Showing papers on "Positronium published in 2010"
TL;DR: In this paper, the authors measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps ${1}^{3}S$-${2}^{ 3}P$ transition.
Abstract: We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps ${1}^{3}S$-${2}^{3}P$ transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, ${E}_{x}=42\ifmmode\pm\else\textpm\fi{}3$ meV, that is consistent with conversion of the confinement energy of Ps in the 2.7-nm-diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures of less than 100 K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited-state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.
109 citations
TL;DR: High formation yield and a meaningful cooled fraction of positronium below room temperature were obtained by implanting positrons in a silicon target in which well-controlled oxidized nanochannels perpendicular to the surface were produced.
Abstract: High formation yield and a meaningful cooled fraction of positronium below room temperature were obtained by implanting positrons in a silicon target in which well-controlled oxidized nanochannels (5-8 nm in diameter) perpendicular to the surface were produced. We show that by implanting positrons at 7 keV in the target held at 150 K, about 27% of positrons form positronium that escapes into the vacuum. Around 9% of the escaped positronium is cooled by collision with the walls of nanochannels and is emitted with a Maxwellian beam at 150 K. Because positronium quantum confinement limits the minimum achievable positronium energy, the tuning of the nanochannel's size is crucial for obtaining positronium gases in vacuum at very low temperature.
96 citations
TL;DR: In this article, the authors measured the ortho-positronium (ortho-Ps) emission energy in vacuum from mesoporous films using the time-of-flight technique.
Abstract: In this paper, we present measurements of the ortho-positronium (ortho-Ps) emission energy in vacuum from mesoporous films using the time-of-flight technique. We show evidence of quantum mechanical confinement in the mesopores that defines the minimal energy of the emitted Ps. Two samples with different effective pore sizes, measured with positron annihilation lifetime spectroscopy, are compared for the data collected in the temperature range $50--400$ K. The sample with smaller pore size exhibits a higher minimal energy ($73\ifmmode\pm\else\textpm\fi{}5$ meV), compared to the sample with bigger pores ($48\ifmmode\pm\else\textpm\fi{}5$ meV), due to the stronger confinement. The dependence of the emission energy with the temperature of the target is modeled as ortho-Ps being confined in rectangular boxes in thermodynamic equilibrium with the sample. We also measured that the yield of positronium emitted in vacuum is not affected by the temperature of the target.
81 citations
TL;DR: Positronium has unexpectedly been found to scatter from a wide variety of atoms and molecules in a way very similar to that of a bare electron moving at the same velocity, despite Ps being neutral and twice the mass.
Abstract: Positronium (Ps), a hydrogen-like atom composed of an electron and its antimatter partner, the positron, is formed in considerable quantities whenever positrons interact with matter. It has unexpectedly been found to scatter from a wide variety of atoms and molecules in a way very similar to that of a bare electron moving at the same velocity, despite Ps being neutral and twice the mass.
80 citations
TL;DR: In this article, the positron-positronium converter composed of regular nanochannels with size in the 5-100 nm range and length about $2\text{ }\ensuremath{\mu}\text{m}$ has been produced in silicon by electrochemical etching.
Abstract: A positron-positronium converter composed of regular nanochannels with size in the 5--100 nm range and length about $2\text{ }\ensuremath{\mu}\text{m}$ has been produced in silicon by electrochemical etching. After controlled oxidation of the inner surface of the channels, a positronium yield up to 45% of the positrons implanted at 1 keV energy was observed. At 1 keV positron implantation energy about 42% of positronium is estimated to be emitted into the vacuum. At 10 keV positron implantation energy, corresponding to a depth of $\ensuremath{\sim}800\text{ }\text{nm}$, the positronium fraction emitted into the vacuum is still 10% in a sample with channels of 4--7 nm and up to $23--25\text{ }\mathrm{%}$ in samples with larger channels. The positronium diffusion length and the fast positronium fraction emitted into the vacuum have been investigated as a function of the channel size tuned in the 5--100 nm range. The dependence of the positronium cooling from the size of the nanochannel is discussed and an evaluation of the expected fraction of thermalized orthopositronium in vacuum is given.
68 citations
TL;DR: It is concluded that, at high Ps densities, the minority spin component is essentially eliminated and the remaining Ps is almost entirely polarized, as required for the formation of a Ps Bose-Einstein condensate.
