用正电子湮没寿命谱法研究铝阳极氧化膜

退火纯铝(99.99%)在15%H_2SO_4中于不同电解条件下进行阳极氧化, 然后测其正电子湮没寿命谱, 寿命谱拟合为三个寿命成份, 指认第二寿命成分为e~+在氧化膜中的湮没。从I_2随电解条件的变化推知膜厚与槽压近似成正比, 在恒温和固定槽压条件下, 膜厚只能达到某极限值。根据τ_2的变化推断出, 使用较高槽压或延长电解时间均使膜的结晶度提高, 它诱发了裂缝或微洞的形成, 导致膜体松散。根据上述结论进一步分析提高膜机械性能的可能途径。     

Application of a pulsed slow-positron beam to polymers

Pulsed slow positrons were produced using a time-varying moderator bias with an interval of 82 ns; 97% of the positrons were compressed within 2 ns width at the target position. Both the positron annihilation lifetime and Doppler broadening of the positron annihilation radiation (DBPR) of polytetrafluoroethylene (PTFE) were measured as a function of the incident energy of slow positrons. It was shown that the lifetime and intensity of the long-lived component of positron annihilation are independent of the positron incident energy above 1.2 keV. However, the width of the Doppler-broadened annihilation γ-ray increased in the energy region below 1.2 keV.     

Characterization of the surface layer of LB-films using a slow positron beam

Langmuir–Blodgett films were studied using a variable energy slow-positron beam. We measured the energy spectra of positron annihilation radiation for Cd and Mg eicosanoid films and obtained the V- and S-parameters as a function of the incident positron energy, E. In the V-E curves of Cd eicosanoid films, there were dips at the positron energy whose mean implantation depth corresponding to the first and second Cd²⁺ layers from the surface. These dips are interpreted as the result of inhibition of Ps formation by the Cd²⁺ ions. The S-parameter was found to be sensitive to chemical composition of the film and also to possible structural change due to heat treatment. Our results suggest that positron beams provide valuable information about the microstructure of the Langmuir-Blodgett films.     

Characterization of multilayer nanofiltration membranes using positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) coupled with a slow positron beam was used to characterize in situ the layer structure and depth profile of the cavity size in thin film composite (TFC) polyamide nanofiltration (NF) membranes prepared by the interfacial polymerization method. Two techniques, using PAS coupled with a slow positron beam of Doppler broadening energy spectra (DBES) and positron annihilation lifetime spectroscopy (PALS) designed to reveal the layer structure and the cavity sizes contained in a multilayer thin film composite NF membrane, were assessed. To the best knowledge of the authors, a characterization of the depth profile of cavities in NF membranes using PAS coupled with a slow positron beam has never been reported. The membranes selected have a composite structure containing three layers: a selective polyamide layer, a transition layer, and a porous support prepared by the phase inversion technique. Furthermore, the cavity size distribution in the selective top layer plays an important role in determining the performance of the NF membranes.     

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.     

Damage‐depth profiling of ion‐irradiated polyimide films with a variable‐energy positron beam

Various polyimide layers [2.2–2.6 μm of hexafluoroisopropylidene bis(phthalic anhydride‐oxydianiline), pyromellitic dianhydride‐oxydianiline, and 3,3′‐4,4′‐biphenyltetracarboxylic dianhydride‐p‐phenylenediamine] spin‐coated on silicon substrates were studied with a variable‐energy positron beam in combination with a Doppler‐broadened annihilation radiation technique. From the experiments, the thickness of the layers was estimated with the VEPFIT routine. These values corresponded well to the values determined from interferometry and ellipsometry. Irradiation of the polyimides with 1 × 10¹⁵ boron ions/cm² at an energy of 180 keV led to a strong chemical modification of the irradiated top layer. This caused the inhibition of positronium formation in the irradiated layer, which was observed as a lowering of the annihilation line S parameter. The thickness of the modified layer was estimated to be 700–800 nm. This value did not agree with the ellipsometric measurements but corresponded to the maximum implantation depth of boron ions calculated with TRIM (Transport of Ions in Matter) code. The positron results appeared somewhat larger than the TRIM estimates. Reasons for these relations are discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3062–3069, 2000     

An application of the coincidence Doppler spectroscopy for substances of chemical interest: phthalocyanine and acetylacetonate complexes

Coincidence Doppler spectroscopy, which is particularly powerful when one is concerned with high momentum components of positron annihilation gamma-rays, has been applied to two different kinds of organo-metalic ligands: metal phthalocyanines and metal acetylacetonates. The energy (momentum) profiles of the annihilation gamma-rays were the same for metal phthalocyanines indicating that positron and/or positronium are not interacting with the metal ions. However, the profiles for the metal acetylacetonates evidently showed a dependence on the kind of metal ions. Discussion is made on the features of positron interaction which are different for phthalocyanines and acetylacetonates.     

