Exploring positron characteristics utilizing two new positron–electron correlation schemes based on multiple electronic structure calculation methods

We make a gradient correction to a new local density approximation form of positron-electron correlation. The positron lifetimes and affinities are then probed by using these two approximation forms based on three electronic-structure calculation methods, including the full-potential linearized augmented plane wave（FLAPW） plus local orbitals approach,the atomic superposition（ATSUP） approach, and the projector augmented wave（PAW） approach. The differences between calculated lifetimes using the FLAPW and ATSUP methods are clearly interpreted in the view of positron and electron transfers. We further find that a well-implemented PAW method can give near-perfect agreement on both the positron lifetimes and affinities with the FLAPW method, and the competitiveness of the ATSUP method against the FLAPW/PAW method is reduced within the best calculations. By comparing with the experimental data, the new introduced gradient corrected correlation form is proved to be competitive for positron lifetime and affinity calculations.

A Review of Studies of Polymeric Membranes by Positron Annihilation Lifetime Spectroscopy

A review is presented of studies of polymeric membranes by applying positron annihilation lifetime spectroscopy （PALS）. PALS has been used to study subnanometer-sized holes, to determine their size distribution and free-volume fractions, and to probe molecular-sized vacancies in glassy polymers. At present, PALS is believed to be a highly effective physical method for the examination of polymeric membranes.

Microstructural Investigation of PM-355 Nuclear Track Detector Subjected to Low-Dose Gamma Irradiation: A Positron Annihilation Lifetime Study

Samples of the PM-355 polymeric solid state nuclear track detector were exposed to low gamma absorbed doses from 1 kGy (0.1 Mrad) up to 9 kGy (0.9 Mrad). Positron annihilation lifetime (PAL) in conjunction with transmission electron microscopy (TEM) and Thermo-gravimetric analysis (TGA) were performed on irradiated and pristine samples at room temperature. The observed lifetime spectra were resolved into three components where the ortho-positronium (o-Ps) lifetime component was associated with the pick-off annihilation of positronium trapped by the free volume. PAL studies of irradiated PM-355 samples showed that ortho-positronium (o-Ps) lifetime increases with an increase in dose up to 4 kGy and decreases at higher doses. In contrast, the intensity of the o-Ps component, I3, decreases with the dose up to about 2 kGy, followed by a much smooth decrease up to 7 kGy, and then it levels off. TGA analysis indicated that the PM-355 detector decomposed in one main breakdown stage. These results are discussed on the basis of chemical and physical changes occurring at the microscopic level in the PM-355 due to irradiation. Crosslinking dominates for doses between 1 and 4 kGy, while the degradation mechanism (Chain scission) prevails for doses up to 9 kGy.

Positron Annihilation Study on TiC Powders

By means of the positron annihilation technique (PAT), superfine TiC powders obtained by ball-milling process were characterized. The results showed that there was a clear correlation between the change in TiC powders and that in the PAT parameters. In the high energy ball-milling process, TiC powders were severely deformed, which caused large increase in the amount of defects and severe distortion of lattice. On the other hand, the extreme refinement of grain size of powders resulted in the increase of grain boundary area and enhancement of system free energy. The spacing between deformed domains in 96 h milled samples is somewhat smaller than those in 48 h milled samples, indicating the size of Type II defects in 96 h milled samples is somewhat smaller than that in 48 h milled samples.

Role of the Reference Frame in Angular Photon Distribution at Electron-Positron Annihilation

The reasons of angular photon distribution occurrence at electron-positron annihilation are considered. It is shown that angular photon distribution is consequence of Doppler’s effect in the reference frame of the electron and positron mass center. In the reference frame bound with moving electron the angular photon distribution is absent. But it is replaced by the Doppler’s shift of photons frequencies. The received results are applied to the analysis of a positron-emission tomograph work.

