Ps Bubble Research Articles

Characterization of the melting process of PTFE using positron annihilation spectroscopy

Poly(tetrafluoroethylene) (PTFE) is a semi-crystalline polymer and the lifetime of ortho-positronium(o-Ps) is known to be able to be separated into two components due to annihilation in the crystal region and in the amorphous region. The melting process of PTFE was investigated using positron annihilation spectroscopy and X-ray diffraction. The results indicated that volume expansion with an increase of temperature is dominantly due to the expansion of the amorphous region and a Ps bubble is formed at melting in both regions. The o-Ps relating to the crystal region definitely remains on the surface of crystal at the time of annihilation. The production of lower energy electrons at melting was deduced by the analysis of the Doppler broadened annihilation photopeak, and the increase in the number of such electrons was found to have great influence on the formation of the o-Ps and annihilation processes of positron and o-Ps.

Positronium bubble formation in room temperature ionic liquids

Positron annihilation age-momentum correlation (AMOC) measurements were performed for a room temperature ionic liquid (IL) to investigate positronium (Ps) bubble formation process. The Ps just after the formation must be squeezed Ps that has larger probability of annihilation with core electrons in liquids. However, it has been believed that the bubble formation in liquids is very fast and difficult to observe the squeezed Ps experimentally. When the bubble formation is slow, it can give broader energy distribution of annihilation gamma-rays at young age region. There have been several experimental results showing young age broadening and some of them are interpreted as the delayed Ps formation. However, if the bubble formation is slow, it also can be a reason of the young age broadening. If the squeezed Ps and the free positron give different momentum distributions by annihilation with core electrons on different atoms, it is possible to detect the annihilation from squeezed Ps at the young age, i.e. the bubble formation can be observed. We found larger high momentum distribution at young age in IL and it might be caused by the delayed bubble formation in IL. Positron annihilation methods can be a tool to investigate the IL molecular dynamics at the time range of pico-nano second and in the scale of sub-nano to nano meter.

Ps Bubble in Liquids

Ortho-positronium lifetime in alkanes and alcohols was measured in a broad range of temperatures. The results were compared with predictions of the bubble model. The bubble radius can be determined from the Tao-Eldrup equation as well as calculated using the model. The depth of potential well and surface tension are the main factors ruling the Ps bubble size. The best fit of the bubble model to the experimental data is obtained assuming the rectangular potential well to be 1 eV deep (equivalent to an infinite one broadened by 0.166 nm) and the microscopic surface tension increased about 3 times comparing to the macroscopic one.

Positronium in a Liquid Phase: Formation, Bubble State and Chemical Reactions

The present approach describes the e+ fate since its injection into a liquid until its annihilation. Several stages of the e+ evolution are discussed: (1) energy deposition and track structure of fast positrons: ionization slowing down, number of ion-electron pairs, typical sizes, thermalization, electrostatic interaction between e+ and the constituents of its blob, and effect of local heating; (2) positronium formation in condensed media: the Ore model, quasifree Ps state, intratrack mechanism of Ps formation; (3) fast intratrack diffusion-controlled reactions: Ps oxidation and ortho-paraconversion by radiolytic products, reaction rate constants, and interpretation of the PAL spectra in water at different temperatures; (4) Ps bubble models. Inner structure of positronium (wave function, energy contributions, relationship between the pick-off annihilation rate and the bubble radius).

Influence of Temperature on Intratrack Processes and Ps Formation and Behaviour in Liquid Water

Positron annihilation lifetime (PAL) spectra are measured in liquid water in the temperature range 2 – 930C. The spectra are treated by taking into account intratrack reactions and assuming that radical reactions with Ps are diffusion-controlled (the respective temperature dependences obeying the Stokes-Einstein law). Equilibrium Ps bubble parameters are obtained.

Energy dissipation and Ps bubble growth in liquids

Dissipative energy losses accompanying growth of the Ps bubble are calculated using the Navier–Stokes equation. This allows to demonstrate a fulfilling of the total energy balance in this process. It is in favor of the adopted approach.

Formation of the Ps bubble in liquid media

The dynamics of the growth of the positronium bubble is considered in hydrodynamic approach. Driving force of the process is the exchange repulsion of the e − constituting Ps from the electrons of the surrounding molecules. Intermolecular forces (Laplace pressure) and friction forces (viscosity) hinder the bubble growth. Numerical calculations are done for a number of liquids having very different surface tensions and viscosities. In liquid He and glycerol the Ps bubble formation time is comparable or even exceeds the para-Ps lifetime.

