Showing papers on "Positronium published in 2011"

Determination of Free-Volume Properties in Polymers Without Orthopositronium Components in Positron Annihilation Lifetime Spectroscopy

Abstract: Existing positron annihilation lifetime spectroscopy (PALS) uses the orthopositronium components, lifetimes, and intensities observed in molecular substrates, such as in polymers, to determine free-volume properties based on an infinitive potential spherical model originally proposed by Tao in 1972. However, in many molecular systems, positronium is either quenched or inhibited by interacting with chemical functional groups and leads to no or nearly no orthopositronium component in PALS. In this Article, a newly modified equation is developed by following the Tao’s quantum model using the positron component (not orthopositronium) of PALS. This modified equation is examined by fitting free-volume results obtained from o-Ps lifetimes with the positron lifetimes in pressure- and temperature-dependent data in polymers and calibrated with known or calculated cavity sizes in zeolite materials. A newly modified positron lifetime-free volume correlation equation is established for the determination of free volume...

Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL revisited: annihilation in a cooling ISM?

Abstract: We analyse SPI/INTEGRAL data on the 511-keV line from the Galactic Centre, accumulated over ∼6 yr of observations. We decompose the X-ray and soft gamma-ray emission of the central part of the Milky Way into relatively compact ‘bulge’ and more extended ‘disc’ components and report their spectral properties. The bulge component shows a prominent 511-keV line and essentially no flux at 1.8 MeV, while the disc component on the contrary contains a prominent 1.8-MeV line and a very weak annihilation line. We show that the spectral shape of the annihilation radiation (the narrow 511-keV line and the associated ortho-positronium continuum) is surprisingly well described by a model of annihilation of hot positrons in a radiatively cooling interstellar medium (ISM). The model assumes that positrons are initially injected into a hot (∼10 6 K), volume-filling ISM, which is allowed to freely cool via radiative losses. The annihilation time in such a medium is longer than the cooling time for temperatures higher than a few 10 4 K. Thus, most of the positrons annihilate only after the gas has cooled down to ∼10 5 K, giving rise to annihilation emission

On the emergence of molecular structure

Abstract: The structure of {a(+/-),a(+/-),b(+/-)}-type Coulombic systems is characterized by the effective ground-state density of the a-type particles, computed via nonrelativistic quantum mechanics without introduction of the Born-Oppenheimer approximation. A structural transition is observed when varying the relative mass of the a- and b-type particles, e.g., between atomic H- and molecular H-2(+). The particle-density profile indicates a molecular-type behavior for the positronium ion, Ps(-).

Positron Scattering From neon and argon

Abstract: High-resolution measurements of positron interactions with Ne and Ar are presented, as well as theoretical treatments. The data extend over a range of 0.3 to 60 eV and comprise measurements of the grand total, positonium formation, and grand total minus positronium formation cross sections. Theoretical treatments of scattering from Ne and Ar are performed under the relativistic optical potential approach, as well as calculations using the convergent close-coupling method. Comparisons of the present measurements and theories are made with previous theoretical and experimental work.

Positronium signature in organic liquid scintillators for neutrino experiments

Abstract: Electron antineutrinos are commonly detected in liquid scintillator experiments via inverse $\ensuremath{\beta}$ decay by looking at the coincidence between the reaction products: neutrons and positrons. Prior to positron annihilation, an electron-positron pair may form an orthopositronium (o-Ps) state, with a mean lifetime of a few nanoseconds. Even if the o-Ps decay is speeded up by spin-flip or pick-off effects, it may introduce distortions in the photon emission time distribution, crucial for position reconstruction and pulse shape discrimination algorithms in antineutrino experiments. Reversing the problem, the o-Ps-induced time distortion represents a new signature for tagging antineutrinos in liquid scintillator. In this article, we report the results of measurements of the o-Ps formation probability and lifetime for the most used solvents for organic liquid scintillators in neutrino physics (pseudocumene, linear alkyl benzene, phenylxylylethane, and dodecane). We characterize also a mixture of pseudocumene $+$1.5 g/l of 2,5-diphenyloxazole, a fluor acting as wavelength shifter. In the second part of the article, we demonstrate that the o-Ps-induced distortion of the scintillation photon emission time distributions represent an optimal signature for tagging positrons on an event by event basis, potentially enhancing the antineutrino detection.

