Showing papers on "Positronium published in 2009"
TL;DR: Total and positronium formation cross sections have been measured for positron scattering from H2O and HCOOH using a positron beam with an energy resolution of 60 meV, and there shows no evidence of any channel coupling effects or scattering resonances for either molecule.
Abstract: Total and positronium formation cross sections have been measured for positron scattering from H2O and HCOOH using a positron beam with an energy resolution of 60 meV (full-width at half-maximum (FWHM)). The energy range covered is 0.5–60 eV, including an investigation of the behavior of the onset of the positronium formation channel using measurements with a 50 meV energy step, the result of which shows no evidence of any channel coupling effects or scattering resonances for either molecule.
57 citations
TL;DR: In this article, a new precise measurement of orthopositronium decay rate has been performed with an accuracy of 150 ppm, which is consistent with the last three results and also the 2nd order correction.
49 citations
TL;DR: In this article, the grand total and positronium-formation cross sections for positrons scattered by helium within the impact energy range from 10 to 60 eV were measured using a high-resolution trap-based pulsed positron beam.
Abstract: Measurements of the grand total and positronium-formation cross sections for positrons scattered by helium within the impact energy range from 10 to 60 eV are presented. All measurements presented here use a high-resolution $(\ensuremath{\sim}70\text{ }\text{meV})$ trap-based pulsed positron beam. Scattering is studied using a high-magnetic field, and absolute measurements of the scattering cross sections are obtained without the need for normalization to other cross sections. We also present single center, convergent close coupling calculations of the total cross section. A detailed study of the cross section to investigate the possibility of scattering resonances and channel coupling has been made. Comparisons with previous cross-section measurements and theoretical calculations are also included.
33 citations
TL;DR: In this article, the formation of positronium in the $2P$ state in collisions of positrons with He, Ar, and Xe atoms have been determined by measuring coincidences between the remnant ion and the Lyman-$\ensuremath{\alpha}$ photon from positRONium.
Abstract: The cross sections for the formation of positronium in the $2P$ state in collisions of positrons with He, Ar, and Xe atoms have been determined by measuring coincidences between the remnant ion and the Lyman-$\ensuremath{\alpha}$ photon from positronium. The maximum fractional contributions of these to the total Ps formation cross sections increase from approximately $0.06\ifmmode\pm\else\textpm\fi{}0.01$ in He to $0.12\ifmmode\pm\else\textpm\fi{}0.04$ in Ar and $0.26\ifmmode\pm\else\textpm\fi{}0.09$ in Xe. In the case of He, good agreement is found with a coupled-state calculation; for Ar and Xe, measurements are compared with a distorted-wave Born approximation.
32 citations
TL;DR: It is shown that dark matter sources cannot account for the observed positronium fraction without extensive propagation, yet with such propagation, standard nucleosynthetic sources can fully accounts for the spatial differences and positrons fraction, leaving no new signal for dark matter to explain.
Abstract: Assuming Galactic positrons do not go far before annihilating, a difference between the observed 511 keV annihilation flux distribution and that of positron production, expected from β^+ decay in Galactic iron nucleosynthesis, was evoked as evidence of a new source and signal of dark matter. We show, however, that the dark matter sources cannot account for the observed positronium fraction without extensive propagation. Yet with such propagation, standard nucleosynthetic sources can fully account for the spatial differences and positronium fraction, leaving no new signal for dark matter to explain.
28 citations
TL;DR: In this article, the positronium negative ion in exponential cosine-screened Coulomb potentials (ECSCP) has been theoretically investigated using highly correlated wave functions with 525 terms and reported binding energies, annihilation rates and several other parameters for various values of the screening parameter.
Abstract: The positronium negative ion (Ps
−) in exponential cosine-screened Coulomb potentials (ECSCP) has been theoretically investigated. Using highly correlated wave functions with 525 terms we report binding energies, annihilation rates and several other parameters for various values of the screening parameter. Our reported results for the unscreened case are in nice agreement with some of the most accurate results in the literature. Further we make a comparative study of the properties of Ps
− in ECSCP with the corresponding properties in screened Coulomb potential (SCP). To the best of our knowledge, such a study on Ps
− in ECSCP is carried out for the first time in the literature.
24 citations
TL;DR: It was found that the primary structural relaxation slows down with temperature, faster than the shrinkage of the hole free volume V(f) would predict, on the basis of the Cohen-Turnbull (CT) free volume theory.
