Showing papers on "Positronium published in 2004"
TL;DR: The electron-positron annihilation spectrum observed by SPI/INTEGRAL during deep Galactic Center region exposure is reported in this paper, which provides the most stringent constraints on the line energy and width.
Abstract: The electron-positron annihilation spectrum observed by SPI/INTEGRAL during deep Galactic Center region exposure is reported. The line energy (510.954$\pm$0.075 keV) is consistent with the unshifted annihilation line. The width of the annihilation line is 2.37$\pm$0.25 keV (FWHM), while the strength of the ortho-positronium continuum suggests that the dominant fraction of positrons (94$\pm$6%) form positronium before annihilation. Compared to the previous missions these deep INTEGRAL observations provide the most stringent constraints on the line energy and width.
Under the assumption of an annihilation in a single-phase medium these spectral parameters can be explained by a warm $T_e\sim 7000-4~10^4$ K gas with the degree of ionization larger than a few $10^{-2}$. One of the wide-spread ISM phases - warm ($T_e \sim 8000$ K) and weakly ionized (degree of ionization $\sim$ 0.1) medium satisfies these criteria. Other single-phase solutions are also formally allowed by the data (e.g. cold, but substantially ionized ISM), but such solutions are believed to be astrophysically unimportant.
The observed spectrum can also be explained by the annihilation in a multi-phase ISM. The fraction of positrons annihilating in a very hot ($T_e \ge 10^6$ K) phase is constrained to be less than $\sim$8%. Neither a moderately hot ($T_e \ge 10^5$ K) ionized medium nor a very cold ($T_e \le 10^3$ K) neutral medium can make a dominant contribution to the observed annihilation spectrum. However, a combination of cold/neutral, warm/neutral and warm/ionized phases in comparable proportions could also be consistent with the data.
119 citations
TL;DR: A brief overview of precision studies of positronium paying special attention to uncertainties of theory as well as comparison of theory and experiment can be found in this article, where the authors also consider in detail advantages and disadvantages of POS tests compared to other QED experiments.
Abstract: As an unstable light pure leptonic system, positronium is a very specific probe atom to test bound state QED. In contrast to ordinary QED for free leptons, the bound state QED theory is not so well understood and bound state approaches deserve highly accurate tests. We present a brief overview of precision studies of positronium paying special attention to uncertainties of theory as well as comparison of theory and experiment. We also consider in detail advantages and disadvantages of positronium tests compared to other QED experiments.
84 citations
TL;DR: In this paper, a many-body theory approach is developed for the problem of positron-atom scattering and annihilation, where strong electron-positron correlations are included nonperturbatively through the calculation of the electron-POSitron vertex function.
Abstract: A many-body theory approach is developed for the problem of positron-atom scattering and annihilation. Strong electron-positron correlations are included nonperturbatively through the calculation of the electron-positron vertex function. It corresponds to the sum of an infinite series of ladder diagrams, and describes the physical effect of virtual positronium formation. The vertex function is used to calculate the positron-atom correlation potential and nonlocal corrections to the electron-positron annihilation vertex. Numerically, we make use of $B$-spline basis sets, which ensures rapid convergence of the sums over intermediate states. We have also devised an extrapolation procedure that allows one to achieve convergence with respect to the number of intermediate-state orbital angular momenta included in the calculations. As a test, the present formalism is applied to positron scattering and annihilation on hydrogen, where it is exact. Our results agree with those of accurate variational calculations. We also examine in detail the properties of the large correlation corrections to the annihilation vertex.
56 citations
TL;DR: An annihilation gamma ray laser was proposed in this article, where a cylinder of high density cold singlet Ps annihilating into a coherent gamma ray burst directed along the axis of the cylinder.
Abstract: An annihilation gamma ray laser could be made by a cylinder of high density cold singlet Ps annihilating into a coherent gamma ray burst directed along the axis of the cylinder. Such a laser would have many important uses and prospects seem fair for making a 1J model in the immediate future. Higher intensity lasers that would be useful for controled fusion are envisioned, but involve so many orders of magnitude increase in our ability to produce and control antimatter that no reasonable statement about the possibilities can be made at this time. This paper describes our vision and we briefly report the present status of the experiments.