Development of positron annihilation spectroscopy to test accelerated weathering of protective polymer coatings

A variable mono-energetic positron beam with a computer-controlled system has recently been constructed at the University of Missouri–Kansas City for weathering studies of polymeric coatings. The beam is designed to measure the S-parameter from Doppler-broadening energy spectra and the sub-nanometer defect properties from positron annihilation lifetimes (PAL). Significant variations of S-parameter and ortho-positronium intensity in coatings, as obtained from the newly built beam and from the Electrotechnical Laboratory’s beam, respectively, are observed as a function of depth and exposure time due to the Xe-light irradiation. A high sensitivity of positron annihilation signal response to the early stage of degradation is observed. Development of positron annihilation spectroscopy to test accelerated weathering of polymeric coatings is discussed.     

Degradation of polymer coating systems studied by positron annihilation spectroscopy. II. Correlation with free radical formation

The photo‐degradation of polymer coating systems due to irradiation by UV and Xenon light sources is studied using positron annihilation spectroscopy and electron spin resonance (ESR). Doppler broadened spectra of positron annihilation, as a function of slow positron implantation energy and ESR spectra, are measured in two types of polyurethane which were exposed, ex situ, to UV irradiation for up to 800 h. The UV irradiation systematically decreases the S parameter as a function of exposure duration and increases the ESR signals. Thus, significant S parameter decrease is correlated with the ESR signal increase resulting from photo‐degradation of polymers due to UV irradiation. Parallel in situ positron annihilation and ESR experiments are performed as a function of Xenon light exposure for up to 100 min. These results show that the photo‐degradation of the polyurethane coatings involves initial free‐radical formation, which is correlated with the subnanometer defects detected by positron annihilation spectroscopy. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1289–1305, 1999     

Three photon annihilations of positrons and positronium in solids with two detectors in coincidence

The two-detector coincidence positron annihilation system at Washington State University was used to observe three-photon annihilation events of positrons. The decay of ortho-positronium (o-Ps) can be distinguished for para-Ps decay and positron two photon annihilations by the unique difference energy distribution. The apparatus was used to detect rare three-photon annihilations of positrons in metals.     

正电子湮没技术在纳米材料中的应

纳米材料在现实应用中表现出越来越优异的性能,其微结构往往对宏观性能有着重要影响。从正电子湮没技术基本原理出发,结合正电子湮没技术在材料微观结构领域的独特优势,介绍了其在纳米金属、合金以及纳米半导体等材料中的微结构研究工作,阐述了纳米材料微结构对基础学科和实际应用的重要意义。     

Metallic Na formation in/on NaCl crystals with irradiation by electron or vacuum ultraviolet photons

Metallic Na was formed in/on NaCl single crystals by irradiating them with a variety of radiation sources, namely, 21 MeV electron pulses, an electron beam of 30 keV and photon fluxes in the VUV region. The physical states were analysed using several methods, optical absorption, lifetime measurement of positron annihilation, Auger electron spectroscopy and UV photoelectron spectroscopy. Metallic Na was obtained in different physical states; clusters were formed in bulk, thin layers (islands) and thick layers on the surface.     

Characterization, transport and sorption properties of poly(thiol ester amide) thin-film composite pervaporation membranes

The effect of acyl chloride chemical structure on the ethanol aqueous solution dehydration through the poly(thiol ester amide) thin-film composite membrane prepared by reacting 2-aminoethanethiol (AETH) with trimesoyl chloride (TMC) or succinyl chloride (SCC) on the surface of the modified asymmetric polyacrylonitrile (mPAN) membrane was investigated. SEM/EDX, ATR-FTIR and water contact angle were applied to analyze the S element, chemical structure, and hydrophilicity of the poly(thiol ester amide) active layer of the composite membrane. In order to estimate the variation in the free volume of the poly(thiol ester amide) active layer and correlate that with the pervaporation performance, positron annihilation spectroscopy (PAS) experiments were conducted, in which a variable monoenergy slow positron beam was used. Doppler broadening S parameters of annihilation radiation energy spectra showed a significant variation with the acyl chloride chemical structures of the poly(thiol ester amide) active layers. The S parameters of the AETH–TMC/mPAN thin-film composite membrane were found to be lower than those of the AETH–SCC/mPAN thin-film composite membrane. In the ethanol aqueous solution dehydration, the AETH–TMC/mPAN thin-film composite membrane exhibited a lower permeation rate and a higher water concentration in the permeate than the AETH–SCC/mPAN. This is in good agreement with the analysis by positron annihilation spectroscopy. The solution effect dominated the pervaporation separation behavior of the poly(thiol ester amide) thin-film composite membrane with TMC substituting for SCC in the poly(thiol ester amide) active layer. The AETH–TMC/mPAN membrane was found to exhibit superior performance compared with some membranes discussed in the literature.     

