B2 structure have been calculated using the DV-Xα electronic structure calculation and compared with experimental results. The calculated positron lifetimes in CoAl can explain the compositional dependence of the positron lifetimes in CoAl. The calculated positron lifetime at the Ti vacancy in CoTi is longer than that at the Co vacancy. The charge transfer from Ti to Co leads to the difference of the localization of the positron wave function at the vacancy. At the Co vacancy site surrounded by the positively charged Ti atoms, the positron wave function does not localize as well as at the Ti vacancy site, which decrease the positron lifetime. The calculated results suggest that the positron lifetimes in intermetallic compounds can not be directly correlated to those of the pure component and the charge transfer between the constituent atoms affects the localization of the positron wave function. Intermetallic compounds are of considerable interest due to their physical and mechanical properties. In intermetal- lic compounds having a wide range of concentrations around the stoichiometry, structural defects are introduced in order to compensate deviations from the stoichiometry. Several prop- erties show compositional dependence originating from the structural defects. Main structural defects are vacancies on the sublattice and anti-structure atoms occupying the other sublattice. Positron annihilation spectroscopy is a sensitive tool for vacancy-type defects in solids. In a perfect crystal, the positron wave function is delocalized in the interstitial region because of the repulsion from the ion cores. If vacancy-type defects exist in solids, the positron wave function is localized at the defect where the electron density is lower than the other regions. The localization of the positron at the defect results in a longer lifetime of the positron compared to the bulk life- time because the positron lifetime is inversely proportional to the electron density where the positron annihilated. Positron annihilation spectroscopy is very useful to study the behav- ior of the structural defects in intermetallic compounds. If the vacancy is mainly responsible for the deviation from the stoichiometry, longer positron lifetimes should be measured compared to the bulk lifetime. However, interpretation of the experimental positron lifetime for intermetallic compounds is more complicated than for pure metals, because several types of vacancies may exist in intermetallic compounds. The dif- ferent types of vacancies may yield the different positron life- times. In order to characterize measured positron lifetimes in intermetallic compounds, a theoretical analysis is indispens- able. However, little theoretical work has been done for inter- metallic compounds. In this work, we have performed calculations of positron lifetimes in intermetallic compounds CoAl and CoTi with the B2 structure based on first-principles electronic structure cal- culations. The calculated results are compared with the exper- imental positron lifetimes in order to identify the defect type in intermetallic compounds and estimate the reliability of the calculated positron lifetimes. 2. Computational Method

Positronium scattering from closed-shell atoms and ions

The scattering of orthopositronium from He, Ne, Ar, ${\mathrm{Li}}^{+},$ ${\mathrm{Na}}^{+},$ and ${\mathrm{K}}^{+}$ is investigated with the fixed-core stochastic variational method. The scattering length for $\mathrm{Ps}\ensuremath{-}\mathrm{He}$ scattering was ${1.57a}_{0},$ a value consistent with the threshold cross sections derived from three positron lifetime experiments. The scattering lengths for the $\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{N}\mathrm{e}$ and $\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{A}\mathrm{r}$ systems were ${1.55a}_{0}$ and ${1.79a}_{0},$ respectively. That there was only a 15% variation among the scattering lengths is compatible with the experiment of Coleman et al. [J. Phys. B 27, 981 (1994)], who found the low-energy cross sections for He, Ne, and Ar to be about the same size. The scattering lengths for Ps scattering from ${\mathrm{Li}}^{+},$ ${\mathrm{Na}}^{+},$ and ${\mathrm{K}}^{+}$ were ${12.9a}_{0},$ ${28.5a}_{0},$ and $\ensuremath{-}{1.9a}_{0}.$ The relatively small magnitude of the ${\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{K}}^{+}$ scattering length strongly supports previous suggestions that the ${\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{K}}^{+}$ system does not support a bound state. The annihilation parameter ${}^{1}{Z}_{\mathrm{eff}}$ has also been computed as part of the analysis. The present values of ${}^{1}{Z}_{\mathrm{eff}}$ are about 2.5--3 times smaller than the accepted experimental values since short-range electron-positron correlations were not taken into consideration when the annihilation matrix element was evaluated.

