Positron Annihilation Mechanism Correlated with Microstructure of C60-Contained Chitosan

Positron annihilation lifetime spectra measurements were performed on C_(60)-containing chitosan as a function of temperature ranging from 290 to 380K.Both the ortho-positronium annihilation lifetimeт_(3) and its intensity Is increase with increasing temperature,however,the values of I_(3) in C_(60)-containing chitosan are smaller than that in pure chitosan,and the slope ofт_(3) with temperature is also suppressed after C_(60) linked.The existing three models for positron annihilation in polymer have been compared to analyze the experiment result.It is realized that only the spur reaction model,combined with the free volume model,can explain it satisfactorily.The ortho-positronium lifetime and intensity were affected by the local physicochemical environment of material,i.e.,free-volume structure,electron scavenging and trapping sites,etc.The linked C_(60) that play a significant chemical inhibition and quenching role on positronium formation and annihilation isfirst observed.The microstructure change brought by C_(60) group has also been discussed.

APPLICATION OF THE POSITRON ANNIHILATION TECHNIQUE

The principles of positron annihilation and four positron experimental techniques are described. The application of positron annihilation technique in material science. atomic physics and other related fields are discussed.

Effects of Flavour-Dependent qq Annihilation on the Mixing Angle of the Isoscalar Octet-Singlet and Schwinger’s Nonet Mass Formula

By incorporating the flavour-dependent quark--antiquark annihilation amplitude into the mass-squared matrix describing the mixing of the isoscalar states of a meson nonet,the new version of Schwinger's nonet mass formula,which holds with a high accuracy for the 0^(-+),1^(--),2^(++),2^(-+)and 3^(--)nonets,is derived and the mixing angle of the isoscalar octet--singlet for these nonets is obtained.In particular,the mixing angle of the isoscalar octet-singlet for the pseudoscalar nonet is determined to take the value of-12.92°,which is in agreement with the value range from-13°to-17°deduced from a rather exhaustive and up-to-date analysis of data.It is also pointed out that the omission of the flavour-dependent qq annihilation effect might be a factor resulting in the invalidity of Schwinger’s original nonet mass formula for the pseudoscalar nonet.

Positron Annihilation Study on the Microstructural Stability in Rapidly Solidified Al_(92.3)Fe_(4.3)V_(0.7)Si_(1.7)Mm_(1.0) Alloy

Rapidly solidified Al92.3Fe4.3V0.7Si1.7Mm1.0 alloy has been studied by positron lifetime spectroscopy and the variations on the intedecial defects with the annealing temperature were revealed by an analysis of the lifetime results. The intedece characteristics derived from the positron-lifetime results could be used to give a satisfactory interpretation of the dependence of mechanical properties on the annealing temperature

Evolution of native point defects in ZnO bulk probed by positron annihilation spectroscopy

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening （CDB） spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600 ℃ annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600℃ annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.

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.

Mapping of the lunar surface by average atomic number based on positron annihilation radiation from Chang'e-1

A map of the average atomic number of lunar rock and soil can be used to differentiate lithology and soil type on the lunar surface.This paper establishes a linear relationship between the average atomic number of lunar rock or soil and the flux of position annihilation radiation(0.512-Me V gamma-ray) from the lunar surface.The relationship is confirmed by Monte Carlo simulation with data from lunar rock or soil samples collected by Luna(Russia) and Apollo(USA) missions.A map of the average atomic number of the lunar rock and soil on the lunar surface has been derived from the Gamma-Ray Spectrometer data collected by Chang'e-1,an unmanned Chinese lunar-orbiting spacecraft.In the map,the higher average atomic numbers(ZA > 12.5),which are related to different types of basalt,are in the maria region;the highest ZA(13.2) readings are associated with Sinus Aestuum.The middle ZA(~12.1) regions,in the shape of irregular oval rings,are in West Oceanus Procellarum and Mare Frigoris,which seems to be consistent with the distribution of potassium,rare earth elements,and phosphorus as a unique feature on the lunar surface.The lower average atomic numbers(ZA < 11.5)are found to be correlated with the anorthosite on the far side of the Moon.

