Ortho-positronium Pick-off Annihilation Research Articles

Ortho-positronium annihilation processes in Y zeolites under different environments by measurements of the energy spectra of the positron annihilation radiation

Measurements of the energy distribution of the positron annihilation radiation in Y zeolites are carried out to elucidate the various ortho-positronium annihilation processes that take place in these microporous materials and relate them to the ortho-positronium lifetime components observed by positron annihilation lifetime spectroscopy in earlier studies. Y zeolites with various Si/Al ratios in the range from 2.6 to 40 are investigated under a controlled atmosphere of N2 gas and after exposure to the atmosphere. The ortho-positronium annihilation modes are characterized by analyzing the γ-ray spectra through the use of the f3γ index, which represents the fraction of 3γ-annihilation, and the S parameter. The proportion of ortho-positronium self-annihilation is found to be no less than 15% in N2 gas, but drastically decreases by the physisorption of water molecules and possibly positronium spin exchange interaction with oxygen in the atmosphere. The presence of water molecules suppresses ortho-positronium intrinsic annihilation and, at the same time, promotes ortho-positronium pick-off annihilation in the pores. Similarly, the S parameter rises due to an increased contribution from para-positronium annihilation and ortho-positronium pick-off annihilation, which is also consistent with the physisorption of water and positronium spin exchange. In conjunction with the previously measured ortho-positronium lifetime components, the specific ortho-positronium annihilation sites and processes in these porous materials under different environments are determined and a complete picture of the ortho-positronium behavior in Y zeolites is obtained.Graphical abstractγ-ray energy spectra of Y zeolites with a Si/Al ratio = 2.6 measured in N2 gas and water-saturated zeolites

Identification of ortho-positronium annihilation sites in Y zeolites by positron lifetime measurements in different environments

The annihilation sites of ortho-positronium formed by positrons incident on Y zeolites with different Si/Al ratios are investigated by positron annihilation lifetime spectroscopy in different environments. Comparison of the measured ortho-positronium lifetimes by exposure of the zeolites to argon, nitrogen, water or vacuum enables us to determine the annihilation process and the specific annihilation site within the zeolite samples. The third lifetime component is ascribed to ortho-positronium pick-off annihilation in or around the apertures of the sodalite cage and in the connecting channels, and is reduced in water because of the physisorbed molecules that hinder the ortho-positronium mobility. The longest fourth lifetime component is attributed to ortho-positronium self-annihilation in the interparticle gaps, but is significantly suppressed in the presence of polar molecules, which suggests a contribution from ortho-positronium pick-off annihilation in the supercage.

A positronium-based systematic study of the physico-chemical properties of zeolite pores

A systematic study of the influence of surface acidity of pores, water content, crystal size and structural type of zeolites on positronium behavior was carried out over a wide range of Si/Al ratios using the positron annihilation lifetime spectroscopy technique. A clear decrease in the ortho-positronium pick-off annihilation lifetime, as well as the total ortho-positronium formation fraction, was detected at lower Si/Al ratios due to the positronium oxidizing effect and positron trapping of the Brønsted acid sites. Upon water adsorption, the suppression of the positronium diffusion by the water molecules adsorbed in the tubular pores and their trapping action of positrons and electrons was found to significantly reduce both the ortho-positronium lifetime and intensity. These results clarified the relation between positronium behavior and the chemical reactivity of the zeolite pores.

PALS and DSC measurements in 8 MeV electron irradiated natural rubber filled with different fillers

The effect of high energy electron irradiation on the microstructure and thermal properties of natural rubber (NR) filled with different fillers at different concentrations are studied. The samples are irradiated with 8 MeV electron beam to a total dose of 100 KGy. The change in free volume size and specific heat due to addition of fillers and irradiation are studied using positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC) respectively. The Positron lifetime spectra are de-convoluted into two components. The longer lived component [Formula: see text] signifies the pick-off annihilation of ortho-positronium (o-Ps) at free volume site which may be related to the radius of the free volume holes. It is observed that the specific heat [Formula: see text] and free volume size are all affected by both irradiation and addition of fillers.

Positron annihilation lifetime study of polyvinylpyrrolidone for nanoparticle-stabilizing pharmaceuticals

Positron annihilation lifetime spectroscopy was applied to characterize free-volume structure of polyvinylpyrrolidone used as nonionic stabilizer in the production of many nanocomposite pharmaceuticals. The polymer samples with an average molecular weight of 40,000gmol−1 were pelletized in a single-punch tableting machine under an applied pressure of 0.7GPa. Strong mixing in channels of positron and positronium trapping were revealed in the polyvinylpyrrolidone pellets. The positron lifetime spectra accumulated under normal measuring statistics were analysed in terms of unconstrained three- and four-term decomposition, the latter being also realized under fixed 0.125ns lifetime proper to para-positronium self-annihilation in a vacuum. It was shown that average positron lifetime extracted from each decomposition was primary defined by long-lived ortho-positronium component. The positron lifetime spectra treated within unconstrained three-term fitting were in obvious preference, giving third positron lifetime dominated by ortho-positronium pick-off annihilation in a polymer matrix. This fitting procedure was most meaningful, when analysing expected positron trapping sites in polyvinylpyrrolidone-stabilized nanocomposite pharmaceuticals.

