Showing papers on "Positronium published in 2003"

Principles and Applications of Positron and Positronium Chemistry

Abstract: Introduction to positron and positronium chemistry, YC Jean et al compounds of positrons and positronium, DM Schrader experimental techniques in positron spectroscopy, PG Coleman organic and inorganic chemistry of the positron and positronium, G Duplatre and I Billard physical and radiation chemistry of the positron and positronium, SV Stephanov and VM Byakov positrons and positronium in the gas phase, DM Schrader positron porosimetry, MH Weber and KG Lynn positron annihilation studies on superconducting materials, CS Sundar positronium in Si and SiO2 thin films, R Suzuki applications to polymers, PE Mallon applications to coatings and paints, YC Jean et al positron annihilation electrospectroscopy, S Amdani et al characterization of nanoparticle and nanopore materials, J Xu age momentum correlation (AMOC), H Stoll et al

High-Resolution Observation of the Solar Positron-Electron Annihilation Line

Abstract: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed the positron-electron annihilation line at 511 keV produced during the 2002 July 23 solar flare. The shape of the line is consistent with annihilation in two vastly different solar environments. It can be produced by formation of positronium by charge exchange in flight with hydrogen in a quiet solar atmosphere at a temperature of ~6000 K. However, the measured upper limit to the 3γ/2γ ratio (ratio of annihilation photons in the positronium continuum to the number in the line) is only marginally consistent with what is calculated for this environment. The annihilation line can also be fitted by a thermal Gaussian having a width of 8.1 ± 1.1 keV (FWHM), indicating temperatures of ~(4-7) × 105 K. The measured 3γ/2γ ratio does not constrain the density when the annihilation takes place in such an ionized medium, although the density must be high enough to slow down the positrons. This would require the formation of a substantial mass of atmosphere at transition-region temperatures during the flare.

Direct measurement of the parapositronium lifetime in alpha-SiO2.

Abstract: The mean lifetime of delocalized parapositronium in alpha-SiO2 has been determined directly for the first time using a newly developed positron lifetime spectrometer based on BaF2 scintillators and a fast digital oscilloscope. The lifetime is found to be 156+/-4 ps, which is much longer than its intrinsic lifetime of 125 ps. This indicates clearly that the primary many-body effect on positronium in alpha-SiO2 is the screening of the Coulomb interaction between the constituent particles by electrons of the medium and that the average distance between the electron and positron in positronium becomes larger than its vacuum value.

van der Waals coefficients for positronium-atom interactions

Abstract: The van der Waals coefficients for positronium interactions with a number of rare gases (He, Ne, Ar, Kr, and Xe) and alkali-metal atoms (Li, Na, K, and Rb) are estimated using a variety of ab initio and semiempirical methods. Dispersion coefficients are also presented for atomic hydrogen and a number of rare-gas and alkali-metal atoms for validation purposes.

Doppler-broadening measurements of positronium thermalization in gases

Abstract: The formation and subsequent thermalization of positronium (Ps) produced at a few eV in gases are investigated using time-resolved Doppler-broadening measurements of the annihilation photons. A static magnetic field quenches the Ps enabling Doppler energy measurements from 25 to 70 ns after the Ps is formed. Varying the gas density permits a significant range of the thermalization process to be observed. Seven different gases are studied, He, Ne, Ar, H{sub 2}, N{sub 2}, isobutane, and neopentane. A classical elastic scattering model fits all the gas data reasonably well. For each gas, an elastic scattering cross section and an average Ps formation energy are determined from the classical model fit. When comparisons can be made, these cross sections are often significantly smaller than most quantum-mechanical-theory predictions and most previous experimental results obtained using the angular correlation technique. Various systematic tests have been applied to the apparatus and the analysis, reinforcing the discrepancy with previous works.

Search for CPT-odd decays of positronium.

Abstract: We have limited a CPT-violating correlation in annihilationsof polarized ortho-positronium. We searched for an asymmetry in thetriple correlations dot k1 cross k2, where k1 and k2 are the two largestphoton momenta, and s is the spin of the positronium. Using theGammasphere array of Compton-suppressed high-purity germanium detectors,we detected 2.65e7 events of ortho-Ps annihila tion. The amplitude of aCPT-violating asymmetry in the data set is found to be 0.0026 plus orminus 0.0031, a factor of 6 smaller than previousexperiments.

Low-energy positron dynamics in small hydrocarbon gases

Abstract: In the present paper we report a set of parameter-free calculations for the scattering of low-energy positrons off gaseous hydrocarbon molecules like C2H2, C2H4, C2H6 and C6H6 The calculations of the corresponding elastic integral cross sections have been carried out within the framework of the symmetry-adapted, single-centre-expansion approach to describe all the bound molecular electrons and the scattered positron, at energies below the thresholds for positronium formation The corresponding multichannel coupled equations have been solved using a Volterra-type set of integral equations The computed cross sections are found to be fairly close to the existing experimental data for all the molecules examined in our work, both in shape and value We have also computed the positron annihilation coefficients, Zeff (T), at the experimentally observed temperature of 300 K and the effect of the breathing vibration in computing the rates of annihilation for different molecular geometries is examined for the case of gaseous C2H2 The present results, among the first non-empirical calculations for polyatomic gases, confirm our earlier findings (Gianturco et al 2001 Phys Rev A 64 32715; Gianturco and Mukherjee 1997 Phys Rev A 56 3638; 2001 Phys Rev A 64 024703) that the Zeff values in such gases are crucially dependent on the inclusion of dynamical coupling between the positron motion and the nuclear vibrations during the annihilation event

Positronium time-of-flight measurements of porous low-k films

Abstract: We demonstrate that positronium (Ps) time-of-flight (TOF) spectroscopy provides a powerful technique for probing the structure of porous low-dielectric constant (low-k) films. The emission of ortho-positronium (o-Ps) from the surface of a series of nano-porous silsesquioxane films was studied with a newly developed TOF spectrometer. Clear Ps emission peaks were observed from the two high porosity films (F38 and KI31), indicating their higher open porosity in comparison with other films. We also observed that the energy of o-Ps emission from F38 is much higher than that from KI31. It was concluded that the open pores in the former film are less tortuous, and o-Ps loses significantly less energy emerging from its surface than from that of latter film.