EFFECT OF POLYMERIZATION DEGREE ON MICROSTRUCTURE OF POLYETHYLENE GLYCOL STUDIED BY POSITRON ANNIHILATION

Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25 - 500). It is observed that all the positron parameters are strongly dependent on N when N≤250, and then tend to constant levels when N≥250. The variations of the longest-lived component (I3,τ3) show that the size of free- volume holes has a minimum at N - 100 and the hole concentration decreases with N. These trends may be due to the variations in crystallinity, the chain ends, entanglements and the Van der Waals interaction between segments. The variations of the second component (I2,τ2) with N reflect the changes of defect properties in crystalline regions of PEG.

Effects of thermal aging on Fe ion-irradiated Fe–0.6%Cu alloy investigated by positron annihilation

Thermal aging effects on surface of 2.5 Me V Fe ion-irradiated Fe–0.6%Cu alloy were investigated using positron annihilation techniques. The samples were irradiated at 573 K to a dose of 0.1 dpa. Their thermal aging was performed at 573 K for 5, 50 and 100 h. From the results of Doppler broadening measurement, an obvious trough could be seen in near-surface region from the S parameters and inflection point form at S–W curves. This indicates changes in the annihilation mechanism of positrons in surface region after thermal aging. Coincident Doppler broadening indicates that the density of Cu precipitates in the thermal aged samples decreased, due to recovery of the vacancies.

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.

Interaction behaviours of solitoffs:fission, fusion, reconnection and annihilation

The interactions between solitoffs are extensively investigated. Besides the known solitoff fission and fusion interac- tions, two new types of solitoff interactions are discovered, named the solitoff reconnection and the solitoff annihilation. Taking the asymmetric Nizhnik-Novikov Veselov equation as an illustrative system, five types of solitoff interactions are graphically revealed on the basis of the analytical solution obtained by the modified tanh function expansion method.

Comparison of the Free Volume of LiCl-Added SPPO Membrane and SPPO-PES Blend Membrane by Positron Annihilation Method

Positron annihilation lifetime spectroscopy （PALS） is a powerful technique for the study of free volume in polymers. The lifetime of ortho-positronium （o-Ps）, a bound state of an electron and a positron, can be used to assess the pore size, while the intensity can be used to characterize the number of pores. Based on the values of the long-lived o-Ps components in the lifetime spectra, the radii and fractional free volumes of sulfonated poly （2,6-dimethyl-1,4- phenyleneoxide） （SPPO） membranes with added LiCl and SPPO-PES （Polyethersulfone） blend were compared. Free volume radii in both kinds of membranes are discussed.

Synthesis, Growing Processes and Physical Properties of CdS Nanoclusters in Y-Zeolite Studied by Positron Annihilation

Direct synthesis of CdS nanoclusters within the pore structure of Y zeolite was made. The location of CdS nanoclusters inside Y zeolite hosts was confirmed by the blue-shifted reflection absorption spectra with respected to that of bulk CdS materials. In this paper, we conducted Positron Annihilation Lifetime Spectrum (PALS) measurements on a series of CdS/Y zeolite samples and concluded that CdS clusters were not located in supercages but in smaller sodalite cages; as the CdS loading concentration increases to 5 wt%, the discrete CdS cubes begin to form bigger superclusters through interaction. The stability of CdS clusters inside the sodalite units is due to the coordination of Cd atoms with the framework oxygen atoms of the double six-ring windows. Moreover, PALS reveals some important information of surface states existing on the interfacial layers between CdS clusters and Y zeolite.

INVESTIGATION OF BEHAVIOUR OF B IN Ni_3Al ALLOYS BY POSITRON ANNIHILATION TECHNIQUE

The behuviour of B atoms added to mono-and poly-crystalline Ni_3AI alloys has been investi- gated hy positron annihilation technique(PA T).If the additive of B was less than 1.37 at-%, some of B atoms solid-solutioned interstitially into matrix and distorted the lattice,thus the lifetime (τ) of positron (e^+) Or matrix dilated:the others of them segregated on vacancy-like defects and formed "filling effect",thus the mean lifetime (τ)and S parameters decreased.If B added was increased up to 2.22 at-%,more defects having larger free volume were induced by horide eutectic occurred along grain boundaries and grain interior,thus the τ in vacancy-like defects.τ and S parameters increased remarkably.

