Showing papers on "Positronium published in 1981"
TL;DR: Positronium is a two-body, leptonic, particle-antiparticle system which possesses self-annihilation channels not directly present in any system studied to date as discussed by the authors.
Abstract: Positronium is a two-body, leptonic, particle-antiparticle system which possesses self-annihilation channels not directly present in any system studied to date. These features make it ideal for the study of the relativistic, two-body problem in quantum electrodynamics. This review will focus on recent experimental advances in fundamental positronium research. In addition, a less detailed discussion of recent theoretical advances is outlined. The review also contains a fairly detailed historial introduction and a section discussing uses of positronium in research not related to tests of quantum electrodynamics.
202 citations
TL;DR: In this article, the properties of several positron-molecule systems were investigated and the stability of the positron complexes were examined both with respect to detachment of a positron and detachment of positronium.
Abstract: LCAO–MO SCF Hartree–Fock theory is employed to investigate the properties of several positron–molecule systems. Three types of molecules are studied: polar molecules with dipole moments greater than 1.625 D, molecules with large quadrupole moments, and nonpolar molecules with high polarizabilities. For those systems which bind positrons, we compare the positron binding energies with the electron affinities. The stability of the positron complexes are examined both with respect to detachment of a positron and detachment of positronium.
45 citations
TL;DR: In this article, the rate of positron detrapping in thermal equilibrium from lattice defects has been calculated by relating it to the specific trapping rate, and the results for vacancies, dislocations and surfaces each show a different temperature dependence for the escape rate.
Abstract: The rate of positron detrapping in thermal equilibrium from lattice defects has been calculated by relating it to the specific trapping rate. The results for vacancies, dislocations and surfaces each show a different temperature dependence for the escape rate. For vacancies a measure of the importance of the detrapping can be obtained from the ratio of the vacancy formation energy to the positron binding energy in the defect. The positronium desorption rate from a surface is also calculated and agreement with experimental results is found.
36 citations
TL;DR: In this paper, a Richardson-Dushman equation for positronium emission from metal surfaces was derived for clean copper surfaces, where the Richardson-dushman equations for thermionic emission was used to obtain the effective reflection coefficient of thermal incident on the copper surfaces.
Abstract: A formula equivalent to the Richardson-Dushman equation for thermionic emission is derived for positronium emission from metal surfaces. Data on clean copper surfaces agree with the thermodynamic equation if one takes an effective reflection coefficient of thermal Ps incident on the copper surfaces to be $\ensuremath{\simeq}=0.9$. For submonolayer converage of sulfur on copper, the positronium emission exceeds the simple thermodynamic limit, and the implications of this result are explored.
36 citations
TL;DR: In this paper, a two-state approximation is applied to positron-lithium-atom collisions considering only the elastic scattering and the ground-state positronium formation, taking the effect of the adiabatic polarization potential into account in both the direct and rearrangement channels.
Abstract: The two-state approximation is applied to positron--lithium-atom collisions considering only the elastic scattering and the ground-state positronium formation. Taking the effect of the adiabatic polarization potential into account in both the direct and rearrangement channels, results for the differential, total, and momentum-transfer cross sections are computed at incident positron energies 0.5--10 eV. While for the elastic scattering the present data show satisfactory agreement with other available calculations, for positronium formation the present results are quite new and give us an idea of the order of the cross section. It is found that the differential cross section for both the elastic scattering and positronium formation shows a deep minimum at low energies of positron impact in agreement with available earlier findings.
33 citations
TL;DR: In this article, the decay rate for the ψ (υ,…) and the positronium in γ U, U being the new neutral spin-1 boson of supersymmetric theories, was evaluated.
23 citations
TL;DR: Positron lifetimes in human red cell ghost membranes have been measured as a function of temperature from 3 degrees C to 25 degrees C and a marked sudden change in the ortho-positronium annihilation rate was found.
20 citations
TL;DR: In this article, the Weinberg-Salam model was used to study the effects of weak interactions in positronium (Ps), such as parity mixing and weak decays of Ps states.
Abstract: Within the context of the Weinberg-Salam (standard) model we study possible effects of weak interactions in positronium (Ps), such as parity mixing and weak decays of Ps states. As expected, weak interaction amplitudes in Ps turn out to be extremely small, their magnitude being characterized byG·me2≃3·10−12 whereG is Fermi's constant andme the electron mass. We show that the standard model forbids parity-violating correlations in a large class of Ps reactions and decays due to CP conservation in the lepton sector. We then consider situations in which parity-odd effects in Ps will occur in the standard model and may even be large enough to be observable. Beyond the context of the standard model we discuss the decay of orthopositronium into a photon and the hypothetical axion under the assumption that the mass of the axion is smaller than twice the mass of the electron.
19 citations
19 citations
TL;DR: In this article, the first measurement of positron drift speeds in H2 gas has been made in the pressure range 10-200 Torr between 0 and 4 Td, using the annihilation parameter Zeff=13.6, the drift speed increases linearly up to 2.9*105 cm s-1 at 1 Td.