Positronium bubble oscillation in room temperature ionic liquids-Temperature dependence

The temperature dependent oscillation of the ortho-positronium pick-off annihilation rate was successfully observed for a room temperature ionic liquid (IL), N,N,N-trimethyl-N- propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI). The fundamental frequencies at 25C and 30C were 5.85GHz and 4.00GHz, respectively. The decay of the oscillation was faster at higher temperature, 30C. Moreover, the higher harmonic frequencies could explain the change of ortho-positronium pick-off annihilation rate successfully. The macroscopic viscosity of the IL could not explain the appearance of the oscillation. It indicated that the positron annihilation methods were very strong tools to study the properties of IL's in sub-nanometer scale that must be very different from the macroscopic properties.

Spin conversion and pick-off annihilation of ortho-positronium in gaseous xenon at elevated temperatures

We present experimental results for the annihilation rate of ortho-positronium in gaseous xenon at 250 kPa using a digital-oscilloscope-based positron annihilation lifetime spectrometer as a function of temperature (300--623 K). Owing to the Zeeman mixing of positronium in a magnetic field, the annihilation rate is divided into two components: one is due to ortho-para postronium spin conversion induced by spin-orbit interaction, while the other is due to pick-off annihilation. The spin-conversion annihilation rate is proportional to ${T}^{2.1}$, where $T$ is the temperature. We attributed this dependence to the fact that the spin conversion occurs only in $p$-wave scattering during positronium--xenon collisions and to the fact that, after thermalization of the ortho-positronium atoms, their velocity profile follows a Maxwell-Boltzmann distribution. The results also show that the pick-off annihilation rate is almost linear in temperature. This increase is a dependence exceptionally stronger than that observed in other gases.

Positron annihilation study on free volume of amino acid modified, starch-grafted acrylamide copolymer

Free volume measurements using positron annihilation lifetime spectroscopy was performed for uncrosslinked and crosslinked starch-grafted polyacrylamide, and their modified amino acid samples including some of their iron(III) complexes. The measurements were performed at room temperature. The analysis of lifetime spectra yielded mostly three lifetime components. It was observed that the values of the short lifetime component τ 1 are slightly higher than the lifetime associated with the self-decay of para-positronium atoms in polymers. The free volume was probed using ortho-positronium pick-off annihilation lifetime parameters. The mean free volume has also been calculated from the lifetime data. The avrage value of this parameter of the crosslinked polymer were found to be higher than those of the uncrosslinked polymer.

Structure, magnetic and positron lifetime studies on CaO–ZrO2–SiO2 glass system doped with vanadium oxide

The structure of CaO–ZrO2–SiO2 glasses doped with V2O5 (0.1–2 mole%) was studied using density and molar volume measurements. It was observed that the molar volume increased while the density decreased with increasing V2O5 content. Magnetic susceptibility measurements showed the diamagnetic behaviour of this glass system. The free volume was probed using ortho-positronium pick-off annihilation lifetime parameters. The mean free volume and fractional free volume have also been calculated from the positron annihilation lifetime data. The overall measured parameters from the structural and/or positron annihilation spectra indicated that the majority of the vanadium ions in the range 0–0.5 mole% of V2O5 are in the V5+ valence state which act as a glass former. On the other hand, the majority of the vanadium ions in the range 0.7–2 mole% of V2O5 are the V3+ and V4+ states acting as a glass modifier.

Investigation of nanocrystalline CoFe 2 O 4 by positron annihilation lifetime spectroscopy

Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in diffuse vacancy clusters or microvoids at the grain boundaries and at the grain-boundary triple points. The longest component corresponds to the pick-off annihilation of ortho-positronium formed at the larger voids. The variations in these lifetimes and their relative intensities with annealing temperature and crystallite size have been studied in detail.

Free volumes in UV-cured polymers based on acrylate oligomers

Polymerization in polymers based on acrylate oligomers was caused by exposure to ultraviolet light. Polymer specimens with different ultraviolet light exposure times have been investigated by positron annihilation lifetime spectroscopy (PALS). The longest lifetime, τ 3, in observed lifetime spectra for positrons was associated with the pick-off annihilation of ortho-positronium ( o-Ps) trapped by free volume in the structure. The aim of this paper was to investigate the influence of the time of UV exposure on free volume sizes in two different polymers based on acrylate oligomers. UV exposure time influences the free volume size: longer time reduces it. The concentration of the free volumes is independent of UV exposure time.

