Showing papers on "Positronium published in 2008"

Ionizing Collisions by Positrons and Positronium Impact on the Inert Atoms

Abstract: Recent progress in the study of positronpositron and positroniumpositronium induced ionizationionization is reviewed within the context of the inert atoms. The focus is on experimental techniques and results, which comprise both integral and differential cross-sections.differential cross-sectionsintegrated cross-sections Measurements for positronium formationpositronium formation and direct ionizationdirect ionization by positron impact, accompanied by singly- or multiply-charged ions, are compared and discussed. First data, integral and differential, on the fragmentationfragmentation of positronium in collision with heliumhelium and xenonxenon are also presented. Comparisons with theories and other projectiles are made where possible and future prospects are considered.

Feshbach-resonance-mediated positron annihilation in small molecules

Abstract: Large positron annihilation cross sections have been observed for a variety of molecules at various energies below the threshold for positronium formation. These large values are due to vibrational Feshbach resonances VFRs , in which the positron attaches to the molecule while exciting a vibration. This leads to rates of annihilation far greater than those expected for a simple collision. The dependence of the annihilation rate on incident positron energy can be used to deduce positron-molecule binding energies. Presented here is a comprehensive study of resonant annihilation in small molecules e.g., hydrocarbons with one or two carbon atoms . In some cases, only fundamental vibrations are important, and theory correctly predicts the annihilation rates as a function of incident positron energy. In other cases, combination and overtone vibrations are shown to support Feshbach resonances. In the subset of these cases where the positron-molecule coupling strengths can be determined for these combination modes, theoretical predictions are in agreement with the measurements. Finally, there are species that do not exhibit VFRs, such as carbon dioxide. This is interpreted as evidence that positrons bind very weakly or not at all to these targets. Several of these molecules exhibit a variety of behaviors that are presently unexplained. The implications of the results presented here for more comprehensive theories of positron annihilation on molecules are discussed.

Positronium cooling into nanopores and nanochannels by phonon scattering

Abstract: We studied the acoustic phonon scattering of a positronium (Ps) atom emitted into and then confined in closed nanopores and open nanochannels of an insulating material at low temperature. Closed nanopores were approximated by a cubic box, while nanochannels by a channel with square geometry. As a case study, calculations were carried out for nanopores and nanochannels in a silica matrix. First-order calculations allowed us to set a lower limit to the size of the pores and of the channels in order to achieve complete Ps thermalization. The Ps energy loss and the cooling time obtained from the Ps-phonon scattering model were compared to those obtained from the classical model considering Ps scattering with surface atoms with an effective mass.

Monte Carlo simulation of non-conservative positron transport in pure argon

Abstract: The main aim of this paper is to apply modern phenomenology and accurate Monte Carlo simulation techniques to obtain the same level of understanding of positron transport as has been achieved for electrons. To this end, a reasonably complete set of cross sections for low energy positron scattering in argon has been used to calculate transport coefficients of low energy positrons in pure argon gas subject to an electrostatic field. We have analyzed the main features of these coefficients and have compared the calculated values with those for electrons in the same gas. The particular focus is on the influence of the non-conservative nature of positronium formation. This effect is substantial, generally speaking much larger than any comparable effects in electron transport due to attachment and/or ionization. As a result several new phenomena have been observed, such as negative differential conductivity (NDC) in the bulk drift velocity, but with no indication of any NDC for the flux drift velocity. In addition, there is a drastic effect on the bulk longitudinal diffusion coefficient for positrons, which is reduced to almost zero, in contrast to the other components of the diffusion tensor, which have normal values. It is found that the best way of explaining these kinetic phenomena is by sampling real space distributions which reveal drastic modification of the usual Gaussian profile due to pronounced spatial differentiation of the positrons by energy.

Efficient emission of positronium negative ions from Cs deposited W(100) surfaces

Abstract: The emission of positronium negative ions from Cs deposited W(100) surfaces has been studied. A dramatic change in the emission efficiency was observed upon coating a W(100) surface with Cs. The conversion efficiency (the fraction of incident slow positrons yielding the ions) of the W(100) target with a 2.2?1014?atoms?cm- 2 Cs coverage, measured over a time interval of 3?103?s immediately after deposition, was found to be 1.25%, which is two orders of magnitude higher than that obtained for the clean, uncoated W(100) surface and 45 times greater than the highest efficiency reported thus far.