POSITRON ANNIHILATION IN OXYGEN-DEFICIENT YBa2Cu3O7-δ CUPRATE AT 77 K AND 300 K

The YBa2Cu3O7-δ cuprate with oxygen-deficiency (δ=0.06~0.68) has been systematically studied by positron lifetime experiment at 77 K and 300 K. It is found that there exists an evident dependence of positron lifetime parameters on oxygen-deficiency δ and an abrupt change near the orthorhombic–tetragonal phase transition. The local electron density n e and vacancy concentration C v are evaluated as a function of oxygen deficiency δ. The effect of the mechanism of charge transfer on local electronic structure is given in detail. The results show the existence of a weak localization of electrons in the tetragonal phase. The positron annihilation mechanism and its relation to superconductivity are also discussed.

Hole carrier localization and Pr substitution in YBCO systems by positron experiment

Pr-substituted YBCO system has been studied by positron lifetime experiment in the range from 0.0 to 1.0. The present results show that the value of short lifetime τ 1 in PrBa 2Cu 3O 7− δ is nearly the same as in the isostructural YBa 2Cu 3O 7− δ and the defect-related positron lifetime component τ 2 decreases as a function of Pr-substitution x. The increase of local electron density n e reflects the electronic intense localization. These experimental results are interpreted in terms of the localization of hole carriers in the Cu–O chains on Pr substitution due to disorder in the Ba and Pr planes by the partly occupation of Ba 2+ ion site with Pr 3+ ion. These will destroy the intrinsic conductivity of the Cu–O chains and so impresses the superconductivity above x=0.6 Pr-substitution concentration.

Direct identification of As vacancies in GaAs using positron annihilation calibrated by scanning tunneling microscopy

We report on the identification of native vacancies in GaAs by positron annihilation with a special emphasis on As vacancy-related defects. In annealed highly Si-doped GaAs, we observe a neutral vacancy defect with a positron lifetime \ensuremath{\tau} of 280--285 ps and a high intensity of the core annihilation, in contrast to Ga vacancies which exhibit a lifetime of \ensuremath{\sim}260 ps and a lower intensity of the core annihilation. This defect is identified by scanning tunneling microscopy measurements to be an As vacancy ${\mathrm{Si}}_{\mathrm{Ga}}$ donor complex. We find that the same defect is also present in low n-doped GaAs, where it was earlier assigned to a neutral As vacancy. The high positron lifetime is explained by a large outward lattice relaxation. Theoretical calculations of the momentum distribution employing free atomic wave functions are in good agreement with the experimental results, provided only relative changes are considered and an outward lattice relaxation is included which yields the experimental positron lifetime. These calculations also yield annihilation parameters for Ga vacancies, in good agreement with the experiment. Our results demonstrate that vacancies in both sublattices of GaAs can directly and unambiguously be identified by positron annihilation once the annihilation characteristics are calibrated by a complementary method. On the basis of this identification, the abundance of As vacancies in GaAs is discussed in terms of stoichiometry and formation energies.

Positron Lifetime Measurements in Nanostructured Ni-Al Samples

ABSTRACTPositron lifetime spectroscopy is an effective tool to study various types of defects in materials including nanostructured ones. The size of free volumes associated with defects can be estimated using the lifetime components found in the measured spectra. Positron lifetime experiments are performed on nanocrystalline Ni-Al samples synthesized by the inert-gas condensation technique. The samples are further characterized by means of X-ray diffraction, electron diffraction and microscopy techniques as well as density measurements. In the lifetime spectra we observe three lifetime components corresponding to different annihilation sites in the samples. These lifetimes are compared with the results of simulations of positron lifetimes in modeled Ni-Al samples obtained using molecular dynamics and Monte Carlo calculations. Finally, we present positron lifetime results for nanocrystalline Ni3Al samples which were produced or annealed at different temperatures.

Characteristic of local electron structure, O–T phase transition and superconductivity for oxygen-deficient YBa 2Cu 3O 7− δ systems

The oxygen deficient YBa 2Cu 3O 7− δ ( δ=0.06∼0.68) has been studied by positron lifetime experiment at normal state 300 K. It is found that there exists an abrupt change of positron lifetime parameter near Orthorhombic–Tetragonal phase transition. The local electron density n e and vacancy concentration C v are evaluated as a function of oxygen deficiency δ. The effect mechanism of local electronic structure on charge transfer and superconductivity is given in detail. The results show the existence of weak localization of electrons in tetragonal phase. The positron annihilation mechanism and its relation with superconductivity are also discussed.

