Showing papers on "Positronium published in 2013"

Perspective of Positron Annihilation Spectroscopy in Polymers

Abstract: Positron annihilation spectroscopy (PAS) is a novel method that provides molecular-level information about complex macromolecular structure in a manner different from, but complementary to, conventional physical and chemical methodology. This paper presents a perspective of PAS in polymeric systems covering 12 aspects: historical, spacial, spherical quantum model, anisotropic structure, voids, positronium chemistry, time, positron annihilation lifetime spectroscopy and data analysis, variable monoenergetic slow positron beam techniques and depth profiling, elemental analysis, multidimensional instrumentation advances in PAS, and free volume and free-volume theories.

Total, elastic, and inelastic cross sections for positron and electron collisions with tetrahydrofuran

Abstract: We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahydrofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies, have been measured for positron collisions with THF. The positron beam used to carry out these experiments had an energy resolution in the range 40–100 meV (full-width at half-maximum). We also present TCS results for positron and electron scattering from THF computed within the independent atom model using the screening corrected additivity rule approach. In addition, we calculated positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations). While our integral and differential positron cross sections are the first of their kind, we compare our TCS with previous literature values for this species. We also provide a comparison between positron and electron-impact cross sections, in order to uncover any differences or similarities in the scattering dynamics with these two different projectiles. © 2013 American Institute of Physics.

$\bar {\sf {H}}^{+}$ ion production from collisions between antiprotons and excited positronium: cross sections calculations in the framework of the GBAR experiment

Abstract: In the framework of the gravitational behaviour of antihydrogen at rest (GBAR) experiment, cross sections for the successive formation of and from collisions between positronium (Ps) and antiprotons ( ) have been computed in the range 0?30?keV energy, using the continuum distorted wave-final state theoretical model in its three-body and four-body formulations. The effect of the electronic correlations in on the total cross sections of production has been studied using three different wave functions for H? (the matter equivalent of ). Ps excited states up to np?=?3, as well as excited states up to nh?=?4, have been investigated. The results suggest that the production of can be efficiently enhanced by using either a fraction of Ps(2p) and a 2?keV () beam or a fraction Ps(3d) and antiprotons with kinetic energy below 1?keV.

Quantification of Linear Entropy for Quantum Entanglement in He, H− and Ps− Ions Using Highly-Correlated Hylleraas Functions

Abstract: The quantum entanglement for the two electrons in three-body atomic systems such as the helium atom, the hydrogen negative ion and the positronium negative ion are investigated by employing highly correlated Hylleraas functions to represent the ground states of such systems. As a measure of the spatial entanglement, the linear entropy of the reduced density matrix is calculated for the ground states. The required four-electron (12-dimensional) integrals are solved analytically such that they are suitable for machine com- putations. Results are compared with other calculations when available.

A simple shape-free model for pore-size estimation with positron annihilation lifetime spectroscopy

Abstract: Positron annihilation lifetime spectroscopy is one of the methods for estimating pore size in insulating materials. We present a shape-free model to be used conveniently for such analysis. A basic model in classical picture is modified by introducing a parameter corresponding to an effective size of the positronium (Ps). This parameter is adjusted so that its Ps-lifetime to pore-size relation merges smoothly with that of the well-established Tao-Eldrup model (with modification involving the intrinsic Ps annihilation rate) applicable to very small pores. The combined model, i.e., modified Tao-Eldrup model for smaller pores and the modified classical model for larger pores, agrees surprisingly well with the quantum-mechanics based extended Tao-Eldrup model, which deals with Ps trapped in and thermally equilibrium with a rectangular pore.

Positron scattering from hydrogen atom embedded in dense quantum plasma

Abstract: Scattering of positrons from the ground state of hydrogen atoms embedded in dense quantum plasma has been investigated by applying a formulation of the three-body collision problem in the form of coupled multi-channel two-body Lippmann-Schwinger equations. The interactions among the charged particles in dense quantum plasma have been represented by exponential cosine-screened Coulomb potentials. Variationally determined hydrogenic wave function has been employed to calculate the partial-wave scattering amplitude. Plasma screening effects on various possible mode of fragmentation of the system e++H(1s) during the collision, such as 1s→1s and 2s→2s elastic collisions, 1s→2s excitation, positronium formation, elastic proton-positronium collisions, have been reported in the energy range 13.6-350 eV. Furthermore, a comparison has been made on the plasma screening effect of a dense quantum plasma with that of a weakly coupled plasma for which the plasma screening effect has been represented by the Debye model. ...