36 citations
TL;DR: In this paper, the dynamics and radiation of positronium were investigated in intense laser fields and two-body quantum mechanical treatment displays the tunneling, free-evolution, and recollision dynamics of electron and positron in both the oscillating laser electric and laser magnetic field components.
Abstract: The dynamics and radiation of positronium are investigated in intense laser fields. Our two-body quantum mechanical treatment displays the tunneling, free-evolution, and recollision dynamics of electron and positron in both the oscillating laser electric and laser magnetic field components. In spite of significant momentum transfer of the numerous incoming laser photons, recollisions of both particles are shown to occur automatically after tunneling ionization, along with substantial x-ray and gamma-ray emission during recombination and annihilation processes.
31 citations
TL;DR: In this paper, the positron lifetime measurements were interpreted in terms of a free-volume approach assuming that the free volume associated with each liquidlike cell of the amorphous phase is divided into free volume holes whose size distribution is given by a normal frequency function.
Abstract: Positron lifetime spectroscopy and wide-angle X-ray scattering have been applied to investigate the structural changes induced by tensile deformation in ultrahigh molecular weight linear polyethylene. The results reveal a correlation between the crystallinity, the positron annihilation characteristics, and the strain of the polymer. The probability of positronium formation increases and the crystallinity of the material decreases with increasing tensile strain. The size distribution of the free-volume holes, where positronium atoms form, is determined from their lifetime distribution. The positron lifetime measurements are interpreted in terms of a free-volume approach assuming that the free volume associated with each liquidlike cell of the amorphous phase is divided into free-volume holes whose size distribution is given by a normal frequency function. The cumulative distribution functions determined from the positron annihilation measurements agrees with those predicted by the free-volume model. This i...
29 citations
TL;DR: In this article, a procedure to analyse resonances in scattering and photoionization calculations is discussed, which is theoretically connected to a general relation between the trace of the time-delay matrix and the energy derivative of the eigenphase sum.
Abstract: A procedure to analyse resonances in scattering and photoionization calculations is discussed. It is theoretically connected to a general relation between the trace of the time-delay matrix and the energy derivative of the eigenphase sum. This procedure is especially useful in analysing resonances with strong background contributions, such as shape resonances lying just above the threshold of a new channel with an overcritical dipole field. In this case the background eigenphase sum diverges towards the threshold, and it can easily change the time delay due to an S-matrix pole into a negative time delay, or time advance. As an application, we analyse results of hyperspherical close-coupling calculations of S-, P-, D-, F-, G- and H-wave resonances in the positronium negative ion Ps− near the Ps(n = 2, 3) thresholds. We find examples of a strong background leading to an overall time advance and the dipole oscillation in the cross section above and near a threshold.
29 citations
01 Jul 2004-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, the authors used a 1/n3 scaling to estimate capture into states with n>3 and showed that the double peak and shoulder structures observed experimentally for Ps formation in Ar, Kr and Xe arise from formation in excited states.
Abstract: Distorted-wave Born approximation calculations for Ps formation in positron impact on He, Ne, Ar, Kr and Xe are reported for the energy range up to 200 eV. Capture into the n=1, 2 and 3 states of Ps is calculated explicitly and 1/n3 scaling is used to estimate capture into states with n>3. The calculations for the heavier noble gases allow for capture not only from the outer np6 shell of the atom but also from the first inner ns2 shell. However, the inner shell capture is found to be very small. Although by no means unambiguous, the calculations provide some support to the conjecture of Larrichia et al. [J. Phys. B 35 (2002) 2525] that the double peak and shoulder structures observed experimentally for Ps formation in Ar, Kr and Xe arise from formation in excited states.
29 citations
01 Jul 2004-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, the current status on measurements of total and positronium fragmentation cross-sections, as well as their comparison with theories, is summarised, and the experimental investigation of its interactions with simple atoms and molecules is reviewed.
Abstract: Progress in the production of a monoenergetic Ps beam and in the experimental investigation of its interactions with simple atoms and molecules is reviewed. The current status on measurements of total and positronium fragmentation cross-sections, as well as their comparison with theories, is summarised.
27 citations
TL;DR: In this article, the single-centred convergent close-coupling method has been applied to positron-helium scattering at energies from the first ionization threshold to 1 keV for calculating one-electron excitation and ionization processes.