Positron transmission into Kapton: The effective mass absorption coefficient of relevance to positron lifetime experiments

The penetration of positrons from a Na22 source into Kapton foils sandwiched between different backscattering materials (Al, Cu, and Pt) has been studied for the geometry characteristic of positron annihilation techniques. Kapton has served as absorber and each of the selected metals as detector. The method, based on measuring the positron lifetimes, makes use of the difference in values of the fractions of positrons annihilating inside and outside the bulk matrix of the detector. A model on positron transmission and backscattering into 2π solid angle was constructed for the purpose. A correction term was added to the classical exponential form of the transmitted intensity, for absorber thickness z smaller than a critical thickness zc above which the transmitted intensity function falls exponentially. Numerical analysis of the data based on the model function has revealed the existence of two effective mass absorption coefficients pertaining, respectively, to the regions below and above zc. The coefficient αs+ for z<zc of relevance to positron lifetime experiments was found to be considerably smaller (by a factor 3) than the coefficient αl+ extracted by exponential regression measurements for sufficiently large absorber thicknesses. The need for new data on the coefficient for backscattering into 2π solid angle led to measuring the latter for the three used metals, yielding in the end the result αs+=14.12 cm2 g−1. The observed gap between αs+ and αl+ is qualitatively discussed.

Positron annihilation study on γ-irradiated cellulose acetate matrix

A cellulose acetate matrix was irradiated in the dose range 0–250 kGy by y radiation from a 60Co source under an argon atmosphere; positron lifetime experiments were performed under the same atmosphere at room temperature. The results reveal a dominant intermediate component τ 2 (∼430 ps) attributable to a crystalline macrocellulosic domain. Important changes are shown by the long-lived pick-off component τ 3 due to the degradation in the polymer matrix.

A study of porous silicon prepared under different HF concentrations by positron annihilation

Two-dimensional angular correlation of positron-electron annihilation radiation and positron-lifetime experiments have been performed on lightly doped porous silicons prepared under different HF concentrations. Positronium formation in the etched pores and positron trapped in voids are observed in both experiments. A surprising result is found that both positron lifetime and momentum spectra show a reduction in the size of the etched pores with decreasing HF concentration in the etching solution. This trend is different from the intuitive expectation that the pore size increases with increasing porosity. Our result can be explained in terms of the formation mechanism of porous film in lightly doped silicon.

Transmission of positrons with aβ + energy distribution through thin films

The transmission through Al foils of isotropically implanted positrons from a22Naβ+ source has been measured. It is shown that the transmission is reasonably well-described using an exponential profile once backscattering is accounted for, except for thicknesses below approximately 7 mg/cm2. Below this thickness, the measured transmission is slightly less than that predicted by the exponential profile. Such a deviation has previously been observed for collimated positrons, suggesting that the implantation profile has no significant dependence on the spatial distribution of the incident positrons. This deviation is critical for the proper interpretation of positron lifetime experiments on thin films using a conventional positron lifetime spectrometer.

A positron annihilation investigation of porous silicon

Positron lifetime experiments have been conducted on 13 different porous silicon layers formed from p+, p, and n+ substrates. Four distinct positron lifetimes could be associated with the porous layers. One lifetime was close to 0.5 ns and is claimed to arise from positrons trapped by small vacancy clusters. Positronium is formed either on the surface with a lifetime of ∼5 ns, or in the pores, from which exceptionally long lifetimes of up to 90 ns originate. Both of these lifetimes arise from the pickoff annihilation of ortho-positronium; the fourth lifetime is the usual 125 ps component associated with para-positronium. The long lifetime component gives rise to 3-γ annihilations which are readily detectable in the energy spectrum of the annihilation quanta. In situ heat treatments of two of the samples provide evidence that gases play a very important role in the passivation of defects, both in the native oxide layer and on the surface, albeit on very different time scales.

Using positron 2D-ACAR as a probe of point defects in GaAs: The As vacancy as a case study

Two-Dimensional Angular Correlation of positron Annihilation Radiation (2D-ACAR) experiments have been performed on n-type GaAs. By combining these results with those from positron lifetime experiments, the momentum distribution of the arsenic vacancy in its neutral (V aAs 0 ) and negative (V As − ) charge states have been extracted. These distributions were all normalized to the respective positron lifetime that characterizes them. The first thing to be noticed is that the momentum distributions of the vacancies, as seen by the positron, are fairly isotropic and structureless. The distribution forV As 0 is more peaked than that ofV As − while the latter is more intense in the large momentum regions of the spectra. From this, it can be inferred that VA. has a smaller open volume thanV As 0 A closer look at the momentum distribution of the vacancies reveals that they are not entirely isotropic, but, in fact, have a bulk-like component. Finally, the experimental results for bulk GaAs andV As − compare well in a qualitative manner with the momentum distributions that result from an ab-initio molecular dynamics calculation.

