Showing papers on "Positronium published in 1987"
TL;DR: In this paper, the authors discuss the possibility of producing antihydrogen in collisions of antiprotons, p with positronium, and discuss the demands which must be met in order to make an experiment of this kind feasible.
Abstract: The authors discuss the possibility of producing antihydrogen in collisions of antiprotons, p with positronium, Ps. Theoretical results of the cross section for this scattering channel are presented for the range of impact velocities from 0 to 4.6*106 m s-1 together with a discussion of the demands which must be met in order to make an experiment of this kind feasible.
109 citations
TL;DR: In this paper, the authors reported accurate variational calculations of the 1,3S and 1.3P phaseshifts for the elastic scattering of electrons (or positrons) by positronium up to the n=2 threshold.
Abstract: The authors report accurate variational calculations of the 1,3S and 1,3P phaseshifts for the elastic scattering of electrons (or positrons) by positronium up to the n=2 threshold. Higher-order phaseshifts are evaluated both in a static-exchange model and in an adiabatic-exchange model to give the total elastic, momentum transfer, ortho-para conversion and elastic differential cross sections. The positions of the minima in the elastic differential cross section are given for several values of k2. New 3P resonances are reported. The 1P continuum wavefunction is used together with an accurate variational wavefunction for the ground state of Ps- to obtain the photodetachment cross section of Ps-, and both length and velocity results are checked by evaluating the sum rule S-1.
81 citations
TL;DR: The emphasis of this work is on the energy levels of the delocalized positron and the electron chemical potential which are now calculated with respect to the same potential reference.
Abstract: The self-consistent electron densities and the corresponding positron states are calculated for several metals and semiconductors in the local-density approximation of the density-functional formalism. The calculations are performed with the linear-muffin-tin-orbital band-structure method. The emphasis of this work is on the energy levels of the delocalized positron and the electron chemical potential which are now calculated with respect to the same potential reference. These energies determine quantities such as the positron and positronium work functions and the deformation potentials which are important parameters in slow-positron-beam experiments. The theoretical results are compared to values extracted from experiments.
78 citations
TL;DR: In this article, the positron annihilation measurements were performed on nanocrystalline iron specimens which are polycrystals with very small crystallite size (5-10 nm) and the lifetimes τ 1 = 180 ± 15 ps, τ 2 = 360 ± 30 ps and long-lived components between 1 and 4 ns were attributed to vacancy-size free volumes in the crystallite interfaces, to microvoids at the intersections of three interfaces, and to positronium formation in larger voids.
69 citations
TL;DR: The Doppler broadening of the two-photon annihilation line arising the intermediate lifetime of positronium in a magnetic field has been measured using a time selecting λ-ray spectrometer as discussed by the authors.
53 citations
TL;DR: Positrons at keV energies, incident on Mo(111) are observed to escape the sample prior to thermalization, and an excess contribution of ''hot'' positronium (Ps) seriously affects positron mobility measurements.
Abstract: Positrons at keV energies, incident on Mo(111) are observed to escape the sample prior to thermalization. Analysis of positron back-diffusion data shows that an excess contribution of ``hot'' positronium (Ps) seriously affects positron mobility measurements. Suppressing the ``hot'' Ps fraction the positron diffusion coefficient is shown to vary as ${T}^{\mathrm{\ensuremath{-}}0.50(4)}$. The positron implantation profile possesses a shape of a derivative of a Gaussian function. The implications of these observations to related experiments are discussed.
46 citations
TL;DR: In this paper, the authors determined the total cross section without ionising channels and compared this with calculations of the angle-integrated elastic and excitation cross sections, showing that the theoretical results for elastic scattering and excitations are in satisfactory agreement with experimental data.
Abstract: From measurements of total, ionisation and positronium formation cross sections the authors have determined the 'total cross section without ionising channels' and compared this with calculations of the angle-integrated elastic and excitation cross sections. The comparison shows that the theoretical results for elastic scattering and excitation are in satisfactory agreement with experimental data. The measurements and calculations of positron ionisation indicate that the positron cross section is smaller than the electron cross section between threshold and about 35 eV, a behaviour which is in accordance with threshold laws derived from the Wannier theory. The partitioning of the total cross section also elucidates the 'convergence problem' of electron and positron scattering on helium, that is, the fact that measured electron and positron total cross sections merge at 200 eV, whereas for the elastic cross sections theory predicts convergence at a much higher energy.
45 citations
TL;DR: In this paper, the total cross sections for 1-400 eV positrons and electrons colliding with NH3 and H2O molecules were measured using a TOF method.