Plasma-polymerized hexamethyldisiloxane films characterized by variable-energy positron lifetime spectroscopy

Nanometer-size holes in plasma-polymerized thin films were characterized by variable-energy positron lifetime spectroscopy for the first time. Hexamethyldisiloxane (HMDSiO) was plasma-polymerized at different discharge powers (30–105 W) and monomer pressures (1.0–4.9 Pa). The positron lifetime spectra of deposited films were collected at positron energies of 1 and 5 keV. All films showed a well-defined long-lived component due to pick-off annihilation of ortho-positronium (o-Ps). The o-Ps lifetime τ3, reflecting the average size of free-volume holes in the film, increased with an increasing ratio of plasma discharge power, W, and monomer flow rate, F. Based on the empirical relationship between the o-Ps lifetime and the cavity radius, hole volumes were estimated to be 0.19–0.36 nm3. We also found that the o-Ps intensity, I3, depends strongly on the same parameter, W/F. Comparison with infrared (IR) absorption spectroscopy data showed that Ps formation is suppressed in films with fewer organic bonds and higher disorder, i.e., those increasingly inorganic in nature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2522–2528, 1999

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.

Positron annihilation and diffraction studies of the photopolymers based on the acrylate oligomers

Abstract Positron annihilation lifetime and X-ray diffraction measurements were made on photopolymers based on acrylate oligomers. Polymerization was caused by exposure to ultra violet light. The measurements were made on photopolymer specimens with different ultra violet light exposure times. The longest positron lifetime, τ 3 , was associated with the pick-off annihilation of orthopositronium (o-Ps) trapped in voids. Interatomic distances and coordination numbers for the investigated samples have been determined from X-ray data. Ultra violet light exposure time affects mainly the sizes of the voids. The nearest neighbour distances and coordination numbers are independent of the ultra violet exposure time.

Positron annihilation lifetime studies of free volume in a polymer electrolyte PEO complexed with NH4I

Positron lifetime spectroscopy has been applied to study the temperature dependence of free-volume properties in a solvent-free polymer-salt complex polyethylene oxide (PEO) doped with ammonium iodide (NH 4 I, with NH 4 + /EO 0.076) in the temperature range of 298 - 353 K. The observed lifetime spectra were resolved into three components and the longest lifetime, τ 3 , was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by the free volume. The lifetime component, τ 3 , and its intensity, I 3 , both showed a significant variation with temperature, which followed a different course in the heating and cooling cycle. Changes in the temperature coefficient of T3 and 13 were observed at T 328 K, the melting point of the sample. This behaviour is correlated to the temperature variation of the electrical conductivity.

Positron annihilation lifetime studies of free volume in a polymer electrolyte PEO complexed with NH4I

Positron lifetime spectroscopy has been applied to study the temperature dependence of free-volume properties in a solvent-free polymer–salt complex polyethylene oxide (PEO) doped with ammonium iodide (NH4I, with NH ≈ 0.076) in the temperature range of 298–353 K. The observed lifetime spectra were resolved into three components and the longest lifetime, τ3, was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by the free volume. The lifetime component, τ3, and its intensity, I3, both showed a significant variation with temperature, which followed a different course in the heating and cooling cycle. Changes in the temperature coefficient of τ3 and I3 were observed at T ≈ 328 K, the melting point of the sample. This behaviour is correlated to the temperature variation of the electrical conductivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 969–976, 1998

Positron Annihilation Lifetime Measurements in Collagen Biopolymer

AbstractLifetime 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 obtained 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 a 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 the positron annihilation spectroscopy technique in the study of biopolymer microstructures.

Thermal Equilibrium Defects in Anthracene Probed by Positron Annihilation

Defects in anthracene were investigated by the positron annihilation technique. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured in the temperature range between 305 K and 516 K. The lifetime of positrons annihilated from the delocalized state was determined to be 0.306 ns around room temperature. Below the melting point, the observed temperature dependence of the line shape parameter S was explained assuming the formation energy of thermal equilibrium defects was 1 eV. Above the melting point, the pick-off annihilation of ortho-positronium in open spaces was observed, where the size of these spaces was estimated to be 0.2 nm3. The annihilation of positrons from the self-trapped state was also discussed.

Free volumes in polystyrene probed by positron annihilation

Free volume characteristics in three samples of monodisperse polystyrene were investigated by positron annihilation technique over a temperature range from 300 to 380 K. The number-average molecular weight of the samples ranged from 5730 to 1,524,000. The observed lifetime spectra were resolved into three components, where the longest lifetime, τ 3 , was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by free volumes. The change of the temperature coefficient of τ 3 was observed at around 350 K, at which the value of τ 3 was a constant value of 2.3 ns for all specimens with different molecular weights. There was no discrete change of τ 3 in intensity, which is corresponding to the number of free volumes. The size of free volume at glass transition was evaluated to be 0.1 nm 3 .

Free volumes in polystyrene probed by positron annihilation

Free volume characteristics in three samples of monodisperse polystyrene were investigated by positron annihilation technique over a temperature range from 300 to 380 K. The number-average molecular weight of the samples ranged from 5730 to 1,524,000. The observed lifetime spectra were resolved into three components, where the longest lifetime, τ3 was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by free volumes. The change of the temperature coefficient of τ3 was observed at around 350 K, at which the value of τ3 was a constant value of 2,3 ns for all specimens with different molecular weights. There was no discrete change of τ3 in intensity, which is corresponding to the number of free volumes. The size of free volume at glass transition was evaluated to be 0.l nm3. © 1996 John Wiley & Sons, Inc.