Showing papers on "Positronium published in 1972"
TL;DR: The behavior of the Ps atom in molecular substances, particularly liquids, is investigated in this article, where the pickoff rates of oPs in various liquid compounds are found to have a simple empirical relationship to the values of the surface tension of the liquids.
Abstract: The behavior of the Ps atom in molecular substances, particularly liquids, is investigated. The pickoff rates of o‐Ps in various liquid compounds are found to have a simple empirical relationship to the values of the surface tension of the liquids. The relationship is found to have a theoretical foundation. The Ps atom is highly localized in a cavity created by the balance of various molecular forces inside the liquid. From the above relationship, other simple relationships between the pickoff rates of o‐Ps and the various properties of the medium, e.g., polarizability, cohesive energy density, etc., and the temperature or pressure changes can be derived and explained. The diffusion of o‐Ps is discussed. A similar approach can also be used for molecular solids.
1,731 citations
TL;DR: In this paper, the authors studied the void, a small vacancy of diameter few tens of Angstroms, which may contain gas atoms and may be disturbed either in a random fashion or arranged in a macro lattice.
Abstract: POSITRON annihilation investigations of defects in crystals have shown that for sufficiently high defect concentrations (typically above about 10−6) all positrons become trapped in the defects before annihilation, thus changing the characteristics of the annihilation process. For example, trapping of positrons may result in the increase in the positron lifetime, a narrowing of the 2-γ angular correlation distribution, and a reduction in the Doppler broadening of the annihilation line. Vacancies in metals1,2, deformation effects in metals3, and defects (F-centres or cation vacancies) in ionic crystals4 have all now been studied by this technique. The trapping of positronium (Ps) in defects in quartz5 and ice6 has also been investigated. Here we report the study of another crystal defect, the void, a small vacancy of diameter few tens of Angstroms. In practice voids may contain gas atoms and may be disturbed either in a random fashion or arranged in a macro lattice7.
176 citations
TL;DR: In this article, the properties and behavior of the positron (positive electron, anti-electron) and of positronium, a "hydrogen atom" containing a positron instead of a proton, are considered from the chemist's viewpoint.
Abstract: In this progress report, the properties and behavior of the positron (“positive electron”, anti-electron) and of the positronium, a “hydrogen atom” containing a positron instead of a proton, are considered from the chemist's viewpoint. Examples are given to demonstrate the development of positronium chemistry, in aqueous solution and in the gaseous, liquid, and solid phases, with its problems and possibilities.
72 citations
TL;DR: In this paper, a detailed analysis of the structure of the zero energy wave function is made, and the wavefunction is used to calculate the electron-positron annihilation rate parameter Zeff, with the result 8.9.
Abstract: Kohn's variational method is used to calculate the positron-hydrogen scattering length and the S wave phase shifts at several energies below the positronium formation threshold, and good agreement is obtained with the results of other recent variational calculations. A detailed analysis of the structure of the zero energy wavefunction is made, and the wavefunction is used to calculate the electron-positron annihilation rate parameter Zeff, with the result 8.9, and the angular correlation of the two annihilation gamma rays. The energy dependence of Zeff is also investigated using the positive energy wavefunctions.
65 citations
TL;DR: In this article, the Fourier transform of the polarization charge around a positron in an electron liquid has been derived, which has three-body correlations built in it, and the calculated annihilation rate of the positron has been shown to merge smoothly into the mean positronium lifetime for larger density.
Abstract: We derive a nonlinear integral equation, which has three-body correlations built in it, for the Fourier transform of the polarization charge around a positron in an electron liquid. The calculated annihilation rate of the positron in the entire metallic density range, besides being in very good agreement with experiment, merges smoothly into the mean positronium lifetime for larger density. To illustrate the large nonlinear effects, the polarization charge around a fixed proton for a density ${r}_{s}=2$ has also been calculated.
48 citations
TL;DR: In this paper, the total scattering cross section for e+ in He has been measured through two energy intervals, 1 to 4'eV and 17 to 26 'eV, and the results are consistent with the calculations of Drachman.
Abstract: The total scattering cross section for e+ in He has been measured through two energy intervals, 1 to 4 eV and 17 to 26 eV. Through the lower elastic-scattering interval our results are consistent with the calculations of Drachman. From below the positronium formation threshold, at 17.8 eV to above the He ionization threshold, our results agree qualitatively with the calculations of Kraidy, who includes in his model virtual positronium and positronium polarization. Just below 17.8 eV the cross section is 0.35 ± 0.04 πa02. In these experiments a high-current linac has been used as a source of positrons and total beam attenuation has been used to determine the cross section.