Angular correlation of positron annihilation radiation in perfluorohexane mixtures with nonpolar liquids

Positron-annihilation linear-slit angular-correlation (AC) spectra in perfluorohexane (C 6 F 14 ) mixtures with hexane and isooctane in the entire concentration interval are reported. Annihilation with electrons in fluorine results in an unusually broad AC-component. The data show that the free positron reacts to form a bound state with C 6 F 14 at low C 6 F 14 concentration. At high C 6 F 14 concentration the positron in Ps annihilates preferentially with electrons in the hydrocarbons, and not with the electrons in C 6 F 14 before this cannot be avoided. Hence, the energy of the Ps bubble state is lower if the surface consists of hydrocarbon molecules

Positronium localization in solid and liquid methane

Positron lifetimes and Doppler broadening of annihilation radiations have been measured on high-purity condensed methane at temperatures ranging from 30 K to 150 K. In the solid phase, the o-Ps lifetime and S parameter increase sigmoidally as a function of temperature. This variation is interpreted as being due to a Ps atom being trapped in thermally activated vacancy-type defects. A vacancy formation enthalpy of 0.10+or-0.02 eV is obtained in solid methane. In the liquid phase, large values for the o-Ps lifetime and intensity, and strong temperature variation of these values are observed. These results are interpreted as being due to the localization of the Ps atom in a bubble state. The temperature dependence of Ps bubble size and microscopic surface tension are evaluated using o-Ps lifetime data.

Quenching of positronium atoms by nitrobenzene in various solutions

A systematic study has been carried out on the quenching of positronium (Ps) atoms by nitrobenzene in various solvents. In order to see the effect of the energy level of Ps, in the localized bubble state, on the quenching capability of nitrobenzene, overall second-order rate constants were plotted against solvent surface tensions. It was found that the rate constant increased with increasing surface tension up to a critical value of ca. 26 dyn cm–1, corresponding to a Ps bubble energy of 1.24 eV. In liquids where the Ps energy is higher than this, the rate constant decreased with increase in solvent viscosity, as is expected for a diffusion-controlled reaction.

TEMPERATURE DEPENDENCE OF CONDENSED METHANE STUDIED BY POSITRON LIFETIME MEASUREMENTS

Positron lifetime spectra have been measured as a function of temperature from 30 K to 150 K in condensed methane. In solid phase, the positronium (Ps) formation in free volume and the Ps yield of 27% were observed. The sigmoidal variation of o-Ps lifetime with temperature was interpreted by Ps trapping in the thermally activated defects. The activated energy Ea = 0.10±0.02 eV was derived in terms of Ps trapping model. In liquid methane with saturated vapor pressure, the results that o-Ps lifetime is 5-7ns and Ps formation probability is 36% indicate the Ps bubble formation. The temperature dependence of the bubble size and surface tension were estimated.

Information on the structure of liquids and solutions obtained by positron annihilation

The low energy positron annihilation research correlated to the research of the structure of liquids and solutions is briefly reviewed. A guide to other review papers is given. One topic is discussed in more detail, namely the bubble state of positronium (Ps) in liquids, and the correlation between the ortho Ps pick off rate and the dispersion and polar components of the solubility parameter. Apparently, the Ps bubble size is independent of the presence of hydrogen bonds.

The temperature dependence of the two positronium bubble states in liquid SF 6

Positron lifetime measurements have been performed on liquid SF 6 in the temperature range from −45°C to 71°C ( T c = 45.65°C). The positron lifetime spectra were resolved into four lifetime components. In the order of increasing lifetimes the four lifetime components are associated with the decay of para-positronium ( p-Ps), free positrons, ortho-positronium ( o-Ps) from a small bubble state, and o-Ps from a large bubble state. The lifetime of o-Ps annihilating from the large bubble state τ 4 increases from 5.7 ns at −45°C to 19.5 ns at 53°C. The lifetime of o-Ps annihilating from the small bubble state τ 3 was found to be 2–2.5 ns in the main part of the temperature range studied. Apparently, this is the first observation of two different o-Ps states in a liquid. The intensity I 4 ( I 3) increases (decreases) from 16.9% (16%) at −45°C to 47.2% (6.4%) at the critical point while above I 3 and I 4 are essentially temperature independent. The large Ps bubble state seems to be similar to the Ps bubble state found in most liquids.

Solvated positron chemistry. II. The reaction of hydrated positrons with Cl −, Br − and I − ions

The reaction of the hydrated positron, e aq + with Cl −, Br −, and I − ions in aqueous solutions was studied by means of positron The measured angular correlation curves for [Cl −, e +], [Br −, e +, and [I −, e +] bound states were in good agreement with th Because of this agreement and the fact that the calculated positron wavefunctions penetrate far outside the X − ions in the [X −, e +] sta propose that a bubble is formed around the [X −, e +] state, similar to the Ps bubble found in nearly all liquids. F −ions did not react w Preliminary results showed that CN − ions react with e aq + while OH −ions are non reactive. The rate constants were 3.9 × 10 10 M −1 s −1, 4.4 × 10 10 M −1 s −1, and 6.3 × 10 10 M −1 s −1 for Cl −, Br −, and I −, respectively, at low (≲ 0.03 M) X − concentrations. A 25% decrease in the rate constant caused by the addition of 1 M ethanol to the I − solutions was i The influence of halide ions on the positronium (Ps) yields in pure water was studied by use of lifetime measurements. The Cl −, Br −, and I − ions reduced the Ps yields at low concentrations (≲ 0.03 M), while F − ions only reduced the Ps-yield However, the Ps yields saturated (e.g. at ≈ 21% ortho-Ps yield in the Cl − case) at higher concentrations. This saturation and the high-concentration effects-in the angular correlation results were interpreted as caused by rather complicated spur effects, wh It is proposed that spur electrons may pick off the positron from the [X −, e + states with an efficiency which depends on the structure of the