Defect analysis with positron annihilation — Applications to Fe aluminides

Measurements of positron annihilation at various temperatures at constant pressure give information about the formation of thermal defects and their enthalpy in metals and alloys. In the FeAl system the formation enthalpy decreases from 1.6–2.0 eV in pure -Fe to 1.37 eV (Fe7at%Al) and 0.91 eV in the D0₃ structure. Additionally, the formation volume of the defects can be determined from measurements varying the pressure at constant temperature. The formation volume increases from 0.88 in Fe7at%Al to 1.42 in Fe40at%Al (=mean atomic volume). The change of the formation volume indicates different types of defects. With the further knowledge of the positron lifetimes in these materials we suggest triple defects as the main defects in this system. In quenching experiments the migration enthalpy is measured using the fast Doppler broadening technique. In Fe₃Al we observe a migration enthalpy of 0.5 eV, which is influenced by an apparing change of order in a temperature range above the D0₃ B₂ transition as determined from the phase diagram by Massalski.Thickness reduction of Fe6.3at%Si single crystals changes positron lifetimes with increasing deformation, particularly by rolling less than 5%.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Positron–electron correlation-polarization potential model for positron binding in polyatomic molecules

Positron binding energies (PBEs) of 41 polyatomic molecules were calculated using the positron–electron correlation-polarization potential (CPP) approach and compared with experimentally measured values. In this approach, the short-range positron–electron potential is modeled using the density-functional expression, whereas the long-range potential is approximated by the attractive polarization potential. The positron–electron CPP model based on local-density approximation yields larger PBEs than experimental values; however, the calculated values can be substantially improved by introducing generalized gradient approximation. We also investigated the conformational dependence of PBEs for representative molecules.     

Radiolabeled γ-AApeptides: a new class of tracers for positron emission tomography

A γ-AApeptide-based tracer for positron emission tomography imaging of integrin α(v)β(3) is reported. Despite its shorter sequence and linear nature, this tracer had comparable integrin α(v)β(3) binding affinity to the cyclic arginine-glycine-aspartic acid peptide but significantly higher resistance to enzymatic degradation and better stability.     

Mössbauer and positron annihilation studies of structural modifications of hemoglobin in solution

The reduction reaction mechanism of carrier-free¹²⁵Sb in HCl solution was studied kinetically. Sb(III) and Sb(v) were separated by solvent extraction using n-benzoyl-n-phenylhydroxylamine(BPHA) in chloroform at different constant time in interval and the reaction rate was determined by measuring the radioactivities of¹²⁵Sb in both organic and aqueous phases. Plot of log[¹²⁵Sb(V)/ [¹²⁵Sb(III)+¹²⁵Sb(V)] against the elasped time do not give straight lines. The curves can be solved to be ABC type reactions by a non-linear squares. On the basis of dependence of the reactions, overall reactions can be expressed as follows;     

Preliminary results of the positron annihilation in zeolites: Peak shape and 3γ-decay

In many sorts of zeolites long-living positron components are manifested, connected presumably with Ps-formation in various kinds of free space in the zeolite structure. Literature values demonstrate, however, a broad scasttr of data obtained for assumedly on the same composition of zeolite indicating the possible influence of tehnology, adsorbates, impurities, water content, etc. In the present work spectral changes of the 511 keV annihilation peak and its vicinity are evaluated, applying for the first time a combination of theS and W parameters and the 3-specific left plateau region measured in zeolites. To ensure reproducible experimental conditions, evaluated samples were measured. The data show individual temperature behaviours of the annihilation parameters in the studied temperature range of 90–450 K for each kind of samples. The temperature dependence becomes reproducible after the first heating run. The results are discussed with respect to the evolution of long-living positron and positronium components.     

Experimental comparison of the relative yield of 3γ/2γ positron annihilation using semiconductor and scintillation detectors

The yield of three photon positron annihilation is measured using semiconductor and scintillation detectors in a comparison for applications in positron emission tomography, particularly in the exploitation of three photon positron annihilation imaging where good energy resolution and good efficiency are required. In this experimental study four detectors, High-purity Germanium (HPGe), Sodium Iodide (NaI(Tl)), Lanthanum Chloride (LaCl₃:10%Ce³⁺) and Lanthanum Bromide (LaBr₃:5%Ce³⁺) were used. The peak-to-peak method was used with a ²²Na source to determine these yields. Aluminium was employed as a reference material as its high electron density reduces positronium formation and lifetimes. Teflon was also used in order to enhance the formation of ortho-positronium, since quenching is low, leading to increased three photon positron annihilation. The relative 3γ/2γ yields obtained were (3.04±0.11)·10⁻², (2.17±0.11)·10⁻², (3.26±0.10)·10⁻² and (2.03±0.11)·10⁻² for LaBr₃:Ce, LaCl₃:Ce, NaI(Tl) and HPGe detectors, respectively. Among these detectors LaBr₃:Ce proved to be the detector of choice for three photon imaging applications as it has both good energy resolution and efficiency.     