Photodetachment of Positronium Negative Ions

Abstract: Photodetachment of the positronium negative ion, a bound state of one positron and two electrons, has been observed. Development of a method to produce the ions efficiently using a Na coated tungsten surface has enabled the first observation of the photodetachment. The obtained lower limit of the photodetachment cross section for the wavelength of 1064 nm is consistent with the theoretical calculations reported so far. The experimental field developed in the present work gives new opportunities to explore the quantum mechanical three-body problem and to develop energy-tunable positronium beams.

Efficient two-step Positronium laser excitation to Rydberg levels

Abstract: Antihydrogen production by charge exchange reaction between Positronium atoms and antiprotons requires efficient excitation of Positronium atoms up to high-n levels (Rydberg levels). A two-step optical excitation, the first from ground to n = 3 and the second from this level to a Rydberg level, is proposed and a suitable laser system is discussed. The requirements on the energy and bandwidth of the excitation laser suggest the use of optical parametric generation technology for both wavelengths. The laser system is composed by two subsystems: one for the generation of 205 nm radiation and the other for the generation of 1670 nm radiation. We have separately developed and tested the laser sources and results are here presented.

Multiterm solution of a generalized Boltzmann kinetic equation for electron and positron transport in structured and soft condensed matter.

Abstract: In this paper, we generalize the semiclassical Boltzmann kinetic equation for dilute gases to consider highly nonequilibrium electrons and positrons in soft condensed matter, accounting rigorously for all types of interactions, including positronium formation, and allowing for both coherent and incoherent scattering processes The limitations inherent in the seminal paper of Cohen and Lekner [M H Cohen and J Lekner, Phys Rev 158, 305 (1967); Y Sakai, J Phys D 40, R441 (2007)] are avoided by solving the kinetic equation using a "multiterm" spherical harmonic representation of the velocity distribution function, as well as formulating a necessarily nonperturbative treatment of nonconservative collisional processes such as positronium formation Numerical calculations of transport properties are carried out for a Percus-Yevick model of a hard-sphere system, and for positrons in liquid argon New phenomena are predicted, including structure-induced negative conductivity and anisotropic diffusion

Positronium formation in Debye plasma

Abstract: Positronium (Ps) formation during positron-hydrogen collisions in Debye plasma has been studied in detail by using the second-order distorted-wave approximation with the inclusion of the adiabatic dipole polarization potential. Reliable results have been reported at several incident positron energies in the range from the Ps formation threshold to 500 eV for different values of the Debye screening parameter μ. Resonances for S-, P- and D-wave partial Ps formation cross sections appear to exist for 0.03 ≤ μ ≤ 0.3. Interesting structures in the differential cross sections are displayed in the surface plots. It is reported here for the first time that there is a huge probability of Ps formation in dense plasma corresponding to Debye screening parameter μ ≥ 0.3.

Positron collisions with molecular hydrogen: cross sections and annihilation parameters calculated using the R-matrix with pseudo-states method

Abstract: The molecular R-matrix with pseudo-states (MRMPS) method is employed to study positron collisions with H2. The calculations employ pseudo-continuum orbital sets containing up to h (l = 5) functions. Use of these high l functions is found to give converged eigenphase sums. Below the positronium formation threshold, the calculated cross sections agree with other high-accuracy theories and generally with the measurements. Calculation of the positron annihilation parameter Zeff with the MRMPS wavefunctions gives values significantly higher than other R-matrix wavefunctions but still do not completely converge with h functions. Extrapolation to higher l-values leads to a predicted value of Zeff for H2 of about 10.4. The MRMPS method is both completely general and ab initio; it can therefore be applied to positron collisions with other molecular targets.

Low-energy positron interactions with xenon

Abstract: Low-energy interactions of positrons with xenon have been studied both experimentally and theoretically. The experimental measurements were carried out using a trap-based positron beam with an energy resolution of ?80?meV, while the theoretical calculations were carried out using the convergent close-coupling method and the relativistic optical potential approach. Absolute values of the grand total, positronium formation and grand total minus positronium formation cross sections are presented over the energy range of 1?60?eV. Elastic differential cross sections (DCS), for selected energies, are also presented both below and above the positronium formation threshold. Fine energy-step measurements of the positronium formation cross section over the energy range of 4.4?8.4?eV, and measurements of the elastic DCS at the energies of 5.33 and 6.64?eV, have been carried out to investigate the ionization threshold regions corresponding to the 2P3/2 and 2P1/2 states of the Xe+ ion. The present results are compared with both experimental and theoretical values from the literature where available.

