Positron Annihilation Lifetime Measurements Research Articles

Guest molecules in Triton X-100 microemulsion: studied by positron annihilation technique and absorption spectroscopy

Positron annihilation lifetime measurements have been performed in water in oil (w/o) microemulsion (ME) of Triton X-100/butanol, n-heptane system in the presence of additives, UO +2 2 and p-nitro phenol (PNP). The microenvironment of the guest molecules in the ME phase was studied by the positronium (Ps) reaction-rate constant. The Ps reaction rate constant values with UO +2 2 in ME changed gradually as a function of water content in ME phase where as a considerable enhancement of Ps reaction rate constants with PNP was observed in the same phase. Furthermore, the solubilization of the probe molecules in the different microphase of the ME system was investigated through the UV-VIS absorption spectra. The results show that UO +2 2 ions were preferentially occupied in the polar water pool of ME but PNP molecules were distributed in the interfacial region (intermediate polarity) of the ME phase.

Compositional and temperature dependence of free volume in segmented copolymer PET/PEO studied by positron annihilation lifetime measurement

Positron lifetime measurements were performed for segmented copolymer poly(ethylene oxide)–poly(ethylene terephthalate) (PEO/PET) as a function of the hard segment content (PET) and the temperature, which is used to study the free volume properties, structural transition and miscibility behaviour of segmented copolymer PEO/PET. From the variation of the ortho-positronium ( o-Ps) lifetimes with changes in the hard segment content, we find that the free volume decreases with increase of the hard segment, this fact means that the hard segment microdomains act as physical cross-links that suppress movement of the chains and that the interactions between the hard segment (PET) and soft segment (PEO) also plays an important role. On the other hand, the temperature dependence of positron lifetimes reveals the existence of two glass transition temperatures. This experimental results show that the hard segment PET is immiscibility with soft segment PEO in segmented copolymer PEO/PET (PEO molecular weight 4000, PET content 30 wt.%). The maximum entropy lifetime method (MELT) is used to detect the distributions of the free volume holes. Large differences in width of the free volume hole distributions as a function of temperature are observed.

A study on free-volume properties of styrene-acrylic ester type copolymers

The positron annihilation lifetime measurements have been performed on a number of amorphous styrene–methyl acrylate copolymers and styrene–butyl methacrylate copolymers. The densities of copolymers were obtained with immersion method by using a capillary pycnometer and the average molecular weights were determined by gel chromatography. The lifetime τ3 of ortho-positronium (o-Ps) pick-off annihilation have been found to correlate with side group volume and polarity of macromolecular chains in the copolymers, and relative intensity I3 is attributed mainly to the electron-attracting groups trapping the spur electrons and positrons. The experimental results have been discussed on the basis of the structural variation of macromolecular chains. In addition, the PALS measurement as a function of time for polystyrene and several styrene–methyl acrylate copolymers has also been performed. The result shows that an electric field is built in polymers during extended positron annihilation spectroscopy measurement, and the field effect is a main factor which causes the decrease in I3 with time. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2476–2485, 1999

Annealing behaviour of reactor pressure-vessel steels studied by positron-annihilation spectroscopy, Mössbauer spectroscopy and transmission electron microscopy

The annealing behaviour of commonly used reactor pressure-vessel steels was studied using positron-annihilation spectroscopy (PAS) (lifetime and Doppler broadening techniques), transmission Mössbauer spectroscopy (MS), integral low-energy electron MS (ILEEMS) and transmission electron microscopy (TEM). The results of these methods applied to different types of steels are discussed. Some significant differences in the experimental results are observed between the Russian (3 types) and Western Europe (5 types) steels and between individual specimens within these two groups. All specimens were annealed in vacuum and studied after this thermal treatment. It was confirmed that the heat affected zone (HAZ) is the most sensitive region for thermal and neutron irradiation-induced embrittlement in the reactor. Positron-annihilation lifetime measurements on HAZ specimens annealed at successively higher temperatures show the drastic increase in the vacancy-type defect formation between 525°C and 600°C. Therefore these specimens were selected for further detailed studies by TEM.

Positron annihilation studies of fatigue in 304 stainless steel

Positron annihilation lifetime measurements were made on well calibrated fatigue samples of SS-304. Measurements were made on a high and low carbon alloy. Two separate lifetimes, indicating two defect site types were resolved in each sample. Significant lifetime changes are observed to occur early in the fatigue cycles.