Abstract: Measurements of positron drift in H2 gas have been made in the pressure range 10-200 Torr between 0 and 4 Td. A shape analysis gives a universal function W/Zeff. Using the annihilation parameter Zeff=13.6, the drift speed W increases linearly up to 2.9*105 cm s-1 at 1 Td. Above 1.4 Td there is evidence for positronium formation. These are the first measurement of positron drift speeds.
18 citations
TL;DR: In this article, a quantitative investigation of the physical properties of positronium in super-strong magnetic fields is presented, and it is shown that, in magnetic fields, the energy values and electromagnetic transitions rates of positRONium can also be reduced by a scaling transformation to those obtained for the hydrogen atom in the limit of an infinite proton mass.
Abstract: As a further application of the authors previous discussion of the general two-body problem in the presence of a homogeneous magnetic field, a quantitative investigation of the physical properties of positronium in super-strong magnetic fields is presented. They show that, in magnetic fields, the energy values and electromagnetic transitions rates of positronium can also be reduced by a scaling transformation to those obtained for the hydrogen atom in the limit of an infinite proton mass. Additionally, the annihilation process is considered, and the B dependence of the gamma decay rates is discussed. For magnetic fields B=2.35*1011 G and 4.70*1012 G these quantities are calculated in the adiabatic approximation and are presented explicitly for the lowest-lying states.
TL;DR: A remarkably narrow component was found in the angular correlation curve of photon pairs resulting from the annihilation of positrons in neutron-irradiated niobium containing voids as discussed by the authors, suggesting that in voids positrons exist in positronium states or in free states and decay primarily by 2-γ annihilation.
TL;DR: In this article, the positronium atom with unpaired electrons was studied in aqueous solutions of NiCl2 through measurement of lifetime and through an experiment of magnetic quenching.
Abstract: The collisions of the positronium atom with unpaired electrons were studied in aqueous solutions of NiCl2 through measurement of lifetime and through an experiment of magnetic quenching. These collisions, by causing transitions between all the Ps states, determined a) a shortening of ortho-Ps lifetime at zero field, 6) a marked reduction of the quenching ratio at high field (H > 10 000 G). The results of our measurements are interpreted by means of the Halpern theory of magnetic quenching and by means of the description of exchange collisions given by Ferrell. The ortho-para conversion rate (at Ni++ion concentration of 6.1019 cm-3) is found to be γ = (0.29 ±0.01) ns-1. The overlap probability |yg(0)|2 of the interacting positron-electron pair in Ps was obtained from a further experiment of magnetic quenching in pure water.
TL;DR: In this article, the positronium fraction was measured in Ge for positron implant energies from 25 to 5025 eV and sample temperatures from 300 to 1050 K. The positron diffusion constant (D/sub +/) in three Ge-single crystals was extracted and an interpretation in terms of a transition from a free positron to a small positron polaron state was proposed.
Abstract: The positronium fraction is measured in Ge for positron implant energies from 25 to 5025 eV and sample temperatures from 300 to 1050 K. A simple one-dimensional diffusion model is fitted to this data to extract the positron diffusion constant (D/sub +/) in three Ge-single crystals. The abrupt decrease in D/sub +/ between 600 and 850 K cannot be explained by the mechanism established for metals: positron-vacancy trapping. An interpretation in terms of a transition from a free positron to a small positron-polaron state is proposed.
TL;DR: In this paper, a positron beam has been used to measure the positronium (Ps) fraction reemitted from a Cu(111)+S sample from 40 K to 350 K for incident energies ranging from 0.5 to 5 keV.
TL;DR: In this article, the density and temperature dependencies of the annihilation rates of oPs in ethane gas were investigated and shown to vary with temperature at low densities of the gas.
Abstract: Results are reported for the density and temperature dependencies of the annihilation rates of o‐Ps in ethane gas. The pickoff quenching rate of o‐Ps in ethane is observed to vary with temperature at low densities of the gas. The observed o‐Ps annihilation behavior with density and temperature of ethane is interpreted in terms of density fluctuations in the gas.
TL;DR: In this article, it was shown that the tunnelling of the positronium atom (Ps) as a whole from some trap to the corresponding level of the complex (PsFe 2+ ).
Abstract: The existence of corrections, due to annihilation, to the usual expressions for the energy levels and decay rates of the m = 0 triplet and singlet states of positronium in a magnetic field are noted. The corrections cause the value of the hyperfine interval n, as obtained from well-known resonance experiments, to increase by 1.8 standard deviations of the current experimental error to n = 203 387.5(1) MHz where n(theor) = 203 400(10) MHz. This shift could be observable in the near future when calculations of n to order a/sup 2/ are completed.
CERN1
TL;DR: In this paper, the decay rate of the triplet state of positronium into axion + gamma was calculated under the assumption that the axion mass is less than 1 MeV.