Introduction of Metastable Vacancy Defects in Electron Irradiated Semi-Insulating GaAs

Positron-lifetime experiments have been performed to investigate the metastability of the point defects produced in the electron irradiation of semi-insulating GaAs. The measurements in darkness indicate the presence of Ga vacancies and Ga antisite defects in a negative charge state. Illumination at 25 K reveals another type of a defect, which has a vacancy in its metastable state. The metastable vacancies can be observed most effectively after illumination with 1.1-eV photons and they are persistent up to the annealing temperature of 80--100 K. The introduction rate of the metastable defects is about 0.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is close to the values reported earlier for the As antisite. The metastable properties of the defects resemble those of the well-known EL2 center in as-grown GaAs. We associate these defects to As antisites, which exhibit the metastability predicted by the theory: in the metastable configuration the As antisite atom relaxes away from the lattice position, leaving a Ga site vacant.

Introduction and recovery of point defects in electron-irradiated Te- and Si-doped GaAs studied by positron lifetime spectroscopy.

Positron lifetime experiments have been performed to study the recovery of point defects in electron-irradiated n-type GaAs, which has been completely compensated in the irradiation. Irradiation temperature was 20 K and isochronal annealings were performed from 77 to 650 K. After all annealings below 450 K positrons detect irradiation-induced Ga vacancies and Ga antisite defects in a negative charge state. The main recovery stage of Ga is at 200--300 K. The Ga antisites anneal out at 500 K. Since the n-type conductivity reappears also after annealing at 500 K, we conclude that the negative Ga antisites have an important role in the compensation of n-type GaAs in the electron irradiation. Removal of compensation at 500 K reveals irradiation-induced As vacancies, which are converted from positive charge state to neutral or negative. The As vacancies introduced in the irradiation have similar ionization levels to those found in as-grown GaAs. The As vacancies recover at 550--650 K and their introduction rate is about 0.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ after annealing at 550 K. The low value of the introduction rate may indicate that the recovery of the As vacancies starts already before 550 K.

Evidence of two kinds of acceptors in undoped semi-insulating GaAs: Positron trapping at gallium vacancies and negative ions.

Positron-lifetime experiments have been performed in Zn-doped p-type and undoped semi-insulating GaAs in the temperature range 20--300 K to investigate native point defects. In p-type materials with hole concentrations of ${10}^{15}$--${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, no evidence of positron trapping is observed. The temperature dependence of the positron lifetime can be explained in terms of lattice expansion associated with positron-phonon coupling. Therefore, we ascribe it to delocalized positrons. In semi-insulating GaAs, two kinds of acceptors are detected with concentrations in the range ${10}^{15}$--${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$: gallium vacancies and negative ions. The temperature dependence of the positron trapping at the Ga vacancy exhibits a slope break at about 130 K. A weakly bound Rydberg-like precursor state is invoked to explain this temperature dependence.

Introduction of metastable vacancy defects in electron-irradiated semi-insulating GaAs.

Positron-lifetime experiments have been performed to investigate the metastability of the point defects produced in the electron irradiation of semi-insulating GaAs. The measurements in darkness indicate the presence of Ga vacancies and Ga antisite defects in a negative charge state. Illumination at 25 K reveals another type of a defect, which has a vacancy in its metastable state. The metastable vacancies can be observed most effectively after illumination with 1.1-eV photons and they are persistent up to the annealing temperature of 80--100 K. The introduction rate of the metastable defects is about 0.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is close to the values reported earlier for the As antisite. The metastable properties of the defects resemble those of the well-known EL2 center in as-grown GaAs. We associate these defects to As antisites, which exhibit the metastability predicted by the theory: in the metastable configuration the As antisite atom relaxes away from the lattice position, leaving a Ga site vacant.

Deformation-induced microvoids in bismuth and possible positronium formation

Positron annihilation spectroscopy is used to study the annealing stages in cold worked single and polycrystalline bismuth. A large drop (onset approximately 240 K) is observed in the Doppler-broadening lineshape measurement, in agreement with previous work. Results of positron lifetime experiments, conducted around the main annealing stage, suggest the formation of positronium at microvoids in cold worked Bi.