Investigation of thermotropic phase transitions in a triple chain amphiphile forming hexagonal columnar and inverse micellar cubic mesophases: a positron annihilation lifetime study

Positron annihilation lifetime (PAL) measurements were performed between 25 and 100 C in order to study thermotropic phase transformations in an amphiphilic liquid crystal material with three hydrophobic alkyl chains. The lifetime spectra were separated into three exponential components, the long-lived of which is attributed to ortho-positronium (o-Ps) annihilation via pick-off processes. The lifetime of o-Ps, tau3, and its relative intensity, I, as well as the average positron lifetime, tau alpha I3tau3, respond to variations in the structural regularities of the environment. The phase transitions, which have the sequence and temperatures (C) Cr 59 Col(H II) 74 Cub(I II) 85 Iso, are well reflected in abrupt changes of, I3 and Tav. The o-Ps lifetime in the crystalline phase of tau3 = 2.35ns may be interpreted as a crystalline packing coefficient of 0.56 or, alternatively, as o-Ps confined in vacancy-type crystal imperfections with a volume of 0.132 nm3. During melting tau3 increases to 3.05 ns, and this is attributed to the preferential formation of Ps bubbles, with a size of 0.207 nm3, in mobile, liquid-like regions of hydrophobic alkyl chains. The bubble size remains at this value during the H-I transition and increases slightly to 0.216 nm3 during the transformation to the isotropic liquid phase. I3 shows variations during all of the phase transitions, which are attributed to changes in the Ps inhibition rate due to electron/positron mobility, and solvation effects varying as a consequence of different degrees of order of the molecular dipoles.

Study on microstructural lattice deformation of polypyrrole by positron annihilation lifetime spectroscopy

Microstructural defects (polarons, bipolarons), responsible for conduction in polypyrrole (PPy) has been studied using positron annihilation lifetime (PAL) spectroscopy, conductivity and electron spin resonance (ESR) measurements. Pyrrole, oxidized at different oxidizing strengths [0.13(N)–2.0(N)] by ammonium ferric sulphate has been used for the preparation of PPy. It is observed from the PAL measurement that of the two lifetime components the longer one originates from annihilation of positron in the defect (polaron and bipolaron) sites and the corresponding intensity is a measure of the defect concentration. Further it is evident from the ESR spectra that the dominant defects are polarons at lower oxidizing strengths [⩽ 0.75(N)] and bipolarons at strengths ⩾ 1.0(N). Conductivity data corroborate the findings.

Effects of varying temperature irradiation on the neutron irradiation hardening of reduced-activation 9Cr–2W martensitic steels

In order to clarify the effects of varying temperature during irradiation on the irradiation hardening of 9Cr–2W steels, tensile tests and positron annihilation lifetime measurements were carried out following the varying temperature irradiation (220/420°C and 340/530°C) utilizing a so called multi-section and multi-division controlled irradiation capsule in JMTR. After the irradiation at 220°C to 0.053 dpa, the steels show irradiation hardening as much as 110 MPa. The hardening was almost completely diminished immediately after the elevation of the irradiation temperature to 420°C. Subsequent irradiation at 420°C up to 0.14 dpa did not cause any hardening. The results of positron annihilation lifetime measurements indicate that microvoids are formed by the irradiation at 220°C but disappear upon elevating the temperature to 420°C and are then formed again by the subsequent irradiation at 420°C up to a total dose of 0.14 dpa. This behavior may be interpreted in terms of decomposition of interstitial loops or migration of small interstitial loops during temperature elevation. There is no good correlation between irradiation hardening and formation of microvoids in neutron-irradiated reduced-activation martensitic steels.

Positron annihilation lifetime measurements in collagen biopolymer

Lifetime measurement in Positron Annihilation Spectroscopy (PAS) is applied to the study of free-volume collagen characteristics as a function of concentration. The lifetimes of positrons were measured by a conventional fast-fast coincidence system. All lifetime data are fitted in three components by using the computer program POSITRON-FIT and resolved. For each concentration, lifetime distributions were analyzed in order to obtain the different components, thus we have observed three components of which one long component τ3. This long lived component can be associated with a pick-off annihilation of ortho-positronium (o-Ps) trapped in free volumes of amorphous region. This investigation shows the potential of positron annihilation spectroscopy in the study of biopolymer microstructures.

