Showing papers on "Positronium published in 1992"
TL;DR: In this article, cross sections for positron collisions with the noble gases He, Ne, Ar, Kr and Xe were calculated in the (truncated) coupled-static approximation.
Abstract: We report cross sections for positron collisions with the noble gases He, Ne, Ar, Kr and Xe. These cross sections are for Ps(1s) formation and elastic scattering, and are calculated in the (truncated) coupled-static approximation. Comparison is made with experiments which measure formation of positronium into all bound states. At their maxima, the theoretical and experimental cross sections are of comparable size but, at higher energies, the theoretical numbers are often much lower than the experimental values, a discrepancy which is difficult to understand. Some very interesting structure is observed in the theoretical cross sections, but the calculations do not show the structure at high energies seen in the experiments. The importance of calculating positronium formation not only by electron capture from the outermost shell of the atom but also from inner shells is highlighted. It is suggested that a sufficiently refined experiment may be able to see shell effects. Structure in the elastic cross section which correlates with behaviour in the positronium channel is pointed out. For He, the effects of polarization and absorption in the elastic and Ps(1s) formation channels is also studied using secondorder optical potentials. Polarization and absorption are found to be very important for this system.
61 citations
TL;DR: In this paper, the authors used a target basis set of the form He(1s, 2s, ks,2p, kp) and investigated the effect upon excitation due to the addition of the positronium basis set.
Abstract: The close-coupling approximation has been used to investigate positronium formation into the n=1 and n=2 levels and helium excitation (11S-21S, 21P) in positron-helium collisions at energies between 31.3 eV and 200 eV. The authors have used a target basis set of the form He(1s, 2s, ks, 2p, kp) and investigated the effect upon excitation due to the addition of the positronium basis set Ps(1s, 2s, 2p). An independent electron model has been combined with a model potential to represent the helium atom. Positronium formation reduces the 21S excitation below 50 eV but the 21P excitation is significantly reduced for all energies up to 150 eV. The present excitation results are in closer agreement with previous close-coupled calculations than with other theoretical methods. The He(n=2) excitation total is in agreement with experimental data below 36 eV. The calculated total positronium formation cross section is in agreement with experiment below 100 eV.
60 citations
TL;DR: The binding energy of positronium chloride is calculated using a model potential for the ten core electrons and a quantum Monte Carlo method for the eight valence electrons and the positron.
Abstract: The binding energy of positronium chloride is calculated using a model potential for the ten core electrons and the quantum Monte Carlo method for the eight valence electrons and the positron. The result is 1.91{plus minus}0.16 eV. Except for three- and four-particle systems, this is the first accurate calculation of the binding energy of a compound containing a positron.
53 citations
TL;DR: In this article, a Laplace inversion technique was employed to analyze the positron lifetime spectra measured under quasi-isotropic external pressures of 0, 4.2, and 14.7 kbar.
Abstract: The free-volume hole distributions in semicrystalline polypropylene was directly measured by positron lifetime annihilation spectroscopy. A Laplace inversion technique was engaged to analyze the positron lifetime spectra measured under quasi-isotropic external pressures of 0, 4.2, and 14.7 kbar into continous lifetime distributions. The hole radii distributions as determined from the ortho-positronium lifetime distributions are found to be between 4.0 and 0.5 A and to have maxima at 3.0, 1.9, and 1.1 A under the external pressures of 0, 4.2, and 14.7 kbar, respectively
52 citations
TL;DR: The isotherm data suggest that pore filling occurs via progressive layer formation at the pore surface followed by an abrupt gas-liquid phase transition at some pressure below the bulk saturated vapor pressure.
Abstract: Positronium annihilation spectroscopy has been used to study capillary condensation of nitrogen adsorbed in mesoporous VYCOR glass. The temperature and pressure dependences of the relative change in the 3γ:2γ annihilation ratio are reported. Both isobars and isotherms exhibit features which are associated with a shifted gas-liquid transition. The isotherm data suggest that pore filling occurs via progressive layer formation at the pore surface followed by an abrupt gas-liquid phase transition at some pressure below the bulk saturated vapor pressure. Such a picture is consistent with the theoretical treatment of adsorption in individual pores
42 citations
TL;DR: A good correlation between the pick-off annihilation lifetime and the free volume, computed as the difference between the specific volume at room temperature and that at 0 K, is reported for molecular liquids and polymers.