TL;DR: In this article, the validity of using nonrelativistic wave functions for lightly-bound states in relativistic situations was explored, and it was shown that wave functions are acceptable for scattering problems, but not necessarily for those involving a decay.
Abstract: We explore the validity of using nonrelativistic wave functions for lightly-bound states in relativistic situations. This is shown to be acceptable for scattering problems, but not necessarily for those involving a decay. In the latter case the wave function would need to have special properties. These do not occur for a deuteron-like wave function but do occur for a positronium-like system, the crucial features being that positronium is bound by zero mass exchange.
TL;DR: In this paper, the authors investigated the inhibition of positronium formation by C6F14 in n-C6H14 and C8H18 by positromum lifetime measurements.
TL;DR: In this article, the positronium yield was measured in mixtures of electron acceptors diluted with a neutral solvent, and the results confirmed the idea that the charge transfer reaction between electrons in positron spur is of importance for positronion formation.
TL;DR: In this article, the authors studied the angular correlation of the γ rays on the decay of the positronium molecule Ps2 into two photons and parapositronium and determined the differential probabilities of the decays.
Abstract: Decays of the positronium molecule Ps2 into para- or orthopositronium Ps in the ground state and photons are investigated. The differential probabilities of the decays are determined. The total probabilitiesw
2γ
(Ps2)
andw
3γ
(Ps2)
of Ps2 annihilation with the production of two and three photons and positronium are calculated to be
$$w_{2\gamma }^{(P_{S_2 } )} = 1.6 \cdot 10^{10} \sec ^{ - 1} ,w_{3\gamma }^{(P_{S_2 } )} = 0.43 \cdot 10^8 \sec ^{ - 1} $$
. The curve of the angular correlation of the γ rays on the decay of Ps2 into two photons and parapositronium is studied. The width of this curve is Δ¦P¦=0.128 a.u. (¦P¦ is the total photon momentum), which corresponds to a deviation of the emission angle of the γ rays from π: θ ≲ 0.934 mrad. The maximum in the distribution of the photons with respect to the momenta ¦P¦ in the center of mass of the annihilating pair is attained at ¦P¦=0.175 a.u. The calculations were made on an M-222 computer, and their accuracy is determined by the choice of the wave function of the positronium molecule and the accuracy in the computer calculation of the integrals.
TL;DR: Positron annihilation technique was applied to the study of structural properties of mixed normal micelles in sodium octanoate-1-pentanol-water systems as mentioned in this paper, and measurements of positron annihilation parameters over a wide range of temperature and surfactant concentrations demonstrate the extreme sensitivity of the technique toward the structural changes associated with the second and third critical concentrations.
Abstract: Positron annihilation technique was applied to the study of structural properties of mixed normal micelles in sodium octanoate-1-pentanol-water systems. Measurements of positron annihilation parameters over a wide range of temperature and surfactant concentrations demonstrate the extreme sensitivity of the technique toward the structural changes associated with the second and third critical concentrations. While the third critical concentration appears to be temperature independent, the second critical concentration is shifted to slightly lower values at higher temperatures (from approx. 1.4 m to approx. 1.0 m on going from 21 to 51/sup 0/C). The plot of the Arrhenius activation energies of positronium formation as a function of surfactant molality indicates some continuous changes in the properties of the micelles below and above the third concentration.
TL;DR: In this paper, the positron annihilation of polymers is discussed and the reactions of positrons and positronium, the effect of external electric and magnetic fields, and the influence of various kinds of radiation, temperature, pressure, phase transitions, etc.
Abstract: The problems of the annihilation of positrons in polymers are discussed and the reactions of positrons and positronium, the effect of external electric and magnetic fields, and the influence of various kinds of radiation, temperature, pressure, phase transitions, etc. are examined. The practical value of the positron method for the investigation of polymers is demonstrated. The bibliography includes 180 references.
TL;DR: In this paper, the dispersion relation for positron-atom forward elastic scattering is obtained for the case when the first ionisation potential of the atom is smaller than the binding energy of positronium.
Abstract: The dispersion relation for positron-atom forward elastic scattering is obtained for the case when the first ionisation potential of the atom is smaller than the binding energy of positronium. The possible applications are discussed.
TL;DR: A new experiment in which positronium is formed in vacuum with a low energy polarized positron beam and an improvement of up to 10(4) in sensitivity to the effect is possible.
Abstract: A positronium-formation experiment with a high sensitivity to a possible relation between the helicity of beta particles emitted in nuclear beta decay and the optical asymmetry of biological molecules is presented. The experiment is based on a mechanism in which the electrons in optically active molecules possess a helicity of less than 0.001, too weak to detect in radiolysis experiments, the sign of which depends on the chirality of the isomer. A helicity-dependent asymmetry is sought in the formation of the triplet ground state of positronium when a low-energy beam of polarized positrons of reversible helicity interacts with an optically active substance coating a channel electron multiplier. Asymmetries between positronium decays observed at positive and negative helicities for the same substance can thus be determined with a sensitivity of 0.0001, which represents a factor of 100 improvement over previous positronium experiments.