Efficient positronium laser excitation for antihydrogen production in a magnetic field

Abstract: Antihydrogen production by charge exchange reaction between positronium (Ps) atoms and antiprotons requires an efficient excitation of Ps atoms up to high-$n$ levels (Rydberg levels). In this study it is assumed that a Ps cloud is produced within a relatively strong uniform magnetic field $(1\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ and with a relatively high temperature $(100\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Consequently, the structure of energy levels are deeply modified by Zeeman and motional Stark effects. A two-step laser light excitation, the first one from ground to $n=3$ and the second from this level to a Rydberg level, is proposed and the physics of the problem is discussed. We derive a simple formula giving the absorption probability with substantially incoherent laser pulses. A 30% population deposition in high-$n$ states can be reached with feasible lasers suitably tailored in power and spectral bandwidth.

Interactions between positronium atoms in porous silica.

Abstract: Interactions between pairs of positronium (Ps) atoms confined in porous silica films have been directly observed for the first time. Because of selection rules, the nature of such interactions should depend on the structure of the porous medium: if a Ps surface state exists, dipositronium (Ps2) molecules may be created, and if there is a continuum of cavity energy levels, spin exchanging collisions may occur. Using two structurally different silica films, we have been able to isolate and study these two processes. Our data indicate that Ps2 formation occurs primarily via a Langmuir-Hinshelwood-type mechanism on the internal pore surfaces, with an interaction length of the order of 7 x 10(-8) cm, and that the effective cross section for nonthermalized Ps-Ps spin exchange quenching in porous silica is around 9 x 10(-15) cm2.

Transport coefficients for positron swarms in nitrogen

Abstract: Monte Carlo simulations of positron transport in nitrogen have been performed based on a complete set of cross sections that has been updated as a result of both experimental and theoretical advances. As the positronium formation cross section becomes dominant at larger mean energies in nitrogen than in argon and hydrogen the effect is much smaller in the case of nitrogen. Nevertheless, the differences between bulk and flux transport coefficients, even in nitrogen, exceed those for electrons, providing another example of the very special case of transport of charged particles under the influence of strong non-conservative collisions.

Atomic resonances involving positrons

Abstract: The recent theoretical studies of atomic resonances involving positrons are reviewed. The topics under discussions are the doubly-excited states of the positronium negative ion and resonance scattering in e + –H, e + –He + , Ps–H, e + –He, e + –Li, e + –Na and e + –K.

Precision Physics of Simple Atoms and Molecules

Abstract: Looking Through Simple Atoms and Molecules at Fundamental Physics.- Looking Through Simple Atoms and Molecules at Fundamental Physics.- Precision Measurements and Fundamental Constants.- The Muon g-: Status and Perspectives.- Guide for Atomic and Particle Physicists to CODATA's Recommended Values of the Fundamental Physical Constants.- Proton Structure and Hydrogen Energy Levels.- Precise Radii of Light Nuclei from Electron Scattering.- Nucleon Form Factor Measurements in Mainz: Past and Future.- Proton Structure Corrections to Hydrogen Hyperfine Splitting.- Atoms with Few Electrons.- Precision Laser Spectroscopy of Li+ and Neutral Lithium.- Halo Nuclei in Laser Light.- Exotic Atoms and Heavy Ions.- Quantum Electrodynamics in Extreme Fields: Precision Spectroscopy of High-Z H-like Systems.- Pionic Hydrogen.- Precision Spectroscopy of Antiprotonic Helium Atoms and Ions - Weighing the Antiproton.- Simple Molecules.- Precision Spectroscopy of Molecular Hydrogen Ions: Towards Frequency Metrology of Particle Masses.- Nuclear Magnetic Dipole Moments from NMR Spectra - Quantum Chemistry and Experiment.- The Negative Ion of Positronium: Decay Rate Measurements and Prospects for Future Experiments.- Savely G. Karshenboim: Guide for Atomic and Particle Physicists to CODATA's Recommended Values of the Fundamental Physical Constants.