Abstract: The total cross sections for 1-400 eV positrons and electrons colliding with NH3 and H2O molecules were measured using a TOF method. The cross sections were obtained by a normalisation method. The results for e+-NH3 and e+-H2O at low energy were much lower than the theoretical values of Jain (1986). The cross sections of positronium formation in these gases were measured.
44 citations
TL;DR: In this paper, a fully covariant two-body equation is applied to the theory of hydrogen, muonium, and positronium spectra, and the major part of the 16 × 16 wave equation is exactly soluble including recoil of both particles to all orders.
Abstract: A fully covariant two-body equation is applied to the theory of hydrogen, muonium, and positronium spectra. Both particles are treated fully relativistically and complete spin algebras for both particles are taken into account. The major part of the 16 × 16 wave equation is exactly soluble including recoil of both particles to all orders. The terms of order α 5 , α 5 are treated perturbatively (although the equation is in principle numerically solvable to all orders). Self-energy (loop) effects are partly considered by an (effective) anomalous magnetic moment, but in a dynamical way using a Pauli coupling from the beginning. The theory simplifies and improves the bound-state QED problems in a number of ways.
42 citations
TL;DR: Using a pulsed position beam made at a 100 MeV electron linac, this paper measured the intensity and velocity distribution of positronium emitted from materials by measuring the time-of-flight of annihilating positrons, augmented by positron lifetime and angular correlation measurements performed with the beam.
Abstract: Low-energy intense positron beams derived from pair production can be made at high-energy electron linacs and such beams are in operation or under installation at several linac facilities. Using a pulsed position beam made at a 100 MeV electron linac, we have measured the intensity and velocity distribution of positronium emitted from materials by measuring the time-of-flight of annihilating positronium. The time-of-flight data are augmented by positron lifetime and angular correlation measurements performed with the beam. Positronium spectra have been measured for a number of metallic samples. Several new observations have been made including details of the energy distribution of positronium emission formed by a thermalized positron and a conduction electron and the production of positronium from energetic positrons scattered out of the sample.
35 citations
TL;DR: In this article, a natural expression for the apparent Galactic center positronium fraction (f) was derived and used to present all previously published data in a consistent manner, and the weighted average of the four reported observations of f amounts to 0.78(+0.06; -0.08).
Abstract: A natural expression for the apparent Galactic center positronium fraction (f) is derived and used to present all previously published data in a consistent manner. The weighted average of the four reported observations of f amounts to 0.78(+0.06; -0.08). This is in agreement with the laboratory simulation result for H2(0.897 + or - 0.003) and with a partially ionized or cold dusty ionized model (80-100 percent). 34 references.
TL;DR: In this paper, the solutions of a covariant two-body equation are applied to the spectra of H, muonium and positronium and the potentials, the recoil terms and the perturbation calculations up to order α 5.
Abstract: The solutions of a covariant two-body equation are applied to the spectra of H, muonium and positronium. In order to compare with the standard results we have expanded the potentials, the recoil terms and the perturbation calculations up to order α 5 . The best known intervals, namely Hfs of H, muonium and positronium and the 2 3 S 1 −1 3 S 1 interval of positronium are obtained to this order in agreement with standard results and experiments. Having thus verified the nonperturbative equation to order α 5 , the recoil and normal and anomalous magnetic moment terms can now be evaluated numerically to all orders.
TL;DR: Positronium velocity distributions using the time-of-flight technique have been determined for positronium emitted from clean, well-annealed, single-crystal samples of Al(111), Cu(100), Ni(100, and Au(100).
Abstract: Positronium velocity distributions using the time-of-flight technique have been determined for positronium emitted from clean, well-annealed, single-crystal samples of Al(111), Cu(100), Ni(100), and Au(100). The positronium energy distribution had a similar shape for all samples and was consistent with the hypothesis that positronium formation leaves behind a single electron hole in the conduction band of the metal. This was verified by explicit calculation for Ni, the first d-band metal to be studied by this technique.
TL;DR: Some lower-lying S-wave resonances in positron-hydrogen scattering associated with the N = 3 positronium (Ps) and N = 4 hydrogen thresholds are investigated by using the method of complex-coordinate rotation.
Abstract: Some lower-lying S-wave resonances in positron-hydrogen scattering associated with the N = 3 positronium (Ps) and N = 4 hydrogen thresholds are investigated by using the method of complex-coordinate rotation. Using Hylleraas-type wave functions with M = 969 terms, several resonances in these energy regions are calculated for the first time. In particular, the lowest resonance associated with the Ps(N = 3) threshold is found to lie at E = -0.077 063 Ry, and the lowest resonance associated with the H(N = 4) threshold is found to lie at E = -0.067 867 Ry. In addition, we also locate a shape resonance lying just above the Ps(N = 3) threshold at E = -0.055 44 Ry.