Positron Annihilation in Perfect and Defective TiO₂Rutile Crystal with Single Particle Wave Function: Slater Type Orbital and Modified Jastrow Functions

Positron annihilation in TiO2 rutile crystal is studied by an assumption that a positron binds with valance electrons of a titanium dioxide to form a pseudo TiO2-positron molecule before it annihilates with these electrons. The orbital modification consisting of explicit electron-positron and electron-electron correlation in each electronic orbital is used for the electrons and positron wave functions. By these wave functions, the calculation results of the positron lifetimes in unmitigated and defective TiO2 crystals are about 170 ps, 266 ps and 243 ps, respectively. These results are in good agreement with experimental data of the positron lifetimes in vacancies of TiO2 from 180 ps to 300 ps.

High-dose neutron induced radiation swelling simulated by heavy ion irradiation and its microscopic study with positron annihilation technique

70 MeV-carbon-ion irradiation is used to simulate the radiation swelling induced by neutron irradiation of 3.2 × 1022 n·cm-2 in domestically-made 316 austenitic stainless steels modified by a 20%-cold-working and Ti-adding from room temper- ature to 802℃. The created swelling is microscopically examined by the positron annihilation lifetime technique. A radiation swelling peak is observed at 580℃ and the corresponding void has an average diameter of 0.7 um which is hardly probed by macroscopic methods.

RADIATION EFFECT OF HOME-MADE STAINLESS STEEL STUDIED BY POSITRON ANNIHILATION

The heavy ion irradiation is used for the first time to simulate the radiation damage produced by reactor neutrons in home-made modified stainless steels. The produced radiation damage is investigated by the positron annihilation spectroscopy. No detectable radiation damage is observed in the home-made modified stainless steels irradiated by 81.6 MeV carbon ions to a total dose of 3.3×1016 cm-2 , indicating that the home-made modified stainless steel of this type has good radiation resistant properties.

Studies on microdomain structure in segmented polyether polyurethaneureas by positron annihilation lifetime and small-angle X-ray scattering

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Study on Defects in Fe-Doped SrTiO3 by Positron Annihilation Lifetime Spectroscopy

SrTi1-xFexO3-δ ceramics were prepared using a traditional solid-state reaction method. From X-ray diffraction(XRD) result, we found that the doped Fe^3+ dissolved in the lattice, and no secondary phase was observed. Cation vacancies in perovskite oxides were identified via positron annihilation lifetime spectroscopy(PALS) measurements. Undoped and Fe-doped SrTiO3 ceramics and single-crystal SrTiO3 were measured by PALS at room temperature. The results show that the main defects in undoped SrTiO3 ceramics are Ti-related defects, and the isolated Ti vacancy lifetime is about 183.4 ps. With the increase of Fe^3+, the concentration of the Ti vacancies decreases accompanied by the appearance of the V^nSr-nV^**O(defect association of Sr vacancies and multiple O vacancies) vacancy defect complexes.

Defects and hyperfine interactions in binary Fe-Al alloys studied by positron annihilation and Mssbauer spectroscopies

The defects, the behavior of 3d electrons and the hyperfine interactions in binary Fe-Al alloys with different Al contents have been studied by measurements of positron lifetime spectra, coincidence Doppler broadening spectra of positron annihilation radiation and M？ssbauer spectra. The results show that on increasing the Al content in Fe-Al alloys, the mean positron lifetime of the alloys increase, while the mean electron density of the alloys decrease. The increase of Al content in binary Fe-Al alloys will decrease the amount of unpaired 3d electrons; as a consequence the probability of positron annihilation with 3d electrons and the hyperfine field decrease rapidly. M？ssbauer spectra of binary Fe-Al alloys with Al content less than 25 at.% show discrete sextets and these alloys make a ferromagnetic contribution at room temperature. The M？ssbauer spectrum of Fe70Al30 shows a broad singlet. As Al content higher than 40 at.%, the M？ssbauer spectra of these alloys are singlet, that is, the alloys are paramagnetic. The behavior of a 3d electron and its effect on the hyperfine field of the binary Fe-Al alloy has been discussed.