Positronium in MOFs: the Atom out of the box

Abstract: Department of Physics, University of Michigan, 48109, USA(Dated: October 7, 2013)Recently, evidence for positronium (Ps) in a Bloch state in self-assembled metal–organic frame-works (MOFs) has been reported [Dutta et al., Phys. Rev. Lett. 110, 197403 (2013)]. In thispaper, we study Ps emission into vacuum from four different MOF crystals: MOF-5, IRMOF-8,ZnO

Convergent-close-coupling formalism for positron scattering from molecules

Abstract: The $ab$ $initio$ convergent-close-coupling method has been extended to positron-molecule collisions within the adiabatic (fixed-nuclei) approximation. Application to molecular hydrogen at energies from 0.1 to 1000 eV has yielded convergent total ionization and grand total cross sections over most of the energy range. We find that very large calculations are required for convergence, even in the case of low-energy elastic scattering, due to the effects of positronium formation. In general, the comparison with experiment is good.

On the Calculation of Resonances in Pre-Born–Oppenheimer Molecular Structure Theory

Abstract: The main motivation for this work is the exploration of rotational–vibrational states corresponding to electronic excitations in a pre-Born–Oppenheimer quantum theory of molecules. These states are often embedded in the continuum of the lower-lying dissociation channel of the same symmetry and thus are thought to be resonances. To calculate rovibronic resonances, the pre-Born–Oppenheimer variational approach of [J. Chem. Phys. 2012, 137, 024104], based on the usage of explicitly correlated Gaussian functions and the global vector representation, is extended with the complex coordinate rotation method. The developed computer program is used to calculate resonance energies and widths for the three-particle positronium anion, Ps–, and the four-particle positronium molecule, Ps2. Furthermore, the excited bound and resonance rovibronic states of the four-particle H2 molecule are also considered. Resonance energies and widths are estimated for the lowest-energy resonances of H2 beyond the b 3∑u+ continuum.

Quantification of linear entropy for quantum entanglement in He, H- and Ps- ions using highly-correlated Hylleraas functions

Abstract: The quantum entanglement for the two electrons in three-body atomic systems such as the helium atom, the hydrogen negative ion and the positronium negative ion are investigated by employing highly correlated Hylleraas functions to represent the ground states of such systems. As a measure of the spatial entanglement, the linear entropy of the reduced density matrix is calculated for the ground states. The required four-electron (12-dimensional) integrals are solved analytically such that they are suitable for machine computations. Results are compared with other calculations when available.

Positron scattering from molecular hydrogen

Abstract: We present results for total and partial cross sections for positron scattering from H${}_{2}$. The total scattering and positronium formation cross sections are reported between 0.5 and 200 eV. Total quasielastic and inelastic scattering cross sections are reported for energies between the positronium formation threshold and 50 eV, with quasielastic differential scattering cross sections reported at 1, 3, 5, 7, and 10 eV. Our results are compared with previous work, both experimental and theoretical, with particular attention paid to the region below the positronium formation threshold, where there are apparent discrepancies in previous work. A discussion of possible reasons for discrepancies between this and previous work is presented, including a focus on known systematic effects in the experimental results.

Evidence of Positronium Bloch States in Porous Crystals of Zn 4 O -Coordination Polymers

Abstract: Positronium (Ps) is shown to exist in a delocalized state in self-assembled metalorganic crystals that have large 1.3--1.5 nm cell sizes. Belonging to a class of materials with record high accessible specific surface areas, these highly porous crystals are the first to allow direct probing with simple annihilation lifetime techniques of the transport properties of long-lived triplet Ps in what is hypothesized to be a Bloch state. Delocalized Ps has unprecedented (high) Ps mobility driven primarily by weak phonon scattering with unusual and profound consequences on how Ps probes the lattice.