Abstract: Long-lived |m|=1 positronium (Ps) atoms are produced in vacuum when high density bursts of positrons with net polarization p0 are implanted into a porous silica film in a 2.3 T magnetic field. We observe a decrease in the |m|=1 population as the density of the incident positron beam is increased due to quenching interactions between oppositely polarized Ps atoms within the target. Saturation of this density dependent quenching indicates that the initial positron spin polarization p0=28±1%, and demonstrates the long term (102s) survival of positron polarization in a Surko-type buffer gas trap. We conclude that, at high Ps densities, the minority spin component is essentially eliminated and the remaining Ps is almost entirely (∼96%) polarized, as required for the formation of a Ps Bose-Einstein condensate. © 2010 The American Physical Society.
50 citations
TL;DR: CP violation in the quark sector has been well established over the last decade, but has not been observed in the lepton sector, and searching for CP violating decay processes in positronium using the angular correlation finds no CP violation.
Abstract: CP violation in the quark sector has been well established over the last decade, but has not been observed in the lepton sector. We search for CP violating decay processes in positronium, using the angular correlation of (S x k{1})(S x k{1}x k{2}), where S is the positronium spin and k{1}, k{2} are the directions of the positronium decay photons. To a sensitivity of 2.2x10{-3}, no CP violation has been found, which is at the level of the CP violation amplitude in the K meson. A 90% confidence interval of the CP violation parameter (C{CP}) was determined to be -0.0023
49 citations
01 Apr 2010
TL;DR: Positronium (Ps) formation measurements in several porous materials as: Vycor, germanate Xerogel, Metal-Organic Frameworks MOF-177 and Aerogel with two densities (20 and 150 mg/cm 3 ) were performed by means of a variable energy positron beam provided with a Ge detector and a positron lifetime spectrometer as discussed by the authors.
Abstract: Positronium (Ps) formation measurements in several porous materials as: Vycor, germanate Xerogel, Metal-Organic Frameworks MOF-177 and Aerogel with two densities (20 and 150 mg/cm 3 ), were performed by means of a variable energy positron beam provided with a Ge detector and a positron lifetime spectrometer. An efficient formation of cooled Ps atoms is a requisite for the production of antihydrogen, with the aim of a direct measurement of the Earth gravitational acceleration g of antimatter, which is a primary scientific goal of AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy). Porous materials are necessary to form a high yield of Ps atoms as well as to cool Ps through collisions with the inner walls of the pores. The different materials were characterized and produce Ps into the pores. Lifetime measurements give an estimation of the typical pores dimension of the substances. A comparative study of the positron lifetime and the Ps fraction values in the above mentioned materials indicates that silica Aerogel, with the appropriate density, is an excellent candidate for an efficient formation of cold Ps atoms for the AEGIS project.
38 citations
TL;DR: For the first time in Ps collisions, a resonantlike structure is observed and it is suggested that the former peak corresponds to the well-known 2Π(u) shape resonance which occurs for electrons at an incident velocity of 0.5 a.u.
Abstract: The total cross sections of positronium (Ps) scattering from a carbon-dioxide molecule have been measured over the range (7--400) eV incident-Ps energy. For the first time in Ps collisions, a resonantlike structure is observed. For the present target, it occurs around 9.5 eV followed by a broader peak at $\ensuremath{\sim}60\text{ }\text{ }\mathrm{eV}$. Following Brawley et al. [Science 330, 789 (2010)] who have observed similarities between the total cross sections of positronium and of electrons incident upon a given target at the same velocity, a corresponding comparison is made for ${\mathrm{CO}}_{2}$. The comparison suggests that the former peak corresponds to the well-known $^{2}\ensuremath{\Pi}_{u}$ shape resonance which occurs for electrons at an incident velocity of 0.5 a.u. Further features are discussed and theoretical input is sought.
36 citations
TL;DR: In this paper, the pore size analysis performed via nitrogen adsorption and positronium annihilation lifetime spectroscopy (PALS) was used to determine the size distribution of nanoporous glass monoliths.
34 citations
TL;DR: The convergent close-coupling method with two-centre expansions has been developed to calculate positron scattering on helium as discussed by the authors, which utilizes a multiconfigurational description of helium wavefunctions.