43 citations
41 citations
TL;DR: In this article, a simple theoretical model based on the necessity of treating the positronium electron and the remaining electrons in the crystal as indistinguishable was proposed, which yields results compatible with both momentum and lifetime data.
Abstract: Previous experiments with positron annihilation in single crystals of quartz have revealed an interesting series of narrow peaks on the momentum distribution of the annihilation photons. Our experiments repeat these observations and also confirm that no very long lifetime exists. We outline a simple theoretical model based upon the necessity of treating the positronium electron and the remaining electrons in the crystal as indistinguishable. The model yields results compatible with both momentum and lifetime data.
39 citations
34 citations
TL;DR: In this paper, a weak beam of energy-analyzed positrons in the range 15'eV to 23 'eV was used to measure positron cross sections on helium.
Abstract: Positron cross sections on helium have been measured by transmission using a weak beam of energy-analyzed positrons in the range 15 eV to 23 eV. The positrons are transmitted in a magnetic field parallel to the axis of the scattering chamber. Comparisons with theory have been made using several sets of phase shifts, and good agreement is found if the latest p- and d-wave phase shifts of Drachman are used. The experiments fail to show any sharp reduction in the transmission at the positronium threshold, from which we deduce an upper limit to the positronium formation cross section of . The agreement between our results and those of Costello et al. in the neighborhood of 17 eV is very good.
31 citations
TL;DR: In previous experiments with positrons annihilating in liquid organic materials (Hogg et al. as discussed by the authors ) the measurements of IN (intensity of the low momentum component from angular correlation data) and I2(intensity of long lived component from lifetime measurements) have resulted in IN = I2/3 within experimental error.
Abstract: In previous experiments with positrons annihilating in liquid organic materials (Hogg et al.) the measurements of IN (intensity of the low momentum component from angular correlation data) and I2 (intensity of the long lived component from lifetime measurements) have resulted in IN = I2/3 within experimental error.For samples of CH4, C6H6, and C6H12 measurements in the liquid state yield the expected equality between IN and I2/3. However, in the solid state for these substances there is a marked disagreement between IN and I2/3. The value of IN, in all cases, is essentially zero suggesting that no positronium is formed, but the value of I2 remains large.
TL;DR: In this article, a series of liquid crystalline compounds were decomposed into two components: the short-lived component (lifetime τ1 and intensity I1) was attributed to free-positron and parapositronium annihilation whereas the longlived component τ2, I2 was considered to be the result of orthopositroniam annihilation, supporting the contention that the positron annihilation process is affected by variations in the structural regularities of the environment.
Abstract: Positron lifetime measurements were carried out in a series of liquid crystalline compounds. The resulting lifetime spectra were decomposed into two components. The short‐lived component (lifetime τ1 and intensity I1) was attributed to free‐positron and parapositronium annihilation whereas the long‐lived component τ2, I2 was considered to be the result of orthopositronium annihilation. Changes of τ2 and I2 during mesomorphic phase transitions are generally very small and can be related to the transition entropies involved, supporting the contention that the positron annihilation process is affected by variations in the structural regularities of the environment. Various mechanisms which could account for the observed changes of the positron lifetimes and their intensities at the phase‐transition points, are discussed.
TL;DR: In this article, an exact bound state solution of the nonlinear field equations of the theory for a particle-antiparticle pair is demonstrated, from the Lagrangian formalism, which is shown to predict all of the experimental facts that are conventionally interpreted in terms of pair annihilation.
Abstract: The general theory developed thus far (Sachs, 1971b, c, d) is applied to two-particle systems. An exact bound state solution of the nonlinear field equations of this theory for a particle-antiparticle pair is demonstrated. From the Lagrangian formalism, this solution is shown to predict all of the experimental facts that are conventionally interpreted in terms of ‘pair annihilation’: (1) the energy-momentum four-vector (and each of the four components, separately) are zero, compared with the energy, 2mc2, of the state when the particle and antiparticle are (asymptotically) free and (2) the dynamical properties of this state of positronium make it appear in experimentation as two distinguishable currents, correlated with a 90° phase difference and polarised in a plane that is perpendicular to the direction of propagation of interaction with other charged matter. The latter features are conventionally interpreted as the two photons which are produced in the annihilation event — however, there are no photons in this theory. The spectral distribution of blackbody radiation is then derived from the properties of an ideal gas of such pairs, in their ground states of null energy-momentum, as observed in a finite cavity.
TL;DR: The mean life τ2 or orthopositronium annihilating in samples of Nylon 6 both dried and containing 05% of H2O has been measured as a function of temperature from 15°C up to 100°C as discussed by the authors.