Positron scattering from hydrogen atom embedded in weakly coupled plasma

Abstract: The positron-hydrogen collision problem in weakly coupled plasma environment has been investigated by applying a formulation of the three-body collision problem in the form of coupled multi-channel two-body Lippmann-Schwinger equations. The interactions among the charged particles in the plasma have been represented by Debye-Huckel potentials. A simple variational hydrogenic wave function has been employed to calculate the partial-wave scattering amplitude. Plasma screening effects on various possible mode of fragmentation of the system e++H(1s) during the collision, such as 1s→1s and 2s→2s elastic collisions, 1s→2s excitation, positronium formation, elastic proton-positronium collisions, have been reported. Furthermore, a detailed study has been made on differential and total cross sections of the above processes in the energy range 13.6-350 eV of the incident positron.

Calculated photoexcitation spectra of positronium Rydberg states

Abstract: Calculations of the photoexcitation spectra of ortho-positronium Rydberg states with principal quantum numbers between 10 and 30 are presented. The effects of Doppler broadening and saturation of the corresponding electric-dipole transitions are studied, together with the role of static and motionally induced electric fields. This is done in the context of recent measurements reported by Cassidy et al. [Phys. Rev. Lett. 108, 043401 (2012)], and with regard to experiments involving the production of antihydrogen by charge-exchange between Rydberg positronium and cold antiprotons.

Low-energy positron and electron scattering from nitrogen dioxide

Abstract: Total cross section (TCS) measurements for positron scattering from nitrogen dioxide (NO2) are presented in the energy range 0.2–40 eV. The TCS, the elastic integral and differential cross sections, and the integral cross section accounting of all the inelastic processes (including positronium formation) have also been computed using the independent atom model with screening corrected additivity rule (IAM-SCAR) for incident energies from 1 to 1000 eV. A qualitative level of agreement is found between the present TCS experiment and theory at the common energies. As no previous measurements or calculations for positron–NO2 scattering exist in the literature, we also computed the TCS for electron collisions with NO2 employing the IAM-SCAR method. A comparison of those results to the present positron cross sections and the earlier electron-impact data and calculations is provided. To investigate the role that chemical substitution plays in positron scattering phenomena, we also compare the present positron–NO2 data with the TCSs measured at the University of Trento for positron scattering from N2O and CO2.

Experimental and theoretical cross sections for positron collisions with 3-hydroxy-tetrahydrofuran

Abstract: Cross section results from a joint experimental and theoretical investigation into positron scattering from 3-hydroxy-tetrahydrofuran (3H-THF) are presented. Total and positronium (Ps) formation cross sections have been measured from 1 to 190 eV using the positron beamline at the Australian National University, which has an energy resolution between 60 and 100 meV. The total cross section (TCS) and the elastic and total inelastic integral cross sections in the energy range between 1 and 1000 eV have been computed within the Independent Atom Model using the Screening Corrected Additivity Rule approach. In addition, we have calculated elastic differential cross sections at selected incident energies. Our computations represent the first theoretical results reported for this target species, while our measured Ps formation cross sections are also novel. Comparison of the present TCS with the previous results from the University of Trento shows a good level of agreement at the lowest energies. We also provide a comparison between the present cross sections for 3H-THF and those from our earlier study on the parent molecule tetrahydrofuran.

Positron life time spectroscopy as a method to study of the defect degree materials with disordered structure

Abstract: The paper presents the use of positron lifetime spectroscopy PALS to study the defect degree of structures of disordered materials, research materials were used as samples: the metallic glass, oxide glass, chalcogenide glass and polymer. As a result of the measurements obtained curves describing the dependence of the number of counts of acts of annihilation as a function of time. The distribution of positron lifetime spectra was carried out on three components. The calculated parameters uptake of positron and positronium in structurally disordered materials studied allow to draw conclusions about the degree and nature of defects of studied materials.

Characterization of positronium properties in doped liquid scintillators

Abstract: Orthopositronium (o-Ps) formation and decay can replace the annihilation process, when a positron interacts in liquid scintillator media. The delay induced by the positronium decay represents either a potential signature for antineutrino detection, via inverse $\ensuremath{\beta}$ decay, or to identify and suppress positron background, as recently demonstrated by the Borexino experiment. The formation probability and decay time of o-Ps depend strongly on the surrounding material. In this paper, we characterize the o-Ps properties in liquid scintillators as function of concentrations of gadolinium, lithium, neodymium, and tellurium dopers used by present and future neutrino experiments. In particular, gadolinium and lithium are high neutron cross section isotopes, widely used in reactor antineutrino experiments, while neodymium and tellurium are double $\ensuremath{\beta}$ decay emitters, employed to investigates the Majorana neutrino nature. Future neutrino experiments may profit from the performed measurements to tune the preparation of the scintillator in order to maximize the o-Ps signature, and therefore the discrimination power.