Abstract: The convergent close-coupling method with two-centre expansions has been developed to calculate positron scattering on helium. The method utilizes a multiconfigurational description of helium wavefunctions. Positronium formation is taken into account explicitly as electron capture into the positronium states. Direct-scattering, positronium-formation and breakup cross sections are calculated at all energies of practical relevance. Convergence in the calculated cross sections is demonstrated by increasing the basis size and orbital quantum number of the included states for each of the centres. Better agreement with experimental data is found when a multiconfigurational description is used for the helium wavefunctions in comparison with the recent frozen-core results.
TL;DR: The porous structure of nanostructured vinylpyrrolidone-divinylbenzene (VP-DVB)/MCM-41 composite was characterized using N(2) adsorption at 77K and positronium annihilation lifetime spectroscopy, atomic force microscopy and Raman spectroscopic.
TL;DR: In this paper, high-purity NiO/Al2O3 catalysts were prepared by mixing NiO and -Al 2O3 nanopowders, and X-ray diffraction patterns were measured to characterize the grain size and crystalline phase of the nanopowsders.
Abstract: High-purity NiO /Al2O3 catalysts were prepared by mixing NiO and -Al2O3 nanopowders. X-ray diffraction patterns were measured to characterize the grain size and crystalline phase of the nanopowders. Positronannihilation spectroscopy was used to study the microstructure and surface properties of the pores inside the NiO /Al2O3 catalysts. The positron lifetime spectrum comprises two short and two long lifetime components. The two long lifetimes 3 and 4 correspond to ortho-positronium o-Ps annihilated in microvoids and large pores, respectively. With increasing NiO content in the NiO /Al2O3 catalysts, both 4 and its intensity I4 show continuous decrease. Meanwhile, the para-positronium p-Ps intensity, obtained from coincidence Doppler broadening spectra, increases gradually with NiO content. The different variation in o-Ps and p-Ps intensity suggests the ortho-para conversion of positronium in NiO /Al2O3 catalysts. X-ray photoelectron spectroscopy shows that Ni mainly exists in the form of NiO. The electron-spin-resonance measurements reveal that the ortho-para conversion of Ps is induced by the unpaired electrons of the paramagnetic centers of NiO.
TL;DR: In this article, two-center convergent close-coupling calculations of positron-lithium collisions were performed using Laguerre-based states and a large number of channels and high partial waves were used to ensure the convergence of the cross sections.
Abstract: We report on two-center convergent close-coupling calculations of positron-lithium collisions. The target is treated as one active electron interacting with an inert ion core. The positronium formation channels are taken into account explicitly utilizing both negative- and positive-energy Laguerre-based states. A large number of channels and high partial waves are used to ensure the convergence of the cross sections. We find the Ramsauer-Townsend minimum in total and elastic cross sections at an impact energy $E$ of about 0.0016 eV. As found previously for H and He, the contributions to the breakup cross section from both the Li and the Ps centers become the same as the threshold is approached.
TL;DR: In this paper, the authors investigated the ECCP resonance states of the positronium negative ion (Ps-) in exponential cosine-screened Coulomb potentials (ECSCP) using stabilization method.
Abstract: The $2s^2~^1{\rm S}^e$
autoionization resonance states of the positronium negative ion (Ps-) in exponential cosine-screened Coulomb potentials (ECSCP) are investigated using stabilization method. Highly correlated wave functions for Ps- are used to take into account the correlation effect of the charged particles. Results for resonance energies and widths are reported for the screening parameter in the range 0.0-0.07 (in a0
-1), to the best our knowledge, for the first time in the literature.
TL;DR: In this paper, the quantum mechanics of two-electron systems are reviewed, starting with the ground state of the helium atom and helium-like ions with central charge Z. The wavefunction proposed by Chandrasekhar is revisited, where the permutation symmetry is first broken and then restored by a counterterm.
Abstract: The quantum mechanics of two-electron systems is reviewed, starting with the ground state of the helium atom and heliumlike ions with central charge Z. The case of Z≥2 is rather straightforward. In contrast, for negative hydrogen ion with Z=1, the stability of H− cannot be achieved using a product of individual electron wavefunctions and requires explicit account of the anticorrelation among the two electrons. The wavefunction proposed by Chandrasekhar is revisited, where the permutation symmetry is first broken and then restored by a counterterm. More difficult problems can be studied using the same strategy such as the stability of hydrogenlike ions for any value of the proton-to-electron mass ratio M/m, the energy of the lowest spin-triplet state of helium and heliumlike ions, and the stability of the doubly excited hydrogen ion with unnatural parity. The positronium molecule, which was predicted years ago and discovered recently, can also be shown to be stable against spontaneous dissociation. Emphasi...