Abstract: Positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature (PVT) experiments were performed to characterize the temperature dependent microstructure of the hole free volume in the low molecular weight glass-former phenyl salicylate (salol). The PALS spectra were analyzed with the new routine LT9.0 and the volume distribution of subnanometer size holes characterized by its mean ⟨vh⟩ and standard deviation σh was calculated. Crystallization of the amorphous sample was observed in the temperature range above 250 K, which leads to a vanishing of the positronium formation. The positronium signal recovered after melting at 303 K. A combination of PALS with PVT data enabled us to calculate the specific density Nh′, the specific volume Vf, and the fraction of holes fh in the amorphous state. From comparison with dielectric measurements in the temperature range above TB=265 K, it was found that the primary structural relaxation slows down with temperature, faster than the shrinkage of the h...
20 citations
TL;DR: The behavior of ortho-positronium (o-Ps) in mesoporous silica films implanted with low-energy positrons has been studied as a function of the film porous volume fraction as mentioned in this paper.
Abstract: The behavior of ortho-positronium (o-Ps) in mesoporous silica films implanted with low–energy positrons has been studied as a function of the film porous volume fraction. A lifetime spectrometer allowed determination of o-Ps annihilation decay both inside and outside of the film. A kinetic model is introduced that permits the determination of the yield and rate of escape of o-Ps into vacuum as well as the annihilation decay rate of the trapped o-Ps in the film. It is shown that these undergo a sudden change at a threshold porous volume fraction, above which the o-Ps escape rate to vacuum varies linearly with volume fraction.
19 citations
TL;DR: In this article, the doubly excited 1,3Pe resonance states of positronium negative ion with Coulomb and screened Coulomb potentials using highly accurate correlated exponential wavefunctions were investigated.
Abstract: We have investigated the doubly excited 1,3Pe resonance states of positronium negative ion with Coulomb and screened Coulomb potentials using highly accurate correlated exponential wavefunctions. For Coulomb interaction, the stabilization and the complex-rotation methods are employed to extract resonance parameters (resonance positions and widths). We have obtained two 1Pe resonances and three 3Pe resonances below the n = 3 Ps threshold. In addition to Feshbach resonances lying below n = 3 Ps threshold, we have calculated one 3Pe shape resonances lying above the Ps (n = 2) threshold. For screened Coulomb (Yukawa) interaction, we employ the stabilization method to extract resonance parameters as functions screening parameter. The resonance energies and widths for 1,3Pe resonance states of Ps− below the n = 3 Ps threshold for different screening parameters ranging from infinity (Coulomb case) to small values are reported, along with the Ps(3S) and Ps(3P) threshold energies. The screened Coulomb results for the 1,3Pe resonance states are reported for the first time in the literature.
17 citations
TL;DR: In this paper, an in-depth look at positron transport in pure argon gas has been performed using a recently established comprehensive set of cross sections and well-established Monte Carlo simulations.
Abstract: Motivated by an increasing number of applications, new techniques in the analysis of electron transport have been developed over the past 30 years or so, but similar methods had yet to be applied to positrons. Recently, an in-depth look at positron transport in pure argon gas has been performed using a recently established comprehensive set of cross sections and well-established Monte Carlo simulations. The key novelty as compared to electron transport is the effect of positronium formation which changes the number of particles and has a strong energy dependence. This coupled with spatial separation by energy of the positron swarm leads to counterintuitive behavior of some of the transport coefficients. Finally new results in how the presence of an applied magnetic field affects the transport coefficients are presented.
17 citations
TL;DR: In this article, it was shown that at incident positron energies below the threshold for positronium atom formation annihilation rates for molecules are much larger than those expected on the basis of simple two-body collisions, and that this phenomenon is due to positron attachment to molecules mediated by vibrational Feshbach resonances.
Abstract: At incident positron energies below the threshold for positronium atom formation annihilation rates for molecules are much larger than those expected on the basis of simple two-body collisions We now understand that this phenomenon is due to positron attachment to molecules mediated by vibrational Feshbach resonances This attachment enhances greatly the overlap of the positron and the molecular electron wave functions and hence increases the probability of annihilation Annihilation spectra as a function of incident positron energy yield measurements positron-molecule binding energies Using this technique, to date, positron-molecule binding energies for thirty molecules have been measured This paper presents a brief review of our current understanding of this process, the significance of these results, and outstanding questions for future research (© 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)
TL;DR: Experimentally determined cross sections for the elastic scattering of positrons by argon and xenon atoms have been found to exhibit a steplike increase at the first inelastic threshold energy, suggesting the existence of an intermediate virtual positronium state which enhances the elastic interaction probability.
