Some applications of positron annihilation lifetime spectroscopy to complex formation and ion association studies

The rate constants for the reactions of positronium with halogen molecules and with halide ions in methanol (I₂, Br₂, Cl⁻, Br⁻) and in dimethylsulphoxide (I₂, Cl⁻) have been measured. The variations of the ortho-positronium lifetime in mixtures of these solutes have been used for quantitative determination of the formation constants of the corresponding polyhalides. These were found to be 2.9 and 5.4 M⁻¹, respectively, for I₂Cl⁻ and I₂Br⁻ in methanol and 2.0 M⁻¹ for I₂Cl⁻ in dimethylsulphoxide. Experiments on acidified AgClO₄ solutions confirmed the formation, at molar concentrations, of the Ag₂ ClO ₄ ⁺ species, very probably a triplet ion association.     

Application of positron annihilation technique to reverse osmosis membrane materials

Positron annihilation lifetime spectroscopy has been adopted as a new approach for studying vacancies of reverse osmosis membrane materials composed of cellulose acetate films and aromatic polyamide resins. The intensity of the ortho-positronium (o-Ps) lifetime increased with the amount of vacancies determined using N₂ isotherm at −195°C. Changes of vacancy profiles induced by heat treatment in the cellulose acetate films were detected using o-Ps. It was found that the positron annihilation technique is applicable to the study of vacancy profiles associated with salt selectivity in typical reverse osmosis membranes.     

Life science research using positron annihilation spectroscopy: UV-irradiated mouse skin

Positron annihilation spectroscopy (PAS) is applied to study mouse skin under different UV irradiations as a function of positron incident energy (0–30 keV). Significant variations in the depth profile of S parameter are observed in a period of hours and of days for UVA and UVB exposures, respectively. The high sensitivity of positron annihilation signals responding to UV irradiation shows that PAS may be developed as a new noninvasive technique for the detection of molecular damage in life science research.     

Evaluation of nonuniformity of polymeric membrane materials by positron annihilation technique

Positron annihilation lifetime distribution was experimentally studied in some polymers, including polymeric membrane materials, in an air and a nitrogen atmosphere. Basic attention was paid to the long-lived distribution component, i.e., to annihilation of orthopositronium (the positron-electron bound system). It was found that ambient oxygen affects the annihilation characteristics of positronium. Newly developed ideas on the mechanisms of formation, localization, and annihilation of positronium in the polymers lead to the conclusion that the distribution of free volumes of different size in the polymeric matrix is not uniform. The number concentration and size of free-volume holes and the size of microirregularities containing these holes were determined     

Water intake of cellulose materials monitored by positron annihilation lifetime spectroscopy

Cellulose - In this study, for the first time, the experimental technique of positron annihilation lifetime spectroscopy (PALS) has been applied to monitor in situ the microstructural changes of...     

Time-differential perturbed angular correlation studies of in-complexes used in nuclear medicine

For determination of the stability of In³⁺ complexes with diethylenetriaminepentaacetic acid (DTPA) and newly synthesized chelates-derivatives of 4-amino-2-hydroxybutyric acid in aqueous solutions of pH 1.0–7.5, at room temperature, the time differential perturbed angular – correlation method was used. Data obtained show that only one of the new ligands forms as strong a complex as DTPA and can be used in water at pH7.     

Thermalization of positronium atoms studied with time-resolved angular correlation of annihilation radiation

Time dependence of the kinetic energy of positronium atoms in the free space between the grains of a pressed tablet (1 g/cm³) of ultrafine silica powder (Cab-O-Sil EH-5) has been measured with time-resolved angular correlation of annihilation radiation (ACAR) apparatus. The apparatus has a momentum resolution of 1.29×10⁻³ mc in full width at half maximum and a time resolution of 2.7 ns in full width at half maximum. It is found that the energy of positronium falls below 0.1e V in 10 ns after the formation.     