TL;DR: In this article, a phenomenological model for the hyperfine splitting of positronium in naphthalene has been presented, where the perturbation effect of the medium on the splitting is not interpretable as a change of the contact density alone, since on passing from vacuum to the medium, the contact and the splitting do not scale together.
Abstract: The contact density |Ψ(0)|2 and the hyperfine splitting of positronium in naphthalene have been determined by means of a magnetic quenching experiment. Results show that the perturbing effect of the medium on the hyperfine splitting is not interpretable as a change of the contact density alone, since on passing from vacuum to the medium, the contact density and the splitting do not scale together. The first is reduced by a factor of 0.25, the second by a factor of 0.067. A simple phenomenological model is presented.
TL;DR: In this article, the momentum distribution of positronium (Ps) in the free space between silica aerogel grains filled with gaseous O2 has been measured as a function of O2 pressure.
Abstract: The momentum distribution of positronium (Ps) in the free space between silica aerogel grains filled with gaseous O2 has been measured as a function of O2 pressure. It reveals that para-Ps with kinetic energy >or approximately=1 eV is efficiently slowed down. This is attributed to successive triplet to or from singlet conversions of the Ps through inelastic collisions with O2. The cross section for this process is estimated to be at least two orders of magnitude larger than that for the conversion through elastic collisions.
TL;DR: The integral transform (generator coordinate) method has been applied to calculate the binding energies and some expectation values for the positronium negative ion and for the recently discovered muonium negative ions.
Abstract: The integral transform (generator coordinate) method has been applied to calculate the binding energies and some expectation values for the positronium negative ion and for the recently discovered muonium negative ion. For both ions the wave functions obtained are compact and accurate.
TL;DR: The cross section for creation of bound states of a fermions and an antifermion, fermionium, in high-energy collisions of electrons with atoms is calculated.
Abstract: The cross section for creation of bound states of a fermion and an antifermion, fermionium, in high-energy collisions of electrons with atoms is calculated. Applications to positronium and to bound states of a muon and an antimuon, ..mu.. fermionium, are discussed.
TL;DR: In this paper, the authors report semiclassical (impact parameter) and classical cross sections for antihydrogen formation in collisions between antiprotons p and positronium atoms Ps computed in the intermediate energy range 2-100 keV lab.
TL;DR: In this paper, the authors studied the structure of water-ethanol, water-n-propanol and water-isopropanol mixtures, their clathrate-likeness and stoichiometry, if any.
Abstract: In our recent paper we found that long lived component of lifetime spectrum τ2, which is connected with pick-off ortho-positronium annihilation and its intensity do not change monotonically with the concentration of organic components in water. The results obtained confirm the thesis of quasi-crystalline, clathrate-like structure of water-tetrahydrofuran and water-cyclohexylamine systems. The attempt of the present work was to study the structure of water-ethanol, water-n-propanol and water-isopropanol mixtures, their clathrate-likeness and stoichiometry, if any. The results of our measurements of positronium lifetime, in particular the component τ2 and the intensity I2 as well as the sound velocity data allow us to suppose that in the systems the liquid clathrate-like hydrates are formed. The stoichiometry of these structures was deduced. The results, although obtained using strongly different experimental methods, i.e., positron annihilation and acoustic measurements, are in very good agreement with the clathrate-like concept of the structure of water-organic mixtures.
TL;DR: In this article, R-matrix calculations for elastic and rotationally inelastic scattering of positrons from CO at energies below the positronium formation threshold were performed within a number of models that represent charge polarisation effects, the rotational motion of the target and the asymptotic potential.
Abstract: R-matrix calculations are presented for elastic and rotationally inelastic scattering of positrons from CO at energies below the positronium formation threshold. The calculations are performed within a number of models that represent charge polarisation effects, the rotational motion of the target and the asymptotic potential. The computed total and momentum transfer cross sections are found to vary widely between the different models employed. A comparison is made between the experimental results for this system and the recent calculations of Jain (1986).
TL;DR: It is concluded that inelastic many-body effects are weak and that the Ps spectra reflect the electron density of states near the surface, which stems from the energy gaps in the projected nearly-free-electron band structure of Al.
Abstract: The momentum distribution of the positronium (Ps) emitted from an Al(100) surface being bombarded by low-energy positrons shows anisotropic lobes that gradually disappear as the initially clean surface is exposed to small amounts of oxygen. A clean Al(111) surface yields a more isotropic spectrum. The simplest model suggests that the anisotropy stems from the energy gaps in the projected nearly-free-electron band structure of Al. We conclude that inelastic many-body effects are weak and that the Ps spectra reflect the electron density of states near the surface.