Interlaboratory comparison of positron annihilation lifetime measurements

Abstract: An interlaboratory comparison for positron annihilation lifetime (PAL) measurements for pure nickel and fused silica was performed. Based on the reported positron (for nickel) and positronium (for fused silica) lifetimes, the uncertainties in the PAL measurements were estimated.

Free Volume Seen by Positronium in Bulk and Confined Molecular Liquid

Abstract: Results of positron annihilation experiments on bulk hexadecane and hexadecane filled into silica gel nano pores will be reported. The lifetime and Doppler broadening measurements were performed at the temperature range (14-300) K. The annihilation data were combined with output of the differential scanning calorimetry at range from 270K up to 300K.

Photodetachment of the positronium negative ion interacting with screened Coulomb (Yukawa) potentials

Abstract: The effect of screened Coulomb (Yukawa) potentials on the photodetachment cross sections of the positronium negative ion is investigated by using the asymptotic form of the bound-state wave function and a plane wave form for the final-state wave function. The required normalization constant is determined from highly accurate, completely non-adiabatic wave functions for the three-particle systems. Photodetachment cross sections for the Ps− ion are calculated for different Debye shielding lengths (D) ranging from infinite (pure Coulomb) to D = 1.81.

Relativistic three-fermion wave equations in reformulated QED with application to positronium and muonium negative ions

Abstract: The variational method, within the Hamiltonian formalism of reformulated QED, is used to determine relativistic wave equations for systems of three fermions of arbitrary mass interacting electromagnetically. The derived interaction kernels of the equations are, in essence, the invariant matrices in the lowest order. The equations are used to obtain relativistic O(?2) corrections to the non-relativistic ground-state energy levels of the positronium and muonium negative ions (e+e?e?, ?+e?e?), as well as H?, using approximate variational three-body wavefunctions. The results are compared with other calculations, where available.

Formation of negative hydrogen ion in positronium-hydrogen collisions

Abstract: The importance of the excited states of Positronium (Ps) in the formation cross sections (both differential and total) of the negative hydrogen ion (H-) are investigated theoretically for the charge transfer reaction, Ps (n = 1, 2) + H ↦ e+ + H- for a wide range of incident energies (e.g., threshold – 500 eV). The calculations are performed in the frame work of a qualitative model, the post collisional Coulomb modified eikonal approximation (CMEA). A comparative study is also made between the capture from ground and excited states of the Ps. The present CMEA model takes account of higher order effects which is essential for a rearrangement process where the first Born type approximation (Coulomb Born for the ionic case) is not supposed to be adequate. At low incident energies, the excited states of Ps (2s, 2p) are found to play a dominant role in the H- formation cross sections. Significant deviations are noted between the present CMEA and the Coulomb Born (CBA) results even at very high incident energies (e.g., Ei = 500 eV), indicating the importance of higher order effects. At high incident energies the present CMEA differential cross section (DCS) exhibits a double peak structure which is totally absent in the CBA and could again be attributed to higher order effects.

Fragmentation of positronium in collision with xenon

Abstract: Cross-sections are presented, both integrated and differential with respect to the longitudinal energy of the ejected positrons, for the fragmentation of Ps in collisions with Xe at 18 and 30 eV impact energy and compared with available theory. The shapes of the positron spectra are also compared with those measured in collisions with He at the same impact energy.

Overview of the present theoretical status of positron‐atom collisions

Abstract: Recent advances in the theoretical treatment of positron‐atom scattering are reviewed with particular emphasis on coupled‐state methods. Resonances appearing above the ionization threshold in positron‐atomic hydrogen collisions are shown to be artefacts of simple approximations. Reliable results can now be obtained for all the main transitions in positron scattering off ground state atomic hydrogen. Calculations now exist for all the alkali metals from Li to Cs. These show a pronounced growth in excited state positronium formation on going up the sequence but with a corresponding collapse in the Ps(1s) formation cross section. Despite a wealth of experimental data, coupled‐state calculations on the noble gases which include positronium formation are rather sparse.