TL;DR: In this article, the elastic scattering of positrons has been measured in the region of the positronium formation threshold for the noble gases He-Xe and Cusplike behavior is observed and, while the features which are observed appear broad, they represent a magnitude of between 4 and 15% of the total elastic cross section.
Abstract: Channel coupling is a phenomenon that has been investigated for many scattering processes, and is responsible for the formation of cusps or steps in the cross sections for open scattering channels at, or near, the onset of a new scattering channel. It has long been speculated that the opening of the positronium formation channel may lead to the formation of such cusp features in the elastic positron scattering cross section. In this work, elastic scattering of positrons has been measured in the region of the positronium formation threshold for the noble gases He-Xe. Cusplike behavior is observed and, while the features which are observed appear broad, they represent a magnitude of between 4 and 15% of the total elastic cross section. No evidence is found of any other features in this region, at least within the uncertainty of the present data, discounting the possibility of scattering resonances.
TL;DR: In this article, elastic-scattering cross sections for positron collisions with helium, neon, argon, krypton, and xenon around the threshold energy of positronium (Ps) formation are presented.
Abstract: Measurements of elastic-scattering cross sections are presented for positron collisions with helium, neon, argon, krypton, and xenon around the threshold energy for positronium (Ps) formation. The elastic cross section falls slowly with increasing energy above the Ps formation threshold in helium and neon, whereas in argon, krypton, and xenon it exhibits an increase, which appears both more prominent and more sustained as the atomic number of the gas increases. It is proposed that this coupling is a result of an intermediate virtual Ps state that enhances branching into the (atom plus positron) final state.
TL;DR: In this paper, the positron-impact excitation of hydrogen atoms embedded in plasma environments is investigated using the close-coupling approximation from the low to intermediate energy region without including any positronium formation channel, and the excitation cross sections for 1s → 2s, 1s→ 2p and 2s-to-2p processes are calculated in a wide Debye parameter range.
Abstract: The positron-impact excitation of hydrogen atoms embedded in plasma environments is investigated using the close-coupling approximation from the low to intermediate energy region without including any positronium formation channel, and the excitation cross sections for 1s → 2s, 1s → 2p and 2s → 2p processes are calculated in a wide Debye parameter range. The screening interactions, described by the Debye–Huckel model, decrease the coupling matrix elements, resulting in the reduction of the excitation cross sections from a few percent to one magnitude of ten. This will alter remarkably the spectroscopy of hydrogen in intensity and position, which should be considered in the simulation and diagnostics under some specific plasma conditions.
TL;DR: In this article, a two-centre convergent close-coupling method, which includes the positronium-formation channel, has been developed for positron scattering on helium.
Abstract: A two-centre convergent close-coupling method, which includes the positronium-formation channel, has been developed for positron scattering on helium. Convergent, pseudoresonance-free cross sections have been obtained. This is only possible if complete expansions are used on both the positronium and helium centres. The method is valid for all projectile energies and all transitions, including breakup, which is associated with excitation of positive-energy helium and positronium pseudostates. Generally, good agreement between calculated cross sections and available experimental data has been found across all incident energies.
TL;DR: In this article, a self consistent interpretation of the positron lifetime experiments in water at different temperatures (2-93 0C) and magnetic fields (H ≤ 2 T) is given.
Abstract: A self consistent interpretation of the positron lifetime experiments in water at different temperatures (2-93 0C) and magnetic fields (H ≤ 2 T) is given. By using the blob model of Ps for-mation we have obtained the contact density in the positronium atom in water, which is in agree-ment with the previous measurements.
TL;DR: Cross sections for ionization excitation of molecules by positron impact have been measured for the first time by scattering a positron beam from CO2 and N2 and the enhancement arises primarily via positronium formation.
Abstract: Cross sections for ionization excitation of molecules by positron impact have been measured for the first time by scattering a positron beam from CO2 and N2. The cross sections have been observed to exceed those for electron impact by up to a factor of approximately 3 for CO2 and approximately 5 for N2. The enhancement arises primarily via positronium formation. The cross sections account for up to approximately 12% and 20% of the total cross sections for positron scattering from N2 and CO2, respectively.
TL;DR: In this paper, a new uptapered cavity is designed for the gyrotron FU CW VA enhancing its tunability and enabling its use for measurements of the hyperfine split of positronium.
