Showing papers on "Positronium published in 1970"
TL;DR: The influence of magnetic field on the narrow peaks in the 2γ angular distribution from e + annihilation in quartz was studied in this paper, and the resulting quenching and polarization effects were interpreted as evidence for positronium-like Bloch states.
41 citations
TL;DR: Using a local effective potential to account for electron exchange, the authors in this paper analyzed the zero-energy scattering of ortho-positronium by helium atoms and found that the results obtained in the static approximation agreed with the static-exchange results of Fraser and Kraidy (1966).
Abstract: Using a local effective potential to account for electron exchange, the authors have analysed the zero-energy scattering of ortho-positronium by helium atoms. The effective potential was chosen so that the results obtained in the static approximation agreed with the static-exchange results of Fraser and Kraidy (1966). Correlation was introduced in two different approximations which agree well. A decrease is found in the scattering length a from 1.72 a0 to 1.39 a0, and an increase of more than a factor of 2 in the quenching rate of the positronium (from Zeff=0.042 to 0.10), as compared with the static-exchange approximation. This gives improved agreement with the experimental value Zeff approximately=0.18.
33 citations
TL;DR: In this article, the mean lifetime τ2 of ortho-positronium annihilating by pickoff depends upon the free volume of the atom and the number of atoms in the atom.
Abstract: In solid polymers the mean lifetime τ2 of ortho‐positronium annihilating by pickoff depends upon the free volume. τ2 has been measured for five samples of relatively monodisperse atactic polystyrene over a temperature range from room temperature to 120°C. The number average molecular weight of the samples ranged from 50 100 to 640 000. For each sample the variation of τ2 with temperature indicated a glass transition about 13 deg below that measured on a differential scanning colorimeter. An increase in the glass transition temperature with molecular weight was observed. The τ2 lifetime, which depends upon the overlap of the positron component of the ortho‐positronium atom with the lattice wavefunction, seems to be very sensitive to the cooperative motion about the chain axis which is associated with the glass transition.
31 citations
TL;DR: It is found that iodine acts as a chemical quencher to positronium, and CCl4 acts as an inhibitor to the formation of positrons.
Abstract: Positron annihilation time spectra on various concentration of iodine and CCl4 adsorbed in silica gels have been studied It is found that iodine acts as a chemical quencher to positronium, and CCl4 acts as an inhibitor to the formation of positronium
28 citations
23 citations
TL;DR: In this paper, the cross sections for electron capture by positrons from molecular hydrogen forming positronium are calculated in the Born approximation for different incident energies, compared with those obtained using a simple peaking approximation.
19 citations
01 Jan 1970
TL;DR: In this article, the authors show that a singlet collision results in emission of two photons in exactly opposite direction and each one having an energv of about 511 = m0 c2) KeV.
Abstract: Positron is the antipartie le of electron and its existence was predicted “by Dirac’s relativistic theory of electron. Positrons were experimentally discovered “by Anderson in cloud chamber photographs of cosmic rays. In a material medium, positrons do not live long because they can either annihilate with electrons in the medium, or can form a metastahle system of a positronium atom which is short-lived. In the first type of decay, called prompt or free annihilation, a positron, after thermalisation, annihilates with an electron in “the medium either througji a singlet collision (spins antiparallel) or through a triplet collision (spins parallel). Selection rules governing the annihilation show that a singlet collision results in emission of two photons in exactly opposite direction and each one having an energv of about 511 (= m0 c2) KeV. This annihilation has a life time ~10-10 s. On the other hand, a triplet collision gives rise to an annihilation into three photons with a lifetime ~10-7 secs. In this case the total rest energy (= 2 m0 c2) of the electron-positron pair is shared between the three photons which are emitted in one plane. The ratio of singlet (2 photon) to triplet (3-photon) annihilation rates was calculated by Ore and Powell2 to “be
$${\lambda _s}/{\lambda _t} = 1115$$
(1)
16 citations
TL;DR: It has been found that the lifetime τ 2 and the intensity I 2 of triplet positronium in light and heavy water show a sudden transition at the respective melting points for the two substances as mentioned in this paper.
15 citations
20 Apr 1970
TL;DR: In this paper, techniques were developed for application of lasers to two fundamental experiments in atomic spectroscopy: Fine structure of positronium n = 2 state; Vibrational spectrum of the hydrogen molecular ion HD(+).
Abstract: : Techniques were developed for application of lasers to two fundamental experiments in atomic spectroscopy: Fine structure of positronium n = 2 state; Vibrational spectrum of the hydrogen molecular ion HD(+). (Author)
13 citations
TL;DR: In this paper, the pickoff annihilation rate of positronium was calculated in a cell approximation for various cell geometries, and the rate can be expressed in terms of two functions $F(S,{v}^{*})$ and $\ensuremath{\xi}(S,{v})$ that depend on a cell potential parameter and the reduced cell volume.
Abstract: In earlier papers [Phys Rev 120, 1289 (1960); 142, 231 (1966)] the pickoff annihilation rate of positronium was calculated in a cell approximation for various cell geometries The rate can be expressed in terms of two functions $F(S,{v}^{*})$ and $\ensuremath{\xi}(S,{v}^{*})$ that depend on a cell potential parameter $S$ and the reduced cell volume ${v}^{*}$ These functions are rederived and previous errors corrected Extensive numerical tabulations are made available and analytical approximations presented
11 citations
TL;DR: It is concluded that cell water in muscle is not ordered in a hexagonal icelike structure, and the hypothesis thatcell water is ordered in an undetermined close packed structure which transforms to the hexagonal ice structure at or near 0 degrees C cannot be ruled out.