Abstract: Experimentally determined cross sections for the elastic scattering of positrons by argon and xenon atoms have been found to exhibit a steplike increase at the first inelastic threshold energy---i.e., that for positronium formation. Rather than supporting the existence of a cusplike behavior predicted theoretically, this feature, which is more pronounced for xenon, suggests the existence of an intermediate virtual positronium state which enhances the elastic interaction probability.
TL;DR: In this paper, the authors used exponential correlated atomic target wave functions to predict positronium (Ps) formation cross sections in positron-helium collisions within the frame work of the distorted-wave approximation at intermediate and high energies of positron impact.
Abstract: Accurate Hylleraas-type correlated helium wave functions are used to predict positronium (Ps) formation cross sections in positron-helium collisions within the frame work of the distorted-wave approximation at intermediate and high energies of positron impact. Exponential correlated atomic target wave functions taking into account up to N=30 basis terms are utilized. Reliable total cross sections for the ground and excited 2s-state Ps formation are reported at intermediate and high energies. The present distorted-wave results are in conformity with the existing theoretical and experimental values available in the literature for intermediate and high-energy positrons. Surface plots of the DWA differential cross section reveal rich structures due to constructive and destructive interference between angular momentum states of the moving Ps.
TL;DR: In this article, the authors examined the prospect for detecting the positronium recombination spectrum and provided formulae to calculate expected line strengths from the e {sup +} production rate and for different conditions in the interstellar medium.
Abstract: Positronium is a short-lived atom consisting of a bound electron-positron pair. In the triplet state, when the spins of both particles are parallel, radiative recombination lines will be emitted prior to annihilation. The existence of celestial positronium is revealed through gamma-ray observations of its annihilation products. These observations, however, have intrinsically low angular resolution. In this paper, we examine the prospects for detecting the positronium recombination spectrum. Such observations have the potential to reveal discrete sources of e {sup +} for the first time and will allow the acuity of optical telescopes and instrumentation to be applied to observations of high-energy phenomena. We review the theory of the positronium recombination spectrum and provide formulae to calculate expected line strengths from the e {sup +} production rate and for different conditions in the interstellar medium. We estimate the positronium emission line strengths for several classes of Galactic and extragalactic sources. These are compared to current observational limits and to current and future sensitivities of optical and infrared instrumentation. We find that observations of the Psalpha line should soon be possible due to recent advances in NIR spectroscopy.
TL;DR: In this article, the authors studied the pick-off process of the triplet bound state ortho -Ps ( o -Ps) with electrons at the walls of the free volumes in polymers.
01 Nov 2009
TL;DR: In this article, the emission of positronium negative ions from W(100) and Mo and Ta polycrystalline surfaces has been investigated and a dramatic change in the emission efficiency upto 1.5% was observed upon coating Ta with Cs.
Abstract: The emission of positronium negative ions from W(100) and Mo and Ta polycrystalline surfaces has been investigated. A dramatic change in the emission efficiency upto 1.5% was observed upon coating Ta with Cs. The efficiency obtained in the present work may be sufficient to enable thorough investigations of the ions.
01 Nov 2009
TL;DR: In this paper, the experimental progress in the experimental study of positron and positronium-induced ionization of atoms and molecules is outlined, including integral and differential cross-sections, as well as formation of positronium in the first excited state.
Abstract: Recent progress in the experimental study of positron- and positronium-induced ionization of atoms and molecules is outlined. Investigations include integral and differential cross-sections, as well as formation of positronium in the first excited state. Future prospects are discussed.
01 Jul 2009
TL;DR: Positron annihilation technique as a probe of subatomic vacancies is described for non-positron polymer scientists in this paper, where the state-of-the-art understandings of how positronium is created and how its mean lifetime is determined in vacancies are described.
Abstract: Positron annihilation technique as a probe of subatomic vacancies is described for non-positron polymer scientists.Although positronium (Ps) is an unparalleled unique probe of inter-molecular vacancies in insulating materials, withboth its intensity and mean lifetime carrying information about the free volume, the processes that lead to these phys-ical quantities are not simple. The state-of-the-art understandings of how Ps is created and how its mean lifetime isdetermined in vacancies are described, and examples of the unique feature as well as the problems and precautionsare illustrated.