Positron annihilation in pivalic acid. Temperature dependence of angular correlation curves

Positron annihilation angular correlation curves have been measured as a function of temperature for trimethylacetic (pivalic) acid in both the brittle and plastic phases. A simple fitting of the data to a sum of three gaussians shows the presence of a narrow component due to para-positronium (p-Ps) annihilation. In the brittle phase the intensity of the narrow component is inconsistent with previous positron lifetime data. A more detailed analysis, requiring consistency with the lifetime data, results in the determination of the shapes of the angular correlation components for free positron-, pick-off-, and p-Ps intrinsic annihilation. The p-Ps component has a width (fwhm) of 3.75 mrad in the brittle phase, probably due to Ps self-trapping or trapping in defects smaller than molecular vacancies. In the plastic phase the width (fwhm) is 3.25 mrad which is ascribed to Ps localization in vacancies and divacancies in accordance with positron lifetime data.     

Some physical aspects of positron annihilation tomography: A critical review

Positron emission tomography (PET) is an imaging modality for medical diagnoses that can determine biochemical and physiological processes in vivo in a quantitative way by using radio-pharmaceuticals labeled with positron emitting radionuclides. This article brings together various aspects of the basic physics, critical design and instrumentation along with the modalities of the application of radiotracers and the radiological protections involved in the processes. A critical discussion on the various aspects of the PET system is also included. Several new advances and scope of future investigations in terms of better sensitivity, local as well as kinetic resolution, specific tracer targeting (including chemical speciation) and better spatial resolution of the PET image are also covered.     

Hole structures of polymers and zeolites probed by two-dimensional angular correlation of annihilation radiation

The two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique has been developed to measure the hole structure of stretched and oriented polymers. The determination of polymer hole structure is demonstrated in a stretched semi-crystalline polyaryl-ether-ether-ketone (PEEK) sample. The atomic scale anisotropy is found to be consistent with the macroscopic scale stretch ratio. Applications of the 2D-ACAR method to image the three-dimensional hole, free-volume, and cavity structures of molecular systems are discussed.     

The time differential perturbed angular correlation study of binding of hafnium to humic acid and its model compound

Time differential perturbed angular correlation (TDPAC) spectroscopy has been used to study the binding of hafnium to mandelic acid, which is a model compound of humic acid. The Fourier transform of TDPAC spectrum of Hf–mandelate complex showed two ω values, namely, ω_Q1 = 77 (±0.4) Mrad/s and ω_Q2 = 111 (±3.1) Mrad/s. The asymmetry parameters for the two sites were found to be 0.7 and 0.3, respectively. The electric field gradient (EFG) around the metal ion was theoretically calculated using the computer code GAMESS. The calculated EFG value was found to be in reasonable agreement with that deduced from experiment. Contrary to Hf–mandelate, the TDPAC spectrum of Hf–humate system did not show characteristic features of discrete binding sites.     

Evaluation of scintillators and semiconductor detectors to image three-photon positron annihilation for positron emission tomography

Positron emission tomography (PET) is rapidly becoming the main nuclear imaging modality of the present century. The future of PET instrumentation relies on semiconductor detectors because of their excellent characteristics. Three-photon positron annihilation has been recently investigated as a novel imaging modality, which demands the crucial high energy resolution of semiconductor detector. In this work the evaluation of the NaI(Tl) scintillator and HPGe and CdZTe semiconductor detectors, to construct a simple three-photon positron annihilation scanner has been explored. The effect of detector and scanner size on spatial resolution (FWHM) is discussed. The characteristics: energy resolution versus count rate and point-spread function of the three-photon positron annihilation image profile from triple coincidence measurements were investigated.     

Path integral methods of positron annihilation

The problem of computing the properties of a low mass quantum particle in equilibrium in a disordered medium is considered. With the advancement of computational speed, statistical methods for sampling a complex phase space are now viable. The Feynman-Kac path integral establishes a connection between a quantum particle and classical polymer consisting of p atoms. This allows the computation of quantum mechanical equilibrium values using well known methods devised for classical systems. Here we review the application of the path integral to the computation the properties of thermalized positron and positronium and introduce some new directions of investigation.     