Interstitial Solution Carbon Concentration and Defects of Ti+Nb ULC-BH Steel by Internal Friction and Positron Annihilation Methods

The interstitial solution carbon concentration and defects in continuously annealed Ti＋Nb bearing ULC- BH （ultra-low carbon bake hardening） steel samples are investigated by multi-functional internal friction apparatus, positron annihilation lifetime spectroscopy （PALS） and coincidence Doppler broadening spectroscopy （CDBS）. The relationship of internal friction peaks, interstitial solution carbon concentrations and movable dislocations in the sam- ples under different conditions is analyzed. A correlation of lifetime component v~ values with interstitial solution car- bon concentrations in the samples for different continuous annealing processes is established, while a correlation of li- fetime component r2 values with multi-vacancies, vacancy clusters, microvoids and other types of defects for various continuous annealing processes is also demonstrated. Furthermore, the average lifetime results illustrate the overall defect densities for various continuous annealing processes. The CDBS analysis reflects the chemical surroundings of the defects at the annihilation sites and reveals that the peak heights of the ratio curves relate to the total number of defects such as interstitial carbon atoms, dislocations, vacancies and other types of defects. The results show that in- ternal friction, PALS and CDBS are effective techniques to identify and characterize the interstitial solution carbon concentration, multi-vacancies, vacancy clusters, microvoids and other types of microscopic defects in annealed Ti＋ Nb bearing ULC-BH steel.

The behavior of 3d electrons and defects in TiAl-based alloys containing V and Cu studied by positron annihilation

Information of defects and 3d electrons in transition metals (Ti,V,Cu) and TiAl-based alloys (Ti50Al50,Ti50Al48V2,Ti50Al48Cu2) can be extracted from the positron lifetime and coincidence Doppler broadening spectra. The results show that the 3d electron signals for the transition metals Ti,V and Cu increase with the number of 3d electrons. The 3d electron signal and the electron density for binary TiAl alloy are relatively low due to the (Ti)3d-(Al)3p interactions. The addition of V and Cu atoms to TiAl alloy leads to the increase in the electron densities in bulk and the defects on grain boundaries simultaneously,as well as the enhancement of the 3d electron signal. The 3d electron signal in the spectrum of Ti50Al48Cu2 alloy is higher than that of Ti50Al48V2 alloy.

Relationship between positron bulk lifetime and lattice constants-research on NaCl-type crystals

The positron lifetimes of some compounds with NaCl-type crystal structure are calculated with the method of atomic superposition approximation （ATSUP） based on the theories of local-density-approximation （LDA） and general- gradient-approximation （GGA）. The systematical results are fitted to a curve as a function of lattice constants. The positron bulk lifetimes of some other compounds with NaCl-type crystal structure, which are deduced from the systematical results, are in agreement with the experimental results given in other literature.

A Study of Substitution Site of Sn in YBa_2(Cu_(1-x)Sn_x)_3O_y Superconductors by Positron Annihilation Technique

It is one of the important ways that the foreign elements can be used as a probe to be substituted into the lattice so as to obtain the information about the electronic and crystal structure of the cuprate oxide superconductors. Obviously, it is essential to determining the real substituted site by foreign elements in discussion of the substitution effect. It is known that the positron annihilation technique (PAT) is highly sensitive to the changes in electronic and defect structure of the materials. Since Jean et al. reported

Determining Effective Mass of Conductive Electron by 2γ-Radiation Spectrum of Positron Annihilation

The movement states of conductive electron determine the thermology and electricity characters of metals. Because of the strong relation between conductive electron and ionic core,the movement states are determined not by the inertial

Thermodynamic behavior of^ (57)Fe implanted into ZrO_2(Y)by CEMS and slow positron beam

Using conversion electron spectroscopy(CEMS) and slow positron beam, the chemical state of 57Fe(100keV,3×1016 cm-2) implanted into ZrO2 containing 0.03 mole fraction Y2O3(ZY3) and its thermodynamic behavior during annealing process at 200 ￣ 500℃ are studied.For as-implanted sample, Fe chemical states of Fe0,Fe2+ and Fe3+ are observed,and assigned to the superparamagnetic metallic iron cluster,iron dimer(and trimer) and complex of the Fe3+ associated with canon vacancy (V) and oxygens respectively.After annealing at 400℃ the complexes of Fe3+-V are mostly dissolved,and the prior phase to α-Fe and α-Fe nano-crystalline cluster are present in the sample.Meanwhile the mixed conducting of oxygen-ions and electrons in the ZY3 sample containing Fe appears,it may correlate with the different iron charge states and their relative amounts,in particular with the α-Fe nano-granule.