TL;DR: In this article, the positronium state in quartz is described by a linear superposition of two states: the first describing the free positron in the crystal and the second corresponding to a positronial Bloch wavefunction in the lattice.
Abstract: The positronium state in quartz is described by a linear superposition of two states: the first describing the free positron in the crystal and the second corresponding to a positronium Bloch wavefunction in the lattice. The condition for positronium formation in the electron gas is deduced by using variational calculations of the positron binding energy to the electron system. The self annihilation parameter $\kappa$ introduced in positron lifetime experiments can be properly justified by using the mixed state. A variational method to calculate $\kappa$ is proposed.
TL;DR: In this paper, the authors measured positronium lifetime and intensity in a set of binary mixtures of C 19 H 40 and C 21 H 44, and found that the lifetime in rigid crystal rigid phases increases with C 21H 44 content as a result of increasing the average interlamellar gap width.
TL;DR: Total cross section measurements for positron scattering from dihydropyran (C(5)H(8)O) are reported, with the energy range of the present study being 0.15-48 eV.
Abstract: We report on total cross section measurements for positron scattering from dihydropyran (C5H8O), with the energy range of the present study being 015−48 eV To the best of our knowledge, there are currently no other corresponding experimental data or theoretical computations against which we can compare our results The effect of this species’ important dipole moment and significant dipole polarizability on the scattering dynamics is considered, as is the opening of the positronium formation channel
TL;DR: In this article, the condition for positronium formation in the electron gas is deduced by using variational calculations of the positron binding energy to the electron system, which can be properly justified by using the mixed state.
Abstract: The positronium state in quartz is described by a linear superposition of two states: the first describing the free positron in the crystal and the second corresponding to a positronium Bloch wavefunction in the lattice. The condition for positronium formation in the electron gas is deduced by using variational calculations of the positron binding energy to the electron system.
The self annihilation parameter κ introduced in positron lifetime experiments can be properly justified by using the mixed state. A variational method to calculate κ is proposed (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, the emission efficiency of positronium negative ions from Cs deposited W(100) surface has been observed, and the upper limit of the binding energy of the ion has been estimated experimentally.
Abstract: Efficient emission of positronium negative ions from Cs deposited W(100) surface has been observed. The emission efficiency measured over a time interval of 3000 s immediately after the deposition was found to be 1.25% for the target with a 2.2 × 1014 atoms/cm2 Cs layer. The emission from Cs deposited polycrystalline Fe surface has also been observed. The upper limit of the binding energy of the ion has been estimated experimentally. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
01 Jan 2009
TL;DR: In this paper, experimental methods employed to investigate positron and positronium impact ionization and fragmentation in collision with atoms and molecules, and associated results are discussed. But the results are limited to the case of ortho-Ps atoms.
Abstract: © Cambridge University Press 2013. Introduction As well as probing matter–antimatter interactions, positrons (as positive electrons) have been employed to highlight charge and mass effects in the dynamics of collisions, including those resulting in the ionization of atoms and molecules (see, e.g., [1]). Positronium (Ps), the hydrogenic atom formed from the binding of a positron and an electron, is readily produced in the scattering of positrons from matter. Ps is quasi-stable with a lifetime against annihilation dependent upon its spin: ground-state para-Ps (1 1S0) has a lifetime τ ≃ 125 ps, whilst ortho-Ps (1 3S1) is considerably longer lived (τ ≃ 142 ns). The beam employed for the scattering work discussed in this chapter consists solely of ortho-Ps atoms. In a dense medium, Ps may undergo several cycles of formation and break-up prior to the annihilation of the positron (see, e.g., [2–6]). A quantitative understanding of this cycle is important also for practical applications such as nanodosimetry relating to positron emission tomography (PET) [e.g., 4]. In this chapter, we consider experimental methods employed to investigate positron and positronium impact ionization and fragmentation in collision with atoms and molecules, and associated results. In the case of positrons, an extensive database now exists of integral cross sections for the inert atoms (see, e.g., [5]), less so for molecules (see, e.g., [6]); differential data remain sparse (e.g., [5]). Our focus will be on the latter two topics as well as studies with positronium projectiles.
TL;DR: In this article, a nonlinear variational linear combination of atomic orbitals was used for the computation of the effect of a high-frequency, intense laser field on the positronium (Ps) quantum states.