Abstract: A good correlation between the ortho-positronium (o-Ps) pick-off annihilation lifetime and the free volume, computed as the difference between the specific volume at room temperature and that at 0 K, is reported for molecular liquids and polymers. Bondi’s group contribution approach was employed for estimating the specific volume at 0 K. The o-Ps lifetimes in molecular solids, such as phenanthrene deviate considerably from the correlation. It is suggested that a bubble may be formed around Ps in liquids and polymers, but not in the low-molecular-weight crystalline solids. A correlation is presented between the diffusion coefficients of Ar and the lifetimes of o-Ps in both liquids and polymers.
41 citations
TL;DR: The Barbieri-Remiddi potential provides a suitable zeroth-order kernel about which perturbation theory is developed and a simple algorithmic method is shown to suffice to extract the Inα -1 terms from most of the interaction terms.
Abstract: The order α 4 Inα -1 f RYD corrections to the n=1 and n=2 energy levels of positronium are calculated using the conventional Bethe-Salpeter formalism. The Barbieri-Remiddi potential provides a suitable zeroth-order kernel about which perturbation theory is developed. A simple algorithmic method which amounts to an expansion of the interaction kernel in powers of the momentum divided by the mass is shown to suffice to extract the Inα -1 terms from most of the interaction terms
33 citations
TL;DR: Theoretical studies of atomic resonances involving positrons are discussed in this paper, where resonances in positron-hydrogen scattering below various hydrogen and positronium thresholds are reviewed.
Abstract: Theoretical studies of atomic resonances involving positrons will be discussed in this talk Investigations on resonances in positron-hydrogen scattering below various hydrogen and positronium thresholds are reviewed, as well as resonances in positronalkali and e+-He+ scattering Resonance phenomena in other atomic systems involving positrons will also be discussed These systems include positronium ions Ps−, positronium molecules Ps2, and positronium hydride PsH
28 citations
TL;DR: In this article, the energy spectra of electrons ejected from an argon target by positron impact have been measured and compared with theoretical expectations, and it was shown that a small ridge is present in some cases at energies compatible with electron capture to continuum states of positronium.
Abstract: Energy spectra of electrons ejected from an argon target by positron impact have been measured and compared with theoretical expectations. No major cusp structure was observed in any of these spectra. Instead a small ridge is present in some cases at energies compatible with electron capture to continuum (ECC) states of positronium. Its magnitude suggests that ECC makes a minor contribution to ionization by a light positively charged projectile.
28 citations
TL;DR: Three-quantum-yield measurements and orthopositronium ({ital o-Ps)-lifetime spectrometry at low temperatures are used to study the interaction of positronium with the surface in fine powders of aluminum oxide and it is found that the surface Ps state is not populated at the expense of the interparticle Ps.
Abstract: Three-quantum-yield measurements and orthopositronium (o-Ps)-lifetime spectrometry at low temperatures are used to study the interaction of positronium with the surface in fine powders of aluminum oxide It is found that electron and/or positron irradiation of the specimen induces surface defects which influence the positronium in three ways: (1) A surface positroniumlike bound state is created, (2) the fraction of o-Ps escaping from the particles is slightly inhibited, and (3) the escaped o-Ps is quenched into two-quantum decay upon collisions with the surface defects It is found that the surface Ps state is not populated at the expense of the interparticle Ps The most likely surface defects are ${\mathrm{Al}}^{2+}$ or ${\mathrm{Al}}^{0}$ due to the migration of irradiation-induced interstitials The techniques of long-lifetime spectrometry and of three-quantum-annihilation-rate measurement could be used to study both the diffusion of bulk defects to the surfaces, and the interactions of o-Ps to surface defects
24 citations
TL;DR: In this paper, the formation of ground-state positronium in collisions of positrons with hydrogen and helium atoms is considered within the distorted-wave Born approximation, and differential and total cross sections are calculated over a broad range of intermediate energies extending from 40 to 500 eV.