Positronium formation via excitonlike states on Si and Ge surfaces

Abstract: Recent experiments combining lifetime and laser spectroscopy of positronium (Ps) show that these atoms are emitted from p-Si(100) at a rate that depends on the sample temperature, suggesting a thermal activation process, but with an energy that does not, precluding direct thermal activation as the emission mechanism. Moreover, the amount of Ps emitted is substantially increased if the target is irradiated with 532 nm laser light just prior to the implantation of the positrons. Our interpretation of these data was that the Ps was emitted via an excitonlike positron-electron surface state, not dissimilar to an electronic surface exciton observed on Si in two-photon photoemission measurements. The hypothesis that this state may be populated by electrons from one of the occupied electronic surface states, either thermally or by laser excitation, is consistent with our observations and suggests that one should expect a high Ps yield at room temperature from an n-doped Si(100) sample, since in this case the same surface states will already be occupied. Here we present data obtained with an n-Si(100) target that supports our model, and also reveals the unexpected result that the kinetic energy of Ps emitted from this material actually decreases when it is heated, an effect we attribute to shifts in the surface energy levels due to the presence of a high density of thermally generated electrons. We show data obtained with a Ge(100) sample that corroborate the idea that the effects we observe are related to surface electron states, and hence should occur for any indirect-band-gap semiconductor with dangling-bond states. Our model is further confirmed by the observation that the Ps yield from p-Si depends linearly on the surface density of the implanted positrons due to the effects of electron-hole pairs created as the positrons slow down. © 2011 American Physical Society.

Porosity and crystallization of water ice films studied by positron and positronium annihilation

Abstract: The growth, evolution, and annealing of closed and interconnected pores in amorphous and crystalline films of water ice grown under a range of conditions have been studied by variable-energy positron annihilation spectroscopy (VEPAS). By measuring positron and positronium-related annihilation parameters as a function of time and film temperature it is shown that VEPAS has the capacity to yield a wealth of depth-dependent information on film morphology, with particularly promising results on the evolution and annealing of subnanometer closed pores---inaccessible to other techniques---and the study of the energetics of structural and phase changes.

Photoemission of positronium from Si.

Abstract: We have observed that the amount of positronium (Ps) emitted from the surface of p-Si(100) is substantially increased if the sample is irradiated with 532 nm laser light just prior to the implantation of positrons. The energy of the emitted Ps has a constant value of ∼0.16eV and is independent of the Si temperature and the applied laser fluence, while the photoemission yield depends on both of these parameters. These observations are consistent with Ps production via a previously observed excitonlike positron surface state that is populated in response to the production of electron-hole pairs in the Si. Possible applications of Ps photoemission include probing surface electron dynamics on Si, the generation of ultrashort Ps or positron pulses using ps lasers, and efficient production of Ps in cryogenic environments. © 2011 American Physical Society.

Binding-energy predictions of positrons and atoms

Abstract: As yet there is no experimental determination of the binding energy of a positron to any neutral atom. However, quantum calculations have given positron binding energies for 12 atoms (including positronium) and have shown that 14 atoms do not bind a positron. We find that the known binding energies can be fitted to a simple expression involving only the polarizabilities, ionization potentials, and the numbers of valence s electrons of the atoms, and we use this relationship to predict positron-atom binding energies for other atoms. Positronium-atom binding is not treated here.

Numerical Modeling of Thermalization of Positrons in Gas-Filled Surko Traps

Abstract: In this paper, we present the results of our Monte Carlo-based numerical simulation of a Penning-Malmberg-Surko positron trap. The results of simulations show the effect that various processes (such as positronium (Ps) formation, annihilation, losses on walls, etc.) have on trapping efficiency. The thermalization profile is shown, along with the evolution of the energy distribution that morphs from a particle beam to a broad swarm-type distribution.

Laser excitation of positronium in the Paschen-Back regime.