Time-resolved determination of ortho-positronium kinetic energy utilizing p-wave scattering during positronium-xenon collisions

Abstract: We report a method to determine time evolution of ortho-positronium ($o$-Ps) kinetic energy with higher accuracy and sensitivity than conventional methods. Our method utilizes the fact that, during Ps-Xe collisions, an $o$-Ps atom can undergo a spin-conversion reaction induced by spin-orbit interaction to decay into two gamma-ray photons and the fact that the reaction rate strongly depends on the $o$-Ps kinetic energy because the reaction occurs only in $p$-wave scattering; thus a small change in the energy leads to a large change in the two-photon annihilation rate. Utilizing this reaction as a ``lens'' to magnify the $o$-Ps kinetic energy, we obtain its time evolution by measuring the time-resolved two-photon annihilation rate with an age-momentum correlation spectrometer. The time evolution of $o$-Ps kinetic energy can be explained by a classical model that assumes elastic collisions in a time range greater than 20 ns and an energy range of 40--60 meV. By applying this method, the Ps-Xe momentum-transfer cross section is found to be $12(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ cm${}^{2}$.

Spin conversion and pick-off annihilation of ortho-positronium in gaseous xenon at elevated temperatures

Abstract: We present experimental results for the annihilation rate of ortho-positronium in gaseous xenon at 250 kPa using a digital-oscilloscope-based positron annihilation lifetime spectrometer as a function of temperature (300--623 K). Owing to the Zeeman mixing of positronium in a magnetic field, the annihilation rate is divided into two components: one is due to ortho-para postronium spin conversion induced by spin-orbit interaction, while the other is due to pick-off annihilation. The spin-conversion annihilation rate is proportional to ${T}^{2.1}$, where $T$ is the temperature. We attributed this dependence to the fact that the spin conversion occurs only in $p$-wave scattering during positronium--xenon collisions and to the fact that, after thermalization of the ortho-positronium atoms, their velocity profile follows a Maxwell-Boltzmann distribution. The results also show that the pick-off annihilation rate is almost linear in temperature. This increase is a dependence exceptionally stronger than that observed in other gases.

Positronium formation in the noble gases

Abstract: We present results based upon our ab initio relativistic optical potential method for the simulation of positronium formation in the noble gases neon, argon, krypton and xenon. The method used in this simulation is based upon an approach, first suggested by Reid and Wadehra, whereby the ionization threshold of the atom is effectively reduced by 6.8 eV, the binding energy of positronium in its ground state. This procedure is then modified in order to account for the fact that positronium formation cross sections tend to zero much more rapidly than the corresponding direct ionization cross sections as the energy of the incident positron increases. The agreement of our positronium formation cross sections with experiment is quite good for argon, krypton and xenon over most of the energy region while for neon our cross sections are generally too small at all energies. Nonetheless, our method predicts positronium formation cross sections in better agreement with experiment than the much more sophisticated theoretical approaches used in the past. We also show that adding positronium formation in this way improves agreement with the measured elastic differential cross section at small scattering angles.

Positronium production in engineered porous silica

Abstract: Positronium (Ps) has been the subject of several experimental and theoretical investigations due to its many scientific applications. In this work high positronium yield was found in engineered porous silica. The studied materials were pellets of swollen MCM-41 and of commercial Davicat 1700, obtained by different compression pressures, with mesopores characterized by different structural and chemical features. The measurements were performed with a variable energy positron beam at room temperature. An estimation of the Ps mean diffusion length was obtained by measuring capped samples. A selected swollen MCM-41 sample (0.39 g/cm3) was measured also at cryogenic temperature (8 K). In this material both the Ps yield and the Ps diffusion length are found to be independent of temperature. The pore surface of the swollen MCM-41 samples is very interesting in comparison to commercial silica as it possesses hydrophobic patches to avoid ice formation at low temperature. Positron lifetime measurements show a high ...