Evaluation of positron annihilation in Beryllium

A band theoretical calculation of angular distribution of annihilation photons (ADAP ) and the positron lifetime, up to and including higher momentum components is reported. The results obtained using biquadratic quasianalytic integration schemes for all Brillouin zone integrals, compare favorably with existing experimental data. There are, however, significant structures in the ADAP curve obtained than in the experimental curves.     

Time differential perturbed angular correlation (TDPAC) study of some hafnium glycolate complexes

Time differential measurements of the 133–482 keV γ—γ angular correlation of ¹⁸¹Ta have been performed in solid hafnium complexes of the type [Hf(glycol)₄] (glycol = p-chloromandelate, p-bromomandelate, b-naphthylglycolate and mandelate). The quadrupolar frequencies (ω_Q) were found to increase within the series. This behaviour was explained by supposing a dissipation of electronic density at the metal level, caused by an increase in intermolecular hydrogen bonding in the complexes throughout the series studied. These results seem to agree with complementary data from IR spectroscopic studies.     

Anisotropy of hole structures in oriented polycarbonate probed by two-dimensional angular correlation of annihilation radiation

Two-dimensional angular correlation of annihilation radiation (2D-ACAR) experiments have been performed on an uniaxially oriented polymer, bisphenol-A polycarbonate. The two-dimensional hole structures of unoriented and oriented polymers, determined from the momentum distributions, are found to be spherical and ellipsoidal, respectively. The average hole radii are determined to be 2.9 Å in the unoriented sample and between 2.3 and 3.8 Å in the oriented sample. Applications of the 2D-ACAR method to image the three-dimensional hole structures of polymeric materials are discussed. © 1996 John Wiley & Sons, Inc.     

Photoelectron microscopy and applications in surface and materials science

We review the recent achievements of photoelectron microscopy (PEM), which is a rapidly developing technique that is significantly advancing the frontiers of surface and materials science. The operation principles of scanning photoelectron microscopes (SPEM), using different photon optic systems to obtain a micro-probe of sub-micrometer dimensions, and of the full-field imaging microscope, using electrostatic lenses for magnification of the irradiated sample area, are presented. The contrast mechanisms, based on photon absorption and photon-induced electron emission, are described and the expected development in the photon and electron optics and detection systems are discussed. Particular attention is paid to the present state-of-art performance of the microscopes collecting photoelectrons (PEs), which carry specific information about the lateral variations in the chemical, magnetic and electronic properties of the material under investigation. Selected results, obtained recently with instruments installed at synchrotron light facilities, are used to illustrate the potential of PEM in characterising micro-phases and dynamic processes with a lateral resolution better than 100 nm.     

Structural heterogeneity in elastic and glassy polymeric materials revealed by positron annihilation and other supplementary techniques

Elastic and glassy polymers are normally considered as homogeneous disordered materials with unimodal (statistical) distribution of elementary free volumes. In this work, we discuss the results of our positron annihilation lifetime (PAL) measurements for a number of elastic and glassy polymers, where observations of the two long-lived ortho-positronium components in the PAL spectra and irregular (nonlinear) variations of annihilation characteristics in the vicinity of glass transition temperature reveal structural heterogeneity of these systems. The conclusion is confirmed by measurements of thermo-stimulated luminescence, thermo-mechanics and mobility of penetrants.     

Some thermodynamic aspects of nitrides in materials science

The energies of combustion in fluorine of gallium nitride and indium nitride in wurzite crystalline structure have been measured in a two-compartment calorimetric bomb, and new standard molar enthalpies of formation have been calculated: Δ_fH_m⁰(GaN(cr) 298.15 K)= –(163.7±4.2) kJ mol^–1 and Δ_fH_m⁰(InN(cr) 298.15 K)= –(146.5±4.6) kJ mol^–1 . Comparison with the recommended values of the Δ_fH_m⁰ nitrides from the literature is also presented.     

Perspectives of positron annihilation spectroscopy in heterogeneous catalysis

The use of polychromatic positron annihilation spectroscopy (PAS) in heterogeneous catalysis is reviewed and its potential applications are discussed. This review covers early and current studies of catalytic oxides and supported metals, refers to the potential use of PAS in the characterization of catalytic sites, and outlines the future of the technique.