Abstract: In this work, a nonlinear variational linear combination of atomic orbitals scheme we introduced recently (Lima et al 2007 Phys. Rev. B 75 073201) for studying hydrogenic systems in semiconductors is adapted for the computation of the effect of a high-frequency, intense laser field on the positronium (Ps) quantum states. Our nonperturbative approach, based upon the Kramers–Henneberger translation transformation, allows us to show that, for sufficiently high intensities, the radiation field induces a wavefunction dichotomy, which causes a reduction on the positronium decay probability (i.e., enhances its lifetime). Our results point out that an enhancement of up to four orders of magnitude in the Ps lifetime can be attained with the current laser technology. Our careful quantitative analysis also corrects some mistakes in the approximate formulae proposed by Ehlotzky in a previous work (Ehlotzky 1988 Phys. Lett. A 126 524).
01 Jan 2009-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, the authors present the level of agreement between theory and experiment for positron-impact ionization of atoms and molecules, and show that for very low impact energies the theory requires the inclusion of the coupling with the positronium formation channel.
Abstract: This paper presents the level of agreement between theory and experiment for positron-impact ionization of atoms and molecules. Distorted-wave theoretical models are able to reproduce the experimental integrated cross sections for atoms and diatomic molecules. The ECC phenomenon seen in the experimental triple differential cross sections can be obtained with a BBK-type model, but for very low impact energies the theory requires the inclusion of the coupling with the positronium formation channel.
TL;DR: In this article, electron spin-spin-correlated pairs of ortho-positronium and hydroxyl radical were generated in positron radiolysis of water molecules.
TL;DR: In this article, the authors proposed a model based on two simple processes that cause enhancement and inhibition of positronium formation to fit the experimental data over a broad temperature range and calculated intrinsic parameters for the polymers under study, such as the shallowly trapped electron recombination energy.
Abstract: Positron annihilation lifetime spectroscopy measurements of polystyrene and its derivatives have been performed in the temperature range 20-300 K. The ortho-positronium lifetime calculated from the lifetime spectrum analysis is related to the mean free-volume hole size. However, the ortho-positronium yield in the me- dium is dependent on the polymer chemistry and also on the irradiation effect induced by the positron source. We proposed a model based on two simple processes that cause, respectively, enhancement and inhibition of positronium formation to fit the experimental data over a broad temperature range. Using this model, intrinsic parameters for the polymers under study, such as the shallowly trapped electron recombination energy and the free-radical recombination energy, were calculated and discussed. V C 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2063-2073, 2009
01 Apr 2009
TL;DR: In this paper, the authors review the recent results on measurements of the cross sections based on obtained from collisional positron traps and subsequent calculations of transport properties of positron swarms that may be used to model thermalization experiments, collisional traps and possible applications of positrons in materials science and biomedicine.
Abstract: Recent experiments, theory and modelling of positron interactions with atoms and molecules are discussed. The first half of the paper is devoted to binary collisions between positrons and crossed beams of atoms or molecules (in this case neon) and the second half deals with ensembles of non-interacting positrons, otherwise known as swarms which are transported through the background gas. We review the recent results on measurements of the cross sections based on obtained from collisional positron traps and subsequent calculations of transport properties of positron swarms that may be used to model thermalization experiments, collisional traps and possible applications of positrons in materials science and biomedicine. It was found that kinetic phenomena occur in positron transport that are mainly the result of the positronium (Ps) formation which has a larger cross section than elastic scattering in most gases and at the same time is a non-conservative process. Most importantly negative differential conductivity (NDC) occurs only for the bulk drift velocity while it does not exist for the flux property, a phenomenon that has not been observed for electrons.
TL;DR: In this article, a comprehensive experimental determination of porosity and pore size distribution in rocks from oil fields formations by deuterium (2H) Nuclear Magnetic Resonance (NMR) and Positronium Annihilation Lifetime Spectroscopy (PALS) was performed.
Abstract: The present work involves a comprehensive experimental determination of porosity and pore size distribution in rocks from oil fields formations by deuterium (2H) Nuclear Magnetic Resonance (NMR) and Positronium Annihilation Lifetime Spectroscopy (PALS). Both techniques yield complementary results; PALS measures the average pore size providing bulk information from which the most abundant pore size can be obtained, and NMR allows for the determination of the relative pore size distribution accurately. Both techniques give complementary information to obtain an absolute pore size distribution.