Low damage reactive ion etching for photovoltaic applications

New concepts in silicon solar cell design require dry processing technologies. For this reason two reactive ion etching (RIE) processes have been developed: one for surface cleaning and one for the removal of phosphorous glass (PSG). However, damage is induced in silicon during reactive ion etching which deteriorates solar cell performance. Damage caused by SF6 RIE cleaning has been investigated by means of secondary ion mass spectroscopy, positron annihilation, and minority charge carrier lifetime measurements. Particles contained in the etch gas can be detected up to a depth of 50–80 nm in the silicon sample. A two layer model of vacancy distribution has been established: A layer of high vacancy concentration (1019 cm−3) up to a depth of 20 nm is followed by a second layer that extends over a depth of 1 μm with a vacancy concentration of 1016 cm−3. Effective minority charge carrier lifetimes decrease to about 10% of the lifetime of the wet etched control during RIE. If a heavily damaged layer of 20 nm is being removed by anodic oxidation, lifetimes return to the initial value. Under certain etching conditions it is possible to anneal plasma induced damage at 400 °C. The influence of RIE induced damage on solar cells is quantified by open circuit voltage analysis: Long process times, addition of oxygen to the etch gas, and high rf power or self-induced dc bias result in a significant decrease in open circuit voltage. Nearly damage free RIE processes have been developed for surface cleaning as well as PSG removal. Dry processed solar cells thus show the same performance as wet etched cells.

Change in free volume during volume phase transition of poly( N-isopropylacrylamide) gel as studied by positron annihilation lifetimes: temperature dependence

Macroscopic volume shrinkage and swelling of a poly( N-isopropylacrylamide), PNIPA, gel on heating was studied and the microscopic structural change during volume phase transition in water was correlated to variations of size, concentration and size distribution of free volume of the PNIPA gel estimated by positron annihilation lifetime measurement. An average free volume size of 0.28 nm radius of the swollen PNIPA gel coincided with that obtained in pure water below a transition temperature (33°C), whereas, in the collapsed state, two kinds of annihilation lifetimes of ortho-positronium were observed. This result implies that a nanoscopic phase separation occurs maybe because of the newly appeared free volumes of respective sizes, 0.18 nm and 0.33 nm in radius, by decreasing the motility of water molecules, accompanied with an aggregation of the polymer network and a significant expansion of the polymer network mesh size, although the macroscopic volume dramatically decreased. Nanoscopic inhomogeneity was discussed in terms of free volume size distribution. These results suggested that an inhomogeneity of the collapsed gel clearly differs from that of the swollen gel.

Carbon Black-polyethylene Composites Studied by Positron Annihilation

Preliminary results of positron annihilation lifetime measurements, performed for carbon black—LDPE composites with three kinds of carbon black (PRINTEX XE2, VULCAN XC-72, and SAKAP 6) are presented. Four components with lifetimes τi ≈120 ps, τ2 340 ps, τ3 .2 ns, τ 2.8 ns occurred to be the best fitted to measured positron lifetime spectra. The presence of carbon black resulted only in reduction of intensities of the components with the lifetimes τ1, τ3, , τ4 (originating in our opinion from the annihilation of the positronium) and increase in the intensity of the component with the lifetime τ 2 . The intensities depend both on the content of carbon black and its kind as well. The strongest influence of the carbon black of the highest specific surface area (PRINTEX XE2) has been observed at the intensities which correlate well with the results of studies of resistivity and tensile strength for the same samples.

Poly(ether imide)s: correlation of positron annihilation lifetime studies with polymer structure and gas permeability

This paper describes the results of positron annihilation lifetime measurements on a series of poly(ether imide)s for which permeabilities and permselectivities for gas separation have been determined previously. Positron lifetimes and intensities vary systematically with changes in polymer structure and correlate with the permeabilities in separation of carbon dioxide and methane. Some apparent scatter in the data might, in part, be associated with differences in the interactions of ortho-positronium with molecular cavities as polymer structures are varied.

Estimation of Pore Size of Porous Materials by Positron Annihilation Lifetime Measurement.

Pore size of several porous materials such as zeolites (MCM-41 and MCM-48) and highly fined grain aggregates of zirconia was estimated by means of positron annihilation lifetime measurement. Observed lifetimes of ortho-positronium (over ∼20 ns, τ 4 and τ 5 ) correspond to the meso-pore sizes assessed by conventional porosimetries due to a nitrogen physisorption and a mercury porosimetry. The long-lived lifetime components included an unassigned lifetime component (τ 5 ), which was not detected by the other porosimetry. The observed ortho-positronium lifetimes are correlated to the mean size of pores. A relation between relative intensities of the long-lived ortho-positronium components (I 4 and I 5 ) and specific surface areas is discussed and a variation of a numerical concentration of pores is correlated to the change of the particle sizes of the zirconia aggregates.