The stability of S-states of unit-charge Coulomb three-body systems: from H- to H2(+).

Abstract: High accuracy non-relativistic quantum chemical calculations of the ground state energies and wavefunctions of symmetric three-particle Coulomb systems of the form {m ± 1 m ± 2 m ∓ 3 } , m 1 = m 2, are calculated using an efficient and effective series solution method in a triple orthogonal Laguerre basis set. These energies are used to determine an accurate lower bound to the stability zone of unit-charge three-particle Coulomb systems using an expression for the width of the stability band in terms of g, the fractional additional binding due to a third particle. The results are presented in the form of a reciprocal mass fraction ternary diagram and the energies used to derive a parameterised function g(a 3), where a 3 =m −1 3 /(m −1 1 +m −1 2 +m −1 3 ) is the reciprocal mass of the uniquely charged particle. It is found that the function is not minimal at a 3 = 0 which corresponds to ∞H− nor is it minimal at the positronium negative ion (Ps−) the system with the least absolute energetic gain by association with a third particle; the function g(a 3) is minimal at m 1/m 3 = 0.49, and a possible physical interpretation in terms of the transition from atomic-like to molecular-like is provided.

Threshold behavior of positronium formation in positron-alkali-metal scattering

Abstract: We consider positron scattering on the alkali-metal atoms of Li, Na, and K at very low energies, where only the elastic scattering and positronium formation in the ground state are the two open channels. Utilizing the recently developed two-center convergent close-coupling method [Lugovskoy, Kadyrov, Bray, and Stelbovics, Phys. Rev. A 82, 062708 (2010)] we investigate the behavior of the cross sections as the impact energy goes to zero and demonstrate their convergence. The study sets quantitative benchmarks for any rigorous theoretical treatment of the collision problems.

Cross section for photoionization of the positronium negative ion at the lowest Feshbach resonance

Abstract: The photoionization cross section for the positronium (Ps) negative ion, Ps−, at the lowest n = 2 Feshbach resonance, estimated by neglecting the influences of the weakly bound outer electron, is σ...

Antihydrogen $(\overline{\rm {H}})$ and muonic antihydrogen $(\overline{\rm {H}}_{\mu })$ formation in low energy three-charge-particle collisions

Abstract: A few-body formalism is applied for computation of two different three-charge-particle systems. The first system is a collision of a slow antiproton, , with a positronium atom: Ps=(e+e−)—a bound state of an electron and a positron. The second problem is a collision of with a muonic muonium atom, i.e. true muonium—a bound state of two muons one positive and one negative: Psμ = (μ+μ−). The total cross section of the following two reactions: and , where is antihydrogen and is a muonic antihydrogen atom, i.e. a bound state of and μ+, are computed in the framework of a set of coupled two-component Faddeev–Hahn-type (FH-type) equations. Unlike the original Faddeev approach the FH-type equations are formulated in terms of only two but relevant components: Ψ1 and Ψ2, of the system's three-body wave function Ψ, where Ψ = Ψ1 + Ψ2. In order to solve the FH-type equations Ψ1 is expanded in terms of the input channel target eigenfunctions, i.e. in this work in terms of, for example, the (μ+μ−) atom eigenfunctions. At the same time Ψ2 is expanded in terms of the output channel two-body wave functions, that is in terms of atom eigenfunctions. Additionally, a convenient total angular momentum projection is performed. Results for better known low energy μ− transfer reactions from one hydrogen isotope to another hydrogen isotope in the cycle of muon catalyzed fusion (μCF) are also computed and presented.

Account of the intratrack radiolytic processes for interpretation of the AMOC spectrum of liquid water

Abstract: Recent development of the Gamma-induced Positron Spectroscopy (GiPS) setup significantly extends applicability of the Age-Momentum Correlation technique (AMOC) for studies of the bulk samples. It also provides many advantages comparing with conventional positron annihilation experiments in liquids, such as extremely low annihilation fraction in vessel walls, absence of a positron source and positron annihilations in it. We have developed a new approach for processing and interpretation of the AMOC-GiPS data based on the diffusion recombination model of the intratrack radiolytic processes. This approach is verified in case of liquid water, which is considered as a reference medium in the positron and positronium chemistry.