TL;DR: Positron lifetime experiments have been performed on H2 gas at temperatures between 77 and 297 K and in the density range from 12-160 Amagat as discussed by the authors. But the results of these experiments are limited.
Abstract: Positron lifetime experiments have been performed on H2 gas at temperatures between 77 and 297 K and in the density range from 12-160 Amagat. The extracted parameters are discussed in terms of current models. In the case of the positronium fraction it has been found that the observed density dependence can, in part, be interpreted using a combined Ore and spur model.
Patent•
03 Apr 1987
TL;DR: In this article, thermalized positrons are derived by moderating high energy positrons obtained from a high energy source, such as 22 Na, and directed by electrostatic lenses to a positronium converter, positioned adjacent a low energy (less than about 50 keV) circulating antiproton beam confined within an ion trap.
Abstract: A process for producing antihydrogen includes providing low energy antiprotons and positronium atoms within an interaction volume. Thermalized positrons are derived by moderating high energy positrons obtained from a high energy source, such as 22 Na. The thermalized positrons are directed by electrostatic lenses to a positronium converter, positioned adjacent a low energy (less than about 50 KeV) circulating antiproton beam confined within an ion trap. Collisions between antiprotons and ortho-positronium atoms generate antihydrogen, a stable antimatter species, with substantial probability.
TL;DR: In this article, the deposition of positrons from the passage of a positronium beam through a Tokamak plasma and the subsequent radial transport of positron to the limiter are simulated.
Abstract: The deposition of positrons from the passage of a positronium beam through a Tokamak plasma and the subsequent radial transport of positrons to the limiter are simulated. The signal created by the annihilation of positrons at the limiter is calculated for several different transport models. Loss processes, such as reformation of positronium and annihilation of positrons by plasma electrons, are examined to determine how they affect the annihilation signal. An analysis of how quantities relevant to electron-mass particle transport can be obtained from a positron transport experiment is included.
TL;DR: A quantum-mechanical theory of positronium formation at surfaces is presented and the theory of the resonant neutralization of ions at a surface is improved for positrons by taking into account the quantum effect of the motion of the positrons near the surface.
Abstract: A quantum-mechanical theory of positronium formation at surfaces is presented. The neutralization probability of positrons implanted into solids escaping from a surface is calculated. The theory of the resonant neutralization of ions at a surface is improved for positrons by taking into account the quantum effect of the motion of the positrons near the surface. The angular distributions and the energy distributions of the emitted positronium are calculated. We give the relationship of the positronium energy distribution and the density of states at the surface.
TL;DR: The differential and total cross sections for the formation of positronium in its ground state from Li and Na atoms by the impact of intermediate-energy positrons are calculated in the first Born and distorted-wave Born approximations.
Abstract: The differential and total cross sections for the formation of positronium in its ground state from Li and Na atoms by the impact of intermediate-energy positrons are calculated in the first Born and distorted-wave Born approximations. Hellmann-type pseudopotentials are used to represent the alkali-metal ion cores. The difference in the use of pseudopotentials and the static potential for the core representation for evaluating various rearrangement cross sections is discussed.
TL;DR: The amplitudes of elementary-atom interaction are derived in the Born approximation under the assumption that a momentum transfer to the atom does not significantly exceed an inverse Bohr radius of the atom.
Abstract: The problem of the interaction of relativistic elementary atoms (Coulomb bound states of elementary particles such as positronium, pionium, etc.) with matter is studied in the reference frame where the atom is initially at rest. An atom of matter is treated as a spinless structureless fast particle. The amplitudes of elementary-atom interaction are derived in the Born approximation under the assumption that a momentum transfer to the atom does not significantly exceed an inverse Bohr radius of the atom. The elementary-atom excitation and ionization processes are considered. The transitions where the spin projection of the atom component is reversed are also studied. In particular the matrix elements for para-ortho and ortho-para transitions are given. The spin structure of the amplitudes is discussed in detail. The sum rules, which allow the calculation of the cross sections summed over atom final states are found. Finally the formulas of the atom interaction cross sections are presented.
TL;DR: A general theory of the direct formation of positronium at metal surfaces is presented using a resonant charge-transfer theory which is similar to the theory of slow ion-beam neutralization.
Abstract: A general theory of the direct formation of positronium at metal surfaces is presented using a resonant charge-transfer theory which is similar to the theory of slow ion-beam neutralization. In contrast to the previous theory (A. Ishii, Surf. Sci. 147, 277 (1984); 147, 295 (1984)) a three-dimensional momentum distribution is considered. Since the momentum parallel to the surface is conserved in the process of positronium formation, the momentum distribution of electrons on the topmost layer of the surface can be obtained from the angle-resolved positronium energy distribution.