Fragmentation of positronium in collision with He atoms: A classical theoretical approach

Abstract: The classical trajectory Monte Carlo method was applied to the description of the fragmentation of the positronium (Ps) in collision with He atoms. The collision system was simplified to a three-body system consisting of the electron and the positron of the Ps, as well as the He atom that was considered as a structureless particle. The interaction of the ${e}^{\ensuremath{-}}$ and ${e}^{+}$ with the He was approximated by a static, fully screened Coulomb potential. The calculations were carried out for collision energies 13, 18, 25, and 33 eV. The obtained total break-up cross sections and the longitudinal energy distributions of the emitted positrons were compared with the recent experimental results of Armitage et al. [Phys. Rev. Lett. $89,$ 173402 (2002)]. The present theory overestimates the measured cross sections by a factor of 1.6\char21{}2.5, but it correctly reproduces the peak found by Armitage et al. in the positron spectra at about half of the residual Ps energy.

Spin-orbit quenching of positronium during atomic collisions.

Abstract: When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is ortho-Ps-->para-Ps conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.

Momentum-transfer cross sections for slow positronium gas collisions

Abstract: The momentum-transfer cross sections for positronium (Ps)–gas molecule scattering in the low energy region are determined by analysing the thermalization process of ortho-positronium (o-Ps) in various gases The momentum distributions of Ps in Ne, H2, CH4, N2, C2H4, n-C4H10, i-C5H12 and neopentane are measured as functions of mean lifetime, by using the one-dimensional angular correlation of annihilation radiation method The average energies obtained for o-Ps with mean lifetimes of 3–90 ns are analysed, to yield the momentum-transfer cross sections for the Ps–gas molecule scattering The cross sections thus determined are σm = (20 ± 8) × 10−16 cm2 for Ne, (16 ± 3) × 10−16 cm2 for H2, (17 ± 6) × 10−16 cm2 for CH4, (37 ± 10) × 10−16 cm2 for N2, (35 ± 8) × 10−16 cm2 for C2H4, (100 ± 30) × 10−16 cm2 for n-C4H10, (110 ± 50) × 10−16 cm2 for i-C5H12 and (180 ± 60) × 10−16 cm2 for neopentane, in the Ps energy range below 03 eV The Ps is thermalized mainly through elastic scattering with diatomic and polyatomic molecules, as in the case of scattering with inert gases

Positronium formation in poly(methyl methacrylate)

Abstract: Positron annihilation age momentum correlation (AMOC) experiments were performed in linear poly(methyl methacrylate), PMMA, at 420 K and 100 K. The lifetime and Doppler broadened line-shape $S(t)$ data were analyzed with Byakov and Stepanov's blob model for the formation of positronium. It is shown that the analysis is consistent with a delayed formation of positronium and also offers an alternative explanation for the observed broadening of the annihilation radiation line shape at short times (a few tens of picuseconds) after positron injection, the so called young age broadening.

Measurements of positronium formation cross sections in positron-Mg collisions

Abstract: We report measurements of positronium (Ps) formation cross sections for 0.1\char21{}60 eV positrons scattered by Mg atoms. There is reasonable agreement between the measured cross sections and recent calculations of those values, both of which indicate an unusually steep increase of the Ps formation cross section from zero to a large peak value as the positron energy is increased from the Ps formation threshold to less than 1 eV above that threshold.

Variational calculations of s-wave positronium–hydrogen scattering

Abstract: A detailed investigation is made of low energy positronium–hydrogen s-wave elastic scattering, using the Kohn variational method, with very flexible scattering wavefunctions including all inter-particle correlations. The singlet and triplet scattering lengths are calculated and found to be a+ = 4.311a 0 and a− = 2.126a 0 respectively. The phase shifts are believed to be within less than 0.5% of the exact values, and they are in good agreement with the results of previous calculations.

Compact boundary-condition-determined wave function for positronium hydride (PsH)

Abstract: A simple, compact, and accurate wave function for positronium hydride is written as a product of Pade’ approximants for electron–nucleus interactions and of Jastrow functions for electron–electron interactions. Most of the parameters are fixed taking into account both the correct cusp conditions when two particles collide and the correct asymptotic behavior when one or two particles go to infinity. The remaining parameters were optimized by variational Monte Carlo calculations. The energy of this single term wave function is −0.786073(6) hartree and favorably compares with very long configuration interaction expansions and even with explicitly correlated function expansions. The exam of the wave function and of various two-dimensional distribution functions shows that the PsH structure is similar to the hydrogen anion structure, with the positron slightly perturbing it and its motion strongly correlated to the electrons that are squeezed towards each other and towards the nucleus.

High Order QED Corrections in Physics of Positronium

Abstract: High-order perturbative corrections to positronium decays and hyperfine splitting are briefly reviewed. Theoretical predictions are compared to the most recent experimental data. Perspectives of future calculations are discussed.

Reply to Comment on ‘Positronium scattering by Ne, Ar, Kr and Xe in the frozen target approximation’

Abstract: A numerical inaccuracy in the static-exchange calculation of Blackwood et al (2002 J. Phys. B: At. Mol. Opt. Phys. 35 2661) for Ps(1s)–Xe scattering is corrected. The corrected result now conforms to the same pattern as He, Ne, Ar and Kr.