Abstract: Zeeman mixing of singlet and triplet $2P$ states of positronium (Ps) atoms, followed by decay back to the ground state, can effectively turn a long-lived triplet atom into a short-lived singlet state, which would seem to preclude laser cooling of Ps in a magnetic field. Here we report experiments which show that, in fact, because of the large splitting of the $n=2$ states in a high magnetic field (the Paschen-Back regime), the amount of such mixing diminishes approximately exponentially with an increasing magnetic field $g0.01\text{ }\text{ }\mathrm{T}$ and is essentially eliminated above $\ensuremath{\sim}2\text{ }\text{ }\mathrm{T}$. Thus, laser cooling of Ps should be feasible at high fields, which will facilitate the production of a Ps Bose-Einstein condensate.

Ground-state energy and relativistic corrections for positronium hydride

Abstract: Variational calculations of the ground state of positronium hydride (HPs) are reported, including various expectation values, electron-positron annihilation rates, and leading relativistic corrections to the total and dissociation energies. The calculations have been performed using a basis set of 4000 thoroughly optimized explicitly correlated Gaussian basis functions. The relative accuracy of the variational energy upper bound is estimated to be of the order of 2$\ifmmode\times\else\texttimes\fi{}$10${}^{\ensuremath{-}10}$, which is a significant improvement over previous nonrelativistic results.

Inner-shell electron capture by impact of positron on the multi-electron atomic targets

Abstract: The first-order Coulomb-Born approximation has been applied to the study of positronium formation through K-shell electron capture in the collision of positron with multi-electron atomic targets. The single-zeta Roothaan-Hartree-Fock wave functions are used to describe the electron initial bound states. The differential and total cross sections are computed for the impact of positron on helium, carbon, neon, sodium and argon atoms, with the formation of positronium in its ground state. For helium atoms, the calculated total cross sections are compared with the available experimental data and other theoretical calculations. The comparison shows a good agreement between the present calculations and the measurements.

Advances towards a new measurement of the 1S-2S transition of positronium

Abstract: In this paper, a new experiment is presented to measure the 1S–2S transition of positronium, the bound state of an electron and a positron. The goal is to improve the current accuracy by a factor of 5 to reach a precision of the order of 0.6 ppb, to check recent QED calculations. This accuracy is challenging, but it seems well within reach in view of the technological advances that have occurred during the last two decades. We will present the details of the experimental set-up, the advances in the production of positronium, the developments of the laser system, and as well our new experimental technique for the detection of Ps in the 2S state.

Physics with many positrons

Abstract: These notes contain speculations about interesting situations that might occur when two or more low-energy positrons interact with each other and/or with various forms of ordinary matter. Topics include many positrons compressed to high density at a field emission tip, a long-lived metastable cold neutral electron positron plasma in a box, dipositronium and other multipositron molecules, the positronium Bose-Einstein condensate, precision measurements on positronium cooled by a pulsed laser method, stimulated emission of annihilation radiation, head-on collisions of two positronium annihilation gamma-ray laser pulses, and possible uses for gamma-ray lasers.

Convergent close-coupling calculations of positron-magnesium scattering

Abstract: The single-center convergent close-coupling method has been applied to positron-magnesium scattering at incident energies from 0.01 to 100 eV. Cross sections are presented for elastic scattering and excitation of $3 {}^{1}P$, as well as for the total ionization and total scattering processes. We also provide an estimate of the positronium formation cross section. The results agree very well with the measurements of the total cross section by Stein et al. [Nucl. Instrum. Methods Phys. Res. Sect. B 143, 68 (1998)], and consistent with the positronium formation measurements of Surdutovich et al. [Phys. Rev. A 68, 022709 (2003)] for positron energies above the ionization threshold. For energies below the positronium formation threshold (0.8 eV) we find a large $P$-wave resonance at 0.17 eV. A similar resonance behavior was found by Mitroy and Bromley [Phys. Rev. Lett. 98, 173001 (2007)] at an energy of 0.1 eV.