Abstract: New uptapered cavity is designed for the gyrotron FU CW VA enhancing its tunability and enabling its use for measurements of the hyperfine split of positronium.
TL;DR: Crystallization of the films was observed at just below 140 K, in agreement with earlier studies, with the topmost 80 nm undergoing a transition consistent with crystallization at 90-100 K.
Abstract: The evolution and annealing of pores in, and the crystallization of, vapor-deposited films of amorphous solid water have been studied by using variable-energy positron annihilation spectroscopy for temperatures in the range 50--150 K. Both positron and positronium annihilation provide insight to the nature of the grown-in pores and their evolution with temperature. Crystallization of the films was observed at just below 140 K, in agreement with earlier studies, with the topmost 80 nm undergoing a transition consistent with crystallization at 90--100 K.
TL;DR: In this paper, the convergent close-coupling method has been applied to positron scattering on a helium atom in the 2 {sup 3}S metastable state, where the positronium (Ps) formation channel is open even at zero scattering energy making the inclusion of the Ps channels especially important.
Abstract: The convergent close-coupling method has been applied to positron scattering on a helium atom in the 2 {sup 3}S metastable state. For this system the positronium (Ps) formation channel is open even at zero scattering energy making the inclusion of the Ps channels especially important. Spin algebra is presented for the general case of arbitrary spins. A proof is given of the often-used assumption about the relationship between the amplitudes for ortho-positronium and para-positronium formation. The cross sections for scattering from 2 {sup 3}S are shown to be significantly larger than those obtained for the ground state.
TL;DR: In this paper, the photodetachment of positronium negative ion (Ps −) interacting with exponential cosine-screened Coulomb potentials (ECSCP) within the framework of dipole approximation was investigated.
Abstract: We have made an investigation to study the photodetachment of positronium negative ion (Ps −) interacting with exponential cosine-screened Coulomb potentials (ECSCP) within the framework of dipole approximation. The dipole transition matrix elements are calculated using the asymptotic form of highly accurate initial wave function of the bound ionic state and plane wave for final electron–positronium state. Results for photodetachment cross section in ECSCP are reported for the screening parameter in the range [0.0,0.35] (in \({a_0^{-1}}\)). Our results for the unscreened case agree nicely with some of the most accurate results available in the literature. Furthermore we make a comparative study of the photodetachment of Ps − in ECSCP with the photodetachment of Ps − in screened Coulomb potential. To the best of our knowledge, such a study on the photodetachment of Ps − in ECSCP is carried out for the first time in the literature.
TL;DR: In this article, the effect of ion irradiation and etching on the microstructure of polyethyleneterephthalate (PET) membrane has been studied using positron annihilation spectroscopy.
TL;DR: In this article, isothermal transformations of supercooled 1-butanol were observed by positron lifetime spectroscopy, showing that the transition to a new (meta)stable structure, in which the ortho -positronium lifetime and intensity reach values that are intermediate between liquid and crystal phases, is discussed.
01 Jan 2010
TL;DR: The positronium formation cross-sections for Xe, CO2 and N-2 have been measured using coincidences between gamma-rays from POS self-annihilation and the resultant ion as mentioned in this paper.
Abstract: The positronium formation cross-sections for Xe, CO2 and N-2 have been measured using coincidences between gamma-rays from positronium self-annihilation and the resultant ion. In the case of Xe, there is excellent agreement with previous experimental determinations. For CO2 there is broad agreement in magnitude with previous measurements in contrast with N-2 where good shape agreement at low energies (<40 eV) is found though the magnitude of the present cross-section is significantly higher.
TL;DR: In this article, a gyrotron, a novel sub-THz light source is adopted with a Fabry-Perot cavity with high finesse and an efficient transportation system in order to obtain sufficient radiation power at 203 GHz.
Abstract: Positronium is an ideal system for the research of Quantum Electrodynamics (QED), especially for QED in bound state. The discrepancy of 3.9σ was found recently between the measured HFS values and the QED prediction of O(α3). It might be due to the contribution of unknown new physics or systematic problems in the all previous measurements. We propose a new method to measure HFS directly and precisely. A gyrotron, a novel sub-THz light source is adopted with a Fabry-Perot cavity with high finesse and an efficient transportation system in order to obtain sufficient radiation power at 203 GHz. The present status of the optimization studies and the current design of the experiment are described.