Abstract: The formation of ground-state positronium in collisions of positrons with hydrogen and helium atoms is considered within the distorted-wave Born approximation. In particular the authors analyse the eikonal and continuum-distorted-wave models in both their post and prior forms. Differential and total cross sections are calculated over a broad range of intermediate energies extending from 40 to 500 eV. The results confirm that the total cross section decreases rapidly with increasing energy, as predicted by the simple first Born approximation, and do not agree with experimental evidence of a slower decay. The computed total cross sections for helium targets agree with those of the close-coupling model for energies above 100 eV. Below 100 eV the authors find a large post-prior discrepancy. The calculated differential cross sections show evidence of a Thomas peak at high energies for the CDW models but not for the eikonal models.
TL;DR: In this article, the authors have made preliminary measurements of positronium (Ps) formation cross sections for 9 to 452 eV positrons scattered from Ar atoms and for 1 to 20 eV PSs scattered from K atoms.
Abstract: We have made preliminary measurements of positronium (Ps) formation cross sections for 9 to 452 eV positrons scattered from Ar atoms and for 1 to 20 eV positrons scattered from K atoms. Our experimental approach involves setting lower and upper limits on Ps formation cross sections using a combination of (1) the detection of the coincidences of 511 keV annihilation gamma rays produced by the decay of para-Ps and by the interaction of ortho-Ps with the wall of the scattering cell in which the Ps is formed, and (2) the determination of scattering cross sections associated with the measured transmission of the positron beam through the gas in our scattering cell with the angular discrimination of our apparatus deliberately made as poor as possible. The constraints placed by these lower and upper limits are used to check for consistency with prior experimental and theoretical results for Ar and to provide the first measurements of Ps formation cross sections for K, which are compared with available theoretical results.
TL;DR: In this paper, a discretized light-cone quantization is applied to gauge theory in 3+1 dimensions and tested by calculating the spectrum and the wave functions for positronium.
Abstract: The method of discretized light-cone quantization is applied to gauge theory in 3+1 dimensions and tested by calculating the spectrum and the wave functions for positronium. Working in a Fock-space expansion, only the electron-positron and the electron-positron-photon states are included. The light-cone Hamiltonian matrix is constructed blockwise, and diagonalized using the Bauer-Rutishauser simultaneous iteration algorithm for sparse matrices. At the current level of accuracy of about 10%, the model reproduces the binding energy and the charge radius of the positronium ground state for the coupling constant of {alpha}=0.3. The distribution functions for fermions and photons are presented for the ground state. In addition the light-cone Schroedinger equation has been solved.
TL;DR: In this article, the lithium atom was described within a single-electron approximation using a model potential, and the effect upon excitation and elastic scattering due to the coupling of positronium formation channels was investigated.
Abstract: The close-coupling approximation has been used to investigate positronium formation into the n=1 and n=2 levels and the excitation of lithium in positron-lithium collisions at impact energies up to 50 eV. The lithium atom is described within a single-electron approximation using a model potential. The authors have used a target basis set of the for Li(2S, 3S, 2p, 3p), and investigated the effect upon excitation and elastic scattering due to the coupling of positronium formation channels by adding the positronium states Ps(1s, 2s, 2p) to the expansion. The convergence of the cross sections was studied with respect to the addition of a Li(p) pseudostate. The sensitivity of the results to the choice of model potential was also examined.
TL;DR: In this article, the authors calculated positronium formation cross sections in the collision of a positron with a helium atom in the energy range from 50 to 300 eV and showed that no existing theory for particle transfer can expalain the high-energy behavior of the measured cross sections.
TL;DR: In this article, a detailed investigation has been made of low energy s-wave positron-lithium scattering using the Kohn variational method in the energy range in which only elastic scattering and positronium formation are possible (0-1.84 eV).