Positronium formation for positron scattering from hydrogen: Maximum positions and scaling law

Abstract: Ground- and excited-state positronium (Ps) formation from the hydrogen atom by positron impact at low and intermediate energies have been studied within the framework of two-center two-channel eikonal final state-continuum initial distorted wave model. A general method for obtaining Ps formation into arbitrary final states has been derived. The present results of Ps (1$s$), Ps $(n=2),$ and total Ps formation cross sections agree very well with previous close-coupling calculations and experimental measurements. The maximum positions of Ps($n$) formation cross sections are found to be in good agreement with the prediction of wave vector matching model introduced by Charlton [J. Phys. B: At. Mol. Opt. Phys. 39, 4575 (2006)]. We also present a scaling law for Ps ($n$) formation cross sections that is valid for all $n$ in the entire energy range.

Compton scattering from positronium and validity of the impulse approximation

Abstract: The cross sections for Compton scattering from positronium are calculated in the range from 1 to 100 keV incident photon energy. The calculations are based on the ${A}^{2}$ term of the photon-electron or photon-positron interaction. Unlike in hydrogen, the scattering occurs from two centers and the interference effect plays an important role for energies below 8 keV. Because of the interference, the criterion for validity of the impulse approximation for positronium is more restrictive compared to that for hydrogen.

Going from classical to quantum description of bound charged particles II: Implications for the atomic physics

Abstract: This paper is the continuation of the analysis of bound quantum systems started in part I (A.L. Kholmetskii, T. Yarman and O.V. Missevitch, Going from classical to quantum description of bound charged particles. I: Basic concepts and assertions), which is based on a novel approach to the transition from classical to quantum description of electrically bound charges, involving the requirement of energy-momentum conservation for the bound electromagnetic (EM) field, when the EM radiation is forbidden. It has been shown that the modified expression for the energy levels of hydrogenic atoms within such a pure bound field theory (PBFT) provides the same gross and fine structure of energy levels, like in the standard theory. At the same time, at the scale of hyperfine interactions, our approach, in general, does evoke some important corrections to the energy levels. Part of such corrections, like the spin-spin splitting in the hydrogen atom, is less than the present theoretical/experimental uncertainties in the evaluation of hyperfine contributions into the atomic levels. But the most interesting result is the appearance of a number of significant corrections (the 1S -2S interval and 1S spin-spin interval in positronium, 1S and 2S -2P Lamb shift in light hydrogenic atoms), which improve considerably the convergence between theoretical predictions and experimental results. In particular, the corrected 1S -2S interval and 1S spin-spin splitting in positronium practically eliminate the existing up-to-date discrepancy between theoretical and experimental data. The re-estimated classic 2S -2P Lamb shift as well as ground-state Lamb shift in the hydrogen atom lead to the proton charge radius r p = 0.841(6) fm (from 2S -2P Lamb shift), and r p = 0.846(22) fm (from 1S Lamb shift), which perfectly agrees with the latest estimation of proton size via the measurement of 2S -2P Lamb shift in muonic hydrogen, i.e. r p = 0.84184(67) fm. Finally, we consider the decay of bound muons in meso-atoms and achieve a quantitative agreement between experimental data and the results obtained through our approach.

Solving the Charging Effect in Insulating Materials Probed by a Variable Monoenergy Slow Positron Beam

Abstract: A variable monoenergy slow positron beam (VMSPB) operating at a high vacuum on insulating materials encounters a problem of significant surface charging effect with time. As a result, positronium formation is inhibited, and the positron annihilation radiation counting rate is reduced; these consequently distorted the experimental positron annihilation and results. To solve such problems, a technique of depositing an ultrathin layer of sputtering noble metals on insulators is developed. We report a successful method of sputtering a few atomic layers of platinum (∼1 nm) on a polyamide membrane to completely remove the charging effect for VMSPB applications in insulators.

Total and ionization cross-sections of N2 and CO by positron impact: Theoretical investigations

Abstract: In this paper we report our calculations on several important total cross-sections (TCSs) of positron impact on isoelectronic N2 and CO molecules, treated in the complex spherical potential formalism. Basically the total (complete) cross-section Q T consists of elastic and inelastic contributions. Our total inelastic cross-section (Q inel) contains ionization and electronic excitations together with positronium formation. Our goal here is to bifurcate Q inel further to deduce total ionization cross-section, using the ‘complex scattering potential–ionization contribution’ (CSP-ic) method of electron–atom/molecule scattering. The present range of positron energy is 15–2000 eV. All the resulting cross-sections are in a good general accord with the existing data. This work highlights the importance of various scattering channels in e + -N2 and e + -CO interactions at intermediate and high energies.