Syntheses of o-iodobenzyl alcohols‒BODIPY structures as potential precursors of bimodal tags for positron emission tomography and optical imaging

Aiming the faster development from bench to bedside of new potential tracers, multimodal tracers for positron emission tomography (PET) and optical imaging (OI) have emerged as a very promising tool. Indeed, they combine the simplicity of use of optical techniques for in vitro/in vivo pre-clinical studies with the various clinical possibilities offered by PET imaging using their radioactive versions. In this context, the preparation of new tags detectable by fluorescence imaging and potentially suitable for PET imaging after a last-step ¹¹C-labeling of the corresponding precursor has been investigated. Various designs and syntheses were explored by linking o-iodobenzyl alcohols and tetramethyl-BODIPY moieties together. Among them, the most promising structure was produced in 30% yield over five steps from a commercially available and inexpensive starting material.     

Efficient synthesis of [¹¹C]ramelteon as a positron emission tomography probe for imaging melatonin receptors involved in circadian rhythms

Ramelteon (TAK-375) is a novel melatonin receptor agonist that is used for clinical treatment of insomnia. The present report describes radiolabeling of ramelteon with the short-lived positron-emitter 11C (T(1/2)=20.4 min) by 2 methods. One method was [11C]methylation of an acetoamide precursor and the other was [11C]acylation of the corresponding amine precursor. First, [11C]methylation method showed the low reproducibility together with the production of many kinds of side products from which the [11C-methyl]Ramelteon was separated with chemical purity of <28% and radiochemical purity of >98%. Whereas, the [11C]acylation method showed high efficiency and reproducibility with a good radiochemical yield (22-43%, decay corrected), high chemical and radiochemical purities (>99% each), and high specific activity (43-162 GBq/μmol) (n=5) after HPLC purification. [11C]Ramelteon is a potential positron emission tomography (PET) probe for imaging the melatonin receptor.     

Three-gamma annihilation of ortho-positronium in NiO/γ-Al2O3 catalysts detected by positron lifetime and coincidence Doppler broadening measurements

The pore structure of NiO/γ-Al₂O₃ catalysts is characterized by positron lifetime and Doppler broadening measurements. A very long lifetime τ₄ of 92 ns is resolved from the positron lifetime spectrum measured for pure Al₂O₃, which could be attributed to the ortho-positronium (o-Ps) lifetime in large pores. It was also found that the fitted lifetime τ₄ and its corresponding intensity I₄ obtained from the lifetime spectra both decrease with narrowing energy window of the stop channel in the fast–fast coincidence lifetime measurement system. This suggests that the ultra long lifetime is primarily due to the self annihilation of o-Ps which emits three gamma-rays. Such 3γ annihilation is further evidenced by measuring the Doppler broadening of annihilation gamma rays in coincidence with the prompt gamma rays (1.28 MeV) emitted from the ²²Na positron source. In NiO/γ-Al₂O₃ catalysts both the lifetime τ₄ and its intensity I₄ decreases with increasing NiO content (from 3 wt% to 40 wt%), which indicates decreasing of the number of 3γ events. The 3γ annihilation parameter analyzed from the coincidence Doppler broadening spectrum shows consistent decrease with increasing NiO content.     

Simultaneous optimization of Gaussian type function exponents for electron and positron with full-CI wavefunction – application to ground and excited states of positronic compounds with multi-component molecular orbital approach

In order to obtain the best wavefunction of positronic compound with molecular orbital (MO) treatment, the full-configuration interaction (full-CI) fully variational MO (FVMO) method is proposed for multi-component systems, in which all the variational parameters in electronic and positronic wavefunctions are optimized under the full-CI scheme. We have applied the full-CI multi-component FVMO method to the ground and positronic-excited states of [H⁻;e⁺] system. Our treatment gives good improvement in the basis functions for positronic compounds owing to the extension of flexibility in the variational space, though the convergence of electron–positron correlation term is slower than that of conventional electron correlation.     

Why is monoalkylation versus bis-alkylation of the Ni(II) complex of the Schiff base of (S)-N-(2-benzoylphenyl)-1-benzylpyrrolidine-2-carboxamide and glycine so selective? MP2 modelling and topological QTAIM analysis of chiral metallocomplex synthons of α-amino acids used for the preparation of radiopharmaceuticals for positron emission tomography

Chiral Ni(II) complexes are used for the preparation of carbon-11 or fluorine-18 enantiomerically pure α-amino acids for positron emission tomography (PET). They enable the selective monoalkylation of a glycine synthon with high stereoselectivity and the preparation of enantiomerically pure α-amino acids with quarternary α-carbon. Molecular modelling of non-, mono- and di-substituted complexes using quantum theory of atoms-in-molecule (QTAIM) topological analysis of electron density allowed us to formulate a new theory explaining the reasons for highly selective monomethylation of the complexes. In the non-substituted complex (GK), the α-carbon atom exhibits a higher atomic volume and a more positive charge in comparison with mono- and di-substituted complexes. This unusual behaviour is accompanied by increasing the bond critical point (BCP) ellipticity of the iminic bond in GK explained by the higher mechanical strain. Both phenomena indicate the increased reactivity and probably originate in more compact core of GK where shorter distances in the internal coordination sphere result in the higher strain of its bonds.