Abstract: A detailed investigation has been made of low energy s-wave positron-lithium scattering using the Kohn variational method in the energy range in which only elastic scattering and positronium formation are possible (0-1.84 eV). The positronium formation cross section is infinite at zero energy, but then falls to be several orders of magnitude smaller than the elastic scattering cross section, which has a peak value of approximately 100 pi a02. The elastic scattering cross sections are almost unaffected by the uncoupling of the positronium channel, but the resulting phaseshifts are significantly more positive, and therefore probably more accurate, than those obtained in the most sophisticated previous study using the uncoupled approximation.
TL;DR: In this paper, the beam production efficiencies with respect to target species and density have been measured, together with a determination of the population of the quantum states of the atoms in the beam.
Abstract: Recent developments in the field of beams of positronium (Ps) atoms formed in charge-exchange reactions of slow positrons with gaseous targets are reported. The beam production efficiencies with respect to target species and density have been measured, together with a determination of the population of the quantum states of the atoms in the beam. Estimates of the total scattering cross section of Ps with He and Ar targets, at intermediate projectile velocities, have been obtained.
TL;DR: In this article, a new treatment of short-range correlation effects (the distributed positron model) is used to calculate a variety of scattering results for low-energy positrons on H2.
Abstract: A new treatment of short-range correlation effects (the distributed positron model) is used to calculate a variety of scattering results for low-energy positrons on H2. The new treatment of correlation is based on a physically motivated approximation that mimics the effect of virtual positronium formation, but requires no semiempirical parameters or non-physical assumptions. The author's total, integrated cross sections are found to be in excellent agreement with measured values. The differential cross sections and eigenphase sums are compared with the results of several recent ab initio calculations.
TL;DR: The modified Faddeev equations in configuration space are applied to solve numerically the problem of the zero-energy elastic scattering of an electron by positronium at zero angular momentum and the results confirm and improve previous variational estimations of these parameters.
Abstract: The modified Faddeev equations in configuration space are applied to solve numerically the problem of the zero-energy elastic scattering of an electron by positronium at zero angular momentum. The singlet and triplet scattering lengths are calculated and zero-energy limits of elastic and ortho-para conversion cross sections are established. The results confirm and improve previous variational estimations of these parameters.
TL;DR: In this paper, the authors investigated the center of mass motion of the positronium atom in a homogeneous magnetic field and showed that the one-dimensional spreading of an ensemble of trajectories obeys a linear diffusion law.
Abstract: The author investigates the classical centre of mass motion of the positronium atom in a homogeneous magnetic field. For the case of vanishing pseudomomentum and angular momentum component along the magnetic field axis the centre of mass moves on a straight line. Its behaviour changes drastically from, for example, quasiperiodic oscillations to random fluctuations if the internal motion changes from regularity to chaos. The author shows that the one-dimensional spreading of an ensemble of trajectories obeys a linear diffusion law. Some particularly interesting examples of the centre of mass motion for Lz not=0 are also discussed.
TL;DR: The formation of {ital n}=2 positronium is investigated using a positron beam incident on a variety of untreated metal foils and crystals and the maximum measured formation fraction is much higher than any previously reported for such targets.
Abstract: The formation of {ital n}=2 positronium is investigated using a positron beam incident on a variety of untreated metal foils and crystals. The maximum measured formation fraction is much higher than any previously reported for such targets. Fully 2% of the incident positrons are observed to form {ital n}=2 positronium at low input energy with little dependence upon the metal substrate of the formation surface. The {ital n}=2 positronium spectra exhibit lifetime components characteristic of two distinct populations with different kinetic energies. The dependence of each of these components on the incident positron energy is studied and interpreted as being consistent with existing theories of positron interactions with metals. The production of {ital n}=2 positronium using positrons transmitted through metal meshes and thin foils is also observed and discussed briefly.
TL;DR: The experimental results do not support the existence of a cusp in the elastic scattering cross section at the positronium formation threshold energy (178 eV) as significant as that previously deduced by Campeanu et al. as discussed by the authors.