TL;DR: In this article, the lifetimes of orthopositronium in solutions of α-α∗-diphenyl-β-pcryl-hydrozyl (DPPH-zyl) and DPPH-zine in methanol and of nitrobenzene and nitromethane in benzene have been measured.
CERN1
TL;DR: In this paper, a two-body bound state equation for a positron and an electron in an interacting electron gas is derived, using a Green's function decoupling scheme and various approximations required to define a bound state.
Abstract: A two-body bound state equation for a positron and an electron in an interacting electron gas is rederived, using a Green's function decoupling scheme and various approximations required to define a bound state are explained. The equation has some new features not discussed before, and the solution rules out the formation of positronium-like state at metallic densities. But the critical r 8 -value where the bound state formation starts is found to be higher than previously reported. § I. Introduction The question whether a positronium-like two-body bound state forms when positrons are injected into metals is often discussed in the literature. 1 )"'"' 3 ) In this work we re-examine the criterion of the bound state formation and show that the r~~-values quoted are too low. r 8 is the usual electron gas parameter con nected to the density of electrons per c.c., n, by
TL;DR: The decay rate of ortho-positronium was measured in degassed molecular liquids and it was found that each atom contributes to the pick-off quenching with an effective number of electrons in the singlet state which does not appear to be perceptibly affected by the structural properties of the molecules as mentioned in this paper.
Abstract: The decay rate of ortho-positronium was measured in degassed molecular liquids. It was found that each atom contributes to the pick-off quenching with an effective number of electrons in the singlet state which does not appear to be perceptibly affected by the structural properties of the molecules. This result is discussed in the framework of the free-volume model proposed by Brandt and co-workers.
TL;DR: In this paper, the authors compare and contrast the mechanical properties of electrons, positrons, and holes in an accelerating crystal lattice and observe that positronium atoms, if they could be condensed, would be superfluid; excitons, when condensed, are not superfluid but change the spatial long range order.
TL;DR: In this article, the authors measured the lifetimes of orthopositronium in several solutions in liquid and solid octanol and benzene and found that the Ps-quenching constant was two to thirty times higher in the solid than in the liquid phase.
TL;DR: Positron lifetime measurements in some ionic compounds like sulphates and hydroxides are reported in this paper, showing that positrons annihilate in these compounds with two lifetimes: τ 1 ≈ 0.3 nsec and τ 2 ≈ 1.5 nsec.
TL;DR: In this article, a quadruple coincidence between the Lyman-α photon and the three ortho-positronium annihilation quanta was used to detect Lymanα radiation from the positronium atom.
Journal Article•
TL;DR: In this paper, the total cross-section for positronium negative ion formation was calculated using the Born approximation and some simplifying assumptions, and the maximum value was estimated to be about 0.09πa02.
Abstract: The total cross-section for positronium negative ion formation in positronium-hydrogen collisions is calculated, using the Born approximation and some simplifying assumptions, for positronium energies up to 90 eV. The maximum value of the cross-section is estimated to be about 0.09πa02 and occurs at the positronium energies of 21 eV. Comparisons with other calculations are also reported. In particular, comparison with the scarse available calculations on ortho-para conversion of positronium shows that at energies greater than 20 eV, positronium ion formation may become a competitive process. Further, it is suggested that positronium ion formation in positronium-hydrogen collisions may serve as a positronium quenching mechanism.
01 Aug 1970
TL;DR: In this paper, the free volume model for the formation and quenching of positronium atoms in molecular materials is discussed in terms of the X-ray diffraction patterns for these materials.
Abstract: The value (τ2) and the intensity (I2) of the delayed component in the lifetime spectra of positrons annihilating in annealed and quenched teflon and in sulphur and crystex (polymer sulphur), at room temperature and at 77° K are reported. These data and the X-ray diffraction patterns for these materials are discussed in terms of the free volume model for the formation and quenching of positronium atoms in molecular materials.
TL;DR: In this article, a method for measuring the momentum correlation in 3γ annihilation of positronium is reported, where the energies of two of the annihilation photons were detected in coincidence by NaI scintillators and registered by a two-parameter analyser.
TL;DR: In this paper, the Sommerfeld-Welker model of a hydrogen atom in a spherical, impenetrable force cavity was used to calculate the basic properties of positronium in an ionic crystal.
Abstract: The Sommerfeld-Welker model of a hydrogen atom in a spherical, impenetrable force cavity is used to calculate the basic properties of positronium in an ionic crystal. The basic properties of positronium (the lifetime against 2γ- and 3γ-annihilation, the count rate of 3γ coincidences, and the magnetic-quenching parameter) are found as functions of the radius of the spherical force cavity.
01 Jan 1970
TL;DR: The positron annihilation photons from positrons annihilating in metal-ammonia solutions indicate the presence of a bound system at the time of annihilation as mentioned in this paper, which can be attributed to either bound (localized) electrons or to the formation of substantial amounts of bound systems induced by the positron (such as positronium).
Abstract: The annihilation photons from positrons annihilating in metal-ammonia solutions indicate the presence of a bound system at the time of annihilation. This can be attributed to either bound (localized) electrons or to the formation of substantial amounts of a bound system induced by the presence of the positron (such as positronium). The situation persists up to concentrations as high as 94 per cent caesium in ammonia. Both alternatives seem improbable.