Abstract: The mean life τ2 or ortho‐positronium annihilating in samples of Nylon 6 both dried and containing 05% of H2O has been measured as a function of temperature from 15°C up to 100°C A sudden change of τ2 in the lifetime‐vs‐temperature curve was found at the glass‐transition temperature The main glass transition for the dry sample is determined to be 50°C and for the sample containing water, 33°C The Tg measured by this method is lower than that measured by other methods This agrees well with the result for polystyrene observed by Stevens and Mao
TL;DR: Positron-annihilation angular-correlation measurements have been made on compressed silicon dioxide powders having mean particle diameters of 50, 120, and 400 \AA{.
Abstract: Positron-annihilation angular-correlation measurements have been made on compressed silicon dioxide powders having mean particle diameters of 50, 120, and 400 \AA{}. The narrow component in the momentum distribution is much more pronounced in the 50- and 120-\AA{} powders than it is in the 400-\AA{} powder. Measurements made in vacuum reveal a narrow component in the momentum distribution which increases with increasing temperature until a saturation temperature is reached. A positronium diffusion constant has been calculated for five temperatures. Similar measurements made in oxygen show an increased narrow component with a nonsaturating temperature dependence. From these measurements a temperature-dependent oxygen enhancement factor has been determined. Both vacuum bakeout and compressing the powder were found to increase the amount of narrow component.
TL;DR: In this paper, a complete calculation of position-hydrogen elastic scattering in a pseudo-state expansion formulation is made and results obtained for s, p, d, etc., partial wave phase shifts.
Abstract: A complete calculation of position-hydrogen elastic scattering in a pseudo-state expansion formulation is made and results obtained for s, p, d, etc., partial wave phase shifts. Good agreement is found between these calculations and those of long range force oriented models for energies below the positronium formation threshold.
TL;DR: The existence of both conversion and chemical positronium quenching by oxygen in liquid nitrogen is proved in this article, and the contributions of conversion and reaction to the total interaction rate can be changed by a variation of the physical conditions of the experiment.
TL;DR: In this paper, a method for the investigation of momentum distribution of photon annihilation from the surface of polycrystalline iron and aluminium, and single-crystaline molybdenum was given.
Abstract: A method is given for the investigation of momentum distribution of γ-quantum annihilation from the metal surfaces. The method uses the interaction between positronium atoms and surface atoms. Possible mechanisms of this interaction are discussed and the conditions under which the method may be applied are defined. The method was used for the investigation of the momentum distribution of photon annihilation from the surface of polycrystalline iron and aluminium, and single-crystalline molybdenum. It is shown that this distribution is parabolic for Fe and Al and differs significantly from the conduction electron distribution in the bulk metal. For Mo crystals it was concluded that there are two states for electrons on the surface of the (100) plane. It is assumed that the experimental definition of the electrons density gradient on the metal–vacuum interface may serve as a criterion which allows to estimate some physical properties of metal and alloy surfaces. The possibilities of the new method are disccussed.
[Russian Text Ignored].
TL;DR: In this article, the half-life of triplet positronium and the radiolysis free ion yield of n-alkanes were investigated and the effect of increasing the molecular chain length on these two phenomena appeared to be simply to increase the density of interacting sites.
Abstract: Two seemingly unrelated phenomena in liquid hydrocarbons have similar trends in their dependences upon the molecular structure of the hydrocarbon. The phenomena are the annihilation half-life of triplet positronium and the radiolysis free ion yield. In n-alkanes the effect of increasing the molecular chain length, upon both phenomena, appears to be simply to increase the density of interacting sites. Branching the alkane chain decreases the strength of molecular interaction with both electrons and positronium, although the relative decrease is much greater for the former than for the latter. The effects of double bonds on the phenomena, after separating out the density effect, are different from each other.
Journal Article•
TL;DR: Positron lifetime measurements have been used to investigate the polymorphism of tristearin this paper, showing that both positronium formation and annihilation are sensitive to the phase changes in tristlein.
TL;DR: Positron lifetime measurements have been used to investigate the polymorphism of tristearin this article, showing that both positronium formation and annihilation are sensitive to the phase changes in tristlein.
TL;DR: It was suggested from the experiments that positrons are captured in liquid or amorphous media, owing to the strong dipolar electric field of polar groups which are free from surrounding molecules as discussed by the authors.
TL;DR: In this article, the parametric resonance in the decay of a system such as positronium is discussed, by means of the Wigner-Weisskopf perturbation theory.
Abstract: The parametric resonance in the decay of a system such as positronium is discussed. It is shown, by means of the Wigner-Weisskopf perturbation theory, that the decay constants depend on the frequency of the external perturbation.