Positronium Formation in Room Temperature Ionic Liquids

Abstract: Positron annihilation age-momentum correlation measurements (AMOC) were performed for room temperature ionic liquids (IL) to investigate positronium (Ps) formation process. The solvation time of free electrons in IL is longer than in usual molecular liquids. And hence IL is very useful materials to investigate fast reactions such as Ps formation. On the other hand, Ps formation is a good tool to investigate the dry electrons. The para-Ps annihilation lifetime is about 125ps and then the effect of dry electrons should be seen on the S(t) parameters made by the age-momentum correlation measurements. The slow Ps formation was observed in the room temperature ionic liquids.

Difficulties in explaining the cosmic photon excess with compact composite object dark matter

Abstract: It has been suggested that dark matter particles are strongly interacting, composite, macroscopically large objects made of well known light quarks (or antiquarks). In doing so it is argued that these compact composite objects (CCOs) provide natural explanations of observed data, such as the 511 keV line from the bulge of our galaxy observed by INTEGRAL, and the excess of diffuse gamma rays in the 1-20 MeV band observed by COMPTEL. Here we argue that the atmospheres of positrons that surround CCOs composed of di-antiquark pairs in the favored color-flavor-locked superconducting state are sufficiently dense as to place stringent limits on the penetration depth of interstellar electrons incident upon them, resulting in an extreme suppression of previously estimated rates of positronium formation, and hence in the flux of 511 keV photons resulting from their subsequent decays. The associated rate of direct electron-positron annihilations, which yield the MeV photons postulated to explain the 1-20 MeV photon excess, is also suppressed. We also discuss how even if a fraction of positrons somehow penetrated the surface of the CCOs, the extremely strong electric fields generated from the bulk antiquark matter would result in the destruction of positronium atoms long before they decay.

e+ + H(ns) → Ps(n′s) + p for arbitrary n and n′

Abstract: Positronium (Ps) formation in an arbitrary s-state from an arbitrary s-state of the hydrogen atom by positron impact has been studied within the framework of distorted-wave theory (DWT) which has recently been derived by the authors. A general formula for obtaining partial-wave distorted-wave amplitudes of positronium formation for all such reactions has been derived in closed analytic forms. This general formalism has then been used to obtain the results for differential and total cross sections in the energy range 20?300 eV. Further, in order to derive the general formula for scattering amplitudes in a closed analytic form, a simple prescription has been made to evaluate the arbitrary order derivatives of the general three- denominator Lewis integral. To the best of our knowledge, the distorted-wave results for the differential and total cross sections for such reactions are reported for the first time in the literature.

Application of the bound state formalism to positronium

Abstract: This report contains a discussion of a bound state formalism for QED and its application to positronium The formalism is based on the Bethe‐Salpeter equation for the two‐to‐two Green’s function The energy levels of positronium are found as poles of this Green’s function A perturbation scheme for the energy levels is set up by comparing the Bethe‐Salpeter equation with a soluble reference equation A useful reference problem is examined, and the reference solution is obtained by comparison with the non‐relativistic Schrodinger‐Coulomb Green’s function Renormalization effects are discussed As an example of the use of this formalism, an outline is given of the calculation of the ground state hyperfine splitting of positronium at the O(mα4) and O(mα5) levels

Exponential correlated basis in Schwinger variational principle for positronium formation in hydrogen

Abstract: An exponential correlated basis set is tested within the framework of multi-channel Schwinger variational principle in the momentum space. It is found that inclusion of only the positron-electron correlation in the basis set is enough to obtain accurate results for all significant partial waves for ground state positronium formation in positron-hydrogen collisions at low and intermediate energies (6.856–75 eV). Results are in conformity with other accurate variational and non-variational calculations as well as observed data available in the literature.

Molecular model for annihilation rates in positron complexes

Abstract: The molecular approach for positron interaction with atoms is developed further. Potential energy curves for positron motion are obtained. Two procedures accounting for the nonadiabatic effective positron mass are introduced for calculating annihilation rate constants. The first one takes the bound-state energy eigenvalue as an input parameter. The second is a self-contained and self-consistent procedure. The methods are tested with quite different states of the small complexes HPs, e + He (electronic triplet) and e + Be (electronic singlet and triplet). For states yielding the positronium cluster, the annihilation rates are quite stable, irrespective of the accuracy in binding energies. For the e + Be states, annihilation rates are larger and more consistent with qualitative predictions than previously reported ones.