Investigation of SPPO Membranes by Positron Annihilation Lifetime Spectroscopy

Positron annihilation lifetime spectroscopy （PALS） is a powerful technique to study the free volume in polymers. The lifetime of ortho-positronium （o-Ps）, a bound state of an electron and a positron, can be used to assess the pore size while the intensity can be used to characterize the number of pores. On the basis of the values of the long-lived o-Ps components in the lifetime spectra, the radii and fractional free volumes in the sulfonated poly （2,6-dimethyl-1,4- phenyleneoxide） （SPPO） membranes with different amounts of LiCl were calculated. It was found that, with the increasing amount of LiCl, the free volume radius and the fractional free volume firstly increased and then decreased. After immersing the membranes in distilled water, the free volume radius and the fractional free volume changed with different water concentrations in the membrane.

A PRELIMINARY STUDY OF EMBRITTLEMENT OF Co-BASED AMORPHOUS ALLOYS BY POSITRON ANNIHILATION

Measurements of the Doppler broadening S-lineshape parameter of positron annihilation and brittleness have been performed for two Co-based amorphous alloy prior to crystallization. It is shown that the brittleness is related to the S-parameter, that is, microdefects may be one of the important factors affecting the embrittlement of the Co-based amorphous alloys.

Spatial correlation properties and phase singularity annihilation of Gaussian Schell-model beams in the focal region

By using the generalized Debye diffraction integral, this paper studies the spatial correlation properties and phase singularity annihilation of apertured Gaussian Schell-model （GSM） beams in the focal region. It is shown that the width of the spectral degree of coherence can be larger, less than or equal to the corresponding width of spectral density, which depends not only on the scalar coherence length of the beams, but also on the truncation parameter. With a gradual increase of the truncation parameter, a pair of phase singularities of the spectral degree of coherence in the focal plane approaches each other, resulting in subwavelength structures. Finally, the annihilation of pairs of phase singularities takes place at a certain value of the truncation parameter. With increasing scalar coherence length, the annihilation occurs at the larger truncation parameter. However, the creation process of phase singularities outside the focal plane is not found for GSM beams.

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.

STUDY OF RECOVERY IN DEFORMED Ni-Cr-Co SUPERALLOYS USING POSITRON ANNIHILATION

The recovery recrystallization of 3 deformed Ni-Cr-Co superalloys with different Co con- tents has been studied using positron annihilation technique and TEM.The alloys with low Co content have more defects occurred than ones with high Co content,and the former may be faster in dislocation recovery than the latter.Thus,the apparent activation energy of recrystallization increases with the increase of their Co content in alloys.The relationship be- tween apparent activation energy and stacking fault energy together with their effect on re- covery were also discussed.

Characterization,spectroscopic investigation of defects by positron annihilation,and possible application of synthesized PbO nanoparticles

Nanocrystalline samples of highly pure lead oxide were prepared by the sol-gel route of synthesis.X-ray diffraction and transmission electron microscopic techniques confirmed the nanocrystallinity of the samples,and the average sizes of the crystallites were found within 20 nm to 35 nm.The nanocrystallites exhibited specific anomalous properties,among which a prominent one is the increased lattice parameters and unit cell volumes.The optical band gaps also increased when the nanocrystallites became smaller in size.The latter aspect is attributable to the onset of quantum confinement effects,as seen in a few other metal oxide nanoparticles.Positron annihilation was employed to study the vacancy type defects,which were abundant in the samples and played crucial roles in modulating their properties.The defect concentrations were significantly larger in the samples of smaller crystallite sizes.The results suggested the feasibility of tailoring the properties of lead oxide nanocrystallites for technological applications,such as using lead oxide nanoparticles in batteries for better performance in discharge rate and resistance.It also provided the physical insight into the structural build-up process when crystallites were formed with a finite number of atoms,whose distributions were governed by the site stabilization energy.