Deterioration of a polyurethane coating studied by positron annihilation spectroscopy: Correlation with surface properties

Deterioration of a polyurethane coating by Florida natural environments as a function of time up to 16 weeks was studied by positron annihilation spectroscopy. Doppler broadening energy spectroscopy (DBES) of annihilation irradiation and positron annihilation lifetime (PAL) were measured as a function of incident positron energy (0–30 keV). A significant decrease in the S‐defect parameter from DBES and the intensity of orthopositronium from PAL was observed as a function of weathering time. This is interpreted as a loss of free volume and holes as a result of the weathering process. The gloss and surface morphology in the same system were measured by glossimetry and atomic force microscopy (AFM), respectively. The gloss decreased and surface roughness increased as a function of weathering time. The AFM images showed a new feature of a spherically coagulated microstructure on the surface after weathering. Direct correlations between the decrease in gloss and the increase in roughness as well as the decrease in the S‐defect parameter from the DBES data and in the free volume from the PAL data were observed. These results were used to discuss the weathering process in terms of chemical and physical changes as a result of photodegradation in protective polymeric systems. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2290–2301, 2001     

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.     

Characterization of film surface treated with ECR plasma by Doppler broadening

Doppler broadened positron annihilation measurements were carried out using the positron beam technique on plasma treated polyethylene films as a function of incident positron energy. In addition, surface properties of the treated films also have been measured using other conventional techniques such as FT-IR, SEM and AFM. The surface tension of the films was also determined using sessile drop method. The S-parameter is seen to decrease on the surface upon plasma treatment that introduces polar groups such as hydroxyl and carbonyl on the surface. The results are discussed.     

The application of Laplace inversion technique in analysis of positron lifetime spectra

In this paper, the Laplace inversion technique, i.e., CONTIN program, has been used to analyze the positron lifetime spectra to obtain continuous annihilation rate distribution (ARD). Two kinds of materials were studied by measuring the positron ARD. In dealuminated Y-type zeolite, five peaks were observed, and the longest component is related to o-Ps lifetime in the secondary pores. In GaAs and in InP semiconductors, the native defects were successfully identified by the difference in positron ARD shape. More evidently, when InP sample was irradiated with high energy heavy ions, the positron ARD showed difference with different irradiation dose. These results indicate that the CONTIN analysis is a good complement to the PATFIT program.Supported by the National Natural Science Foundation of China.     

Modeling positrons in molecular electronic structure calculations with the nuclear-electronic orbital method

The nuclear-electronic orbital (NEO) method was modified and extended to positron systems for studying mixed positronic-electronic wavefunctions, replacing the mass of the proton with the mass of the positron. Within the modified NEO framework, the NEO-HF (Hartree-Fock) method provides the energy corresponding to the single-configuration mixed positronic-electronic wavefunction, minimized with respect to the molecular orbitals expressed as linear combinations of Gaussian basis functions. The electron-electron and electron-positron correlation can be treated in the NEO framework with second-order perturbation theory (NEO-MP2) or multiconfigurational methods such as the full configuration interaction (NEO-FCI) and complete active space self-consistent-field (NEO-CASSCF) methods. In addition to implementing these methods for positronic systems, strategies for calculating electron-positron annihilation rates using NEO-HF, NEO-MP2, and NEO-FCI wavefunctions were also developed. To apply the NEO method to the positronium hydride (PsH) system, positronic and electronic basis sets were optimized at the NEO-FCI level and used to compute NEO-MP2 and NEO-FCI energies and annihilation rates. The effects of basis set size on NEO-MP2 and NEO-FCI correlation energies and annihilation rates were compared. Even-tempered electronic and positronic basis sets were also optimized for the e+LiH molecule at the NEO-MP2 level and used to compute the equilibrium bond length and vibrational energy.     

Positron lifetime spectroscopy as a probe of nanoporosity of cement-based materials

Hydrating cement pastes prepared with water-to-cement ratio equal to 0.5 in weight were investigated through positron annihilation lifetime spectroscopy over a period of 4 weeks. Two series of samples were prepared using different pre-treatment procedures, in order to assess the influence of sample manipulations on positron annihilation measurements. The technique is sensitive to changes in the pore structure with ageing time. Moreover, from the positron data an estimate of the first two moments of radii distribution of the nanopores was obtained.     

Study on the microstructure and mechanical properties for epoxy resin/montmorillonite nanocomposites by positron

Positron annihilation lifetimes have been measured for epoxy resin/organic montmorillonite (OMMT) nanocomposites. Effects of different dispersion states of nano-layered OMMT on the positron annihilation parameters and the mechanical properties were studied. We found that the ortho-positronium (o-Ps) intensity decreased with increasing OMMT content, which indicated that the interaction between the host and nanofillers restrained the segmental motion, resulting in a decrease of the free volume. On the other hand, it is very interesting to observe a good correlation between the interfacial interaction and mechanical properties, suggesting that the dispersion states of OMMT and interfacial property between clay layers and matrix played an important role in determining the mechanical properties.