Analysis of the sintering process of 2 mol% yttria-doped zirconia by positron annihilation lifetime measurements

Analysis of the sintering process of 2 mol% yttria-doped zirconia by positron annihilation lifetime measurements

Observation of Vacancies by Positron Trapping in Cerium‐Doped Zirconolites

Positron annihilation lifetime measurements on cerium‐doped zirconolites have given strong indications of positron trapping in samples that have previously been deduced to contain cation vacancies, mainly from X‐ray absorption spectroscopy, thermogravimetry, and microanalysis. The increases in lifetime of the majority lifetime component are ∼20% for samples deduced to contain a few percent of vacant cation sites. Lanthanum‐doped SrTiO3 samples, in which it is generally agreed that A‐site vacancies are produced via the lanthanum doping, have been used as a model system and similar increases in lifetime have been observed for samples that contain more than ∼0.1 formula units of lanthanum.

Estimation of free volume in polystyrene-polyphenylene ether blend probed by the positron annihilation lifetime technique

Variations of size, content and size distribution of free volumes in linear polystyrenes (PS) synthesized by radical processes, with the addition of miscible polyphenylene ether (PPE) in PS, were observed by positron annihilation lifetime measurements. The average free volume radius of polystyrene was proved to increase from 0.30 nm (PPE: 0%) to 0.34 tun (PPE: 100%) and the average intensity, relevant to free volume content, decreased with the increase of PPE. The size distributions of free volume of various PS to PPE ratios also reveal that average free volume size increases and that smaller free volumes decrease by the increase of PPE ratio. The evidence that the size distribution curve of PS and PPE mixture (1:1) is more similar to that of PS suggests that free volumes produced in PS are stable with the addition of PPE and that more positroniums annihilate in smaller free volumes generated in PS than in bigger ones formed by PPE, since numbers of free volume overwhelm those of PPE.

Electric-field dependence of positronium formation in liquids and polymers

Positron annihilation lifetime measurements have been performed to study the effect of an external electric field on the probability of positronium (Ps) formation in several liquids (isooctane, hexane, cyclohexane) and polymers [polystyrene, poly(4-methyl-1-pentene), low- and high-density polyethylene, polypropylene, polycarbonate]. The ortho-positronium intensity ${I}_{3}$ for all the samples except polar polycarbonate decreased with increasing external electric-field strength in the range of 0--60 kV/cm. Based on the Onsager equation, the average initial spatial separation between a positron and an electron in the terminal positron spur has been deduced to be 10--44 nm for the nonpolar polymers, 21--52 nm for the degassed liquids, and 19--37 nm for the air-saturated liquids, indicating that the average thermalization distance of the positron is larger than that of an excess electron. It is shown that the time required for Ps formation is on the order of 10 ps.

The micro structural study of 15Kh2MFA and 15Kh2NMFA reactor pressure vessel steels using positron-annihilation spectroscopy, mössbauer spectroscopy and transmission electron microscopy

The micro structure of the non-irradiated low-alloyed steels (15Kh2MFA and 15Kh2NMFA) was studied using different spectroscopic methods as the positron annihilation lifetime and Doppler broadening techniques, the Mössbauer spectroscopy, and the Integral low-energy electron Mössbauer spectroscopy as well as the Transmission electron microscopy. Differences in the microstructural parameters of these types of RPV steels are well detectable using all methods. It was confirmed that the heat-affected zone of these steels is the most sensitive place for thermal embrittlement in the reactor. Positron-annihilation lifetime measurements on the successive annealed specimens (XTA, YTA), which simulated the heat-affected zone, showed the rapid increase in the vacancy-type defects formation in the temperature region 525–600°C. Therefore these specimens were studied using Transmission electron microscopy in more detail.

Application of Positron Annihilation Spectroscopy to Study Recovery and Recrystallization of Commercial Pure Al and Al-1wt% Mn Alloys

Isochronal annealing of cold worked commercial pure aluminum (99.5%) and Al-1wt% Mn alloys was demonstrated between room temperature and 823 K. The stages of recovery and recrystallization were studied using microhardness and positron annihilation lifetime measurements. Apositive correlation was established between the variation of the intensity of long lived component and Vickers microhardness with annealing temperature during the two stages of recovery, which were identified in both aluminum and Al(Mn). The retardation of the recovery stages and recrystallization in Al(Mn) alloys is interpreted in terms of precipitation of aluminum-manganese particles as well as manganese-vacancy interaction.