Abstract: Measurements of that part of the total positron-helium cross section not associated with positronium formation have been made for positron energies between 14 and 28 eV The experimental results do not support the existence of a cusp in the elastic scattering cross section at the positronium formation threshold energy (178 eV) as significant as that previously deduced by Campeanu et al (1987)
TL;DR: In this paper, a second-order Born approximation of the positronium formation in the energy range 100 eV-2 keV was used to obtain the integrated capture cross-sections in the case of the ground and the 2-ital s excited state.
Abstract: Positronium formation in the {ital n}=1 and {ital n}=2 levels in {ital e}{sup +}-He collision has been studied using a second-order Born approximation. Differential and integrated capture cross sections are reported in the energy range 100 eV--2 keV. The predicted total positronium-formation cross sections differ from measured values, the theoretical results being lower. The first-order Born results are not expected to be valid in the energy range considered. A Thomas mechanism is found to be valid in the case of the ground as well as the 2{ital s} excited state. A similar structure in the differential cross section for 2{ital p}-state capture has not been found.
TL;DR: It is shown how the originally homogeneous eigenvalue equation for the positronium bound states is augmented by an inhomogeneity due to the coupling to the two-photon sector to reproduce the well-known result that QED has a continuous spectrum at all energies.
Abstract: A Hamiltonian variational method is used in quantum field theory to describe photon-photon scattering in quantum electrodynamics (QED) at energies where the cross section is dominated by the formation of positronium states of symmetry {ital J}{sup {ital P}{ital C}}=0{sup {minus}+}. It is shown how the originally homogeneous eigenvalue equation for the positronium bound states is augmented by an inhomogeneity due to the coupling to the two-photon sector. Thus our calculation reproduces the well-known result that QED has a continuous spectrum at all energies and that the positronium bound states appear as resonances in the {gamma}-{gamma} scattering channel. Consequences for photon-photon scattering experiments are discussed briefly.
TL;DR: A theory of ion desorption due to positronium formation at a solid surface is presented in this article, where the positron wave function is calculated by using a LEED program with the potential for positron calculations.
TL;DR: In this article, the cross sections for photoionization of positronia in strong magnetic fields were derived within the Born approximation (B ˜ 10 12 -10 13 G, ω B = eB/m e c > ω ', the frequency of the photon in the positronium rest frame).
Abstract: The cross sections for the photoionization of positronia in strong magnetic fields are derived within the Born approximation (B ˜ 10 12 -10 13 G, ω B = eB/m e c > ω ', the frequency of the photon in the positronium rest frame). Further, the total ionization rate of the positronium by thermal photons in strong magnetic fields is computed. We discuss the application of this process in light of the recent suggestions that in the vicinity of the pulsars the high-energy photons can be captured by the strong magnetic fields and be transformed into bound positronia.
TL;DR: In this paper, the strong magnetic field effect on the chemical bound-state formation of positronium with nitrobenzene in n−hexane was further examined as a function of temperature, from 264 to 328 K.
Abstract: The strong magnetic field effect on the chemical bound‐state formation of positronium with nitrobenzene in n‐hexane previously studied at 294 K has been further examined as a function of temperature, from 264 to 328 K. The model proposed before to explain the data, implying the definition of triplet and singlet positronium bound‐state, appears to be valid at all temperatures studied. The variation with temperature of the parameters descriptive of the bound‐state are derived; the hyperfine splitting is constant at 1.06×10−5 eV, while the contact density parameter, ηc, shows an abrupt change from 0.43 to 1.56 between 286 and 292 K. These experiments give new insight on the structure of the bound‐state formed, which may be considered as a charge‐transfer complex.
TL;DR: In this paper, the depolarization, relative to the direction of a magnetic field, which accompanies the slowing-down of positrons in the two enantiomers of camphor, down to the positronium formation energy, has been found to be greater (27%) in L-camphor.
Abstract: The depolarization, relative to the direction of a magnetic field, which accompanies the slowing-down of positrons in the two enantiomers of camphor, down to the positronium formation energy, has been found to be greater (27%) in L-camphor. The measuring method was based on the Ps relative yield, in the magnetic field, in triplet and singlet state. All other annihilation features, as well as the intrinsic properties of Ps once formed, do not display significant differences between the two enantiomers.