Abstract: The single-centred convergent close-coupling method has been applied to positron-helium scattering at energies from the first ionization threshold to 1 keV for calculating one-electron excitation and ionization processes. Total scattering, positronium formation plus single ionization (unresolved), and target excitation cross sections have been calculated. It is found that the often-used frozen-core approximation for the target structure leads to a small but systematic overestimation of the cross sections. Generally, the agreement between the present and most recent calculations, as well as experiment, is satisfactory.
26 citations
TL;DR: In this article, the positron annihilation lifetime spectroscopy (PALS) was used in conjunction with transmission electron microscopy (TEM) for microstructural studies of silica gel powder.
Abstract: Microstructural studies of silica gel powder were carried out using positron annihilation lifetime spectroscopy (PALS) in conjunction with transmission electron microscopy (TEM). It is argued that the two distinct long-lived components found (labeled by τ3 and τ4) may be ascribed to ortho positronium annihilation in microcavities within the grains and intergranular mesoscopic pores, respectively. In the latter type of void, a significant fraction decays via the three-photon mode. A simple physical picture of positronium annihilation in the larger pores is put forward, while the situation vis-a-vis the smaller cavities is shown to be well described by a modification of the currently prevailing model for the pick-off process. The simple parametrization finally arrived at provides a sharpening of the use of the positron as a useful probe for microstructural study of porous substances. It is emphasized that two different positron annihilation mechanisms prevail in the microcavities and mesopores.
01 Jul 2004-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, the authors present recent data on positron annihilation below and above the threshold for positronium formation in atoms and molecules, using a trap-based beam with an energy resolution of 25 meV (FWHM).
Abstract: This paper presents recent data on positron annihilation below and above the threshold for positronium formation in atoms and molecules. Use of a trap-based beam allows experimentation down to 50 meV and up to tens of electron volts with an energy resolution of 25 meV (FWHM). Above positronium formation, results are presented for absolute cross-section measurements in argon. Below positronium formation, absolute Zeff spectrum in argon, xenon and the alkane molecules are presented. Data are compared to other experimental and theoretical results where available. The effect of fluorination of hydrocarbons on Zeff below the threshold for Ps formation as well as other outstanding questions are discussed. � 2004 Elsevier B.V. All rights reserved.
TL;DR: In this article, the binding energies of highly excited positronium atoms (Ps* ) created via charge exchange collisions were chosen by a laser. But the binding energy was not determined by the initial laser excitation.
01 Jul 2004-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, a coupled state framework was proposed for scattering by H(1s), He(1 1 S) and Li(2s) using pseudostates and allowing for excitation of both the Ps and the atom.
Abstract: New results are presented for Ps(1s) scattering by H(1s), He(1 1 S) and Li(2s). Calculations have been performed in a coupled state framework, usually employing pseudostates, and allowing for excitation of both the Ps and the atom. In the Ps(1s)–H(1s) calculations the H − formation channel has also been included using a highly accurate H − wave function. Resonances resulting from unstable states in which the positron orbits H − have been calculated and analysed. The new Ps(1s)–He(1 1 S) calculations still fail to resolve existing discrepancies between theory and experiment at very low energies. The possible importance of the Ps − formation channel in all three collision systems is discussed.
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09 Jul 2004TL;DR: In this article, a three-dimensional or two-dimensional photonic bandgap (PBG) structure containing at least one cavity (10) was proposed to capture and store electrically neutral excited positronium (Ps*) (16).
Abstract: A device is provided that can capture and store electrically neutral excited species (16) of antimatter or exotic matter (a mixture of antimatter and ordinary matter), in particular excited positronium (Ps*) (16). The antimatter trap comprises a three-dimensional or two-dimensional photonic bandgap (PBG) structure (14) containing at least one cavity (10) therein. The species (16) are stored in the cavity (10) or in an array (110) of cavities (10). The PBG structure (14) blocks premature annihilation of the excited species (16) by preventing decays to the ground state and by blocking the pickoff process. A Bose-Einstein Condensate form of Ps* (16) can be used to increase the storage density. The long lifetime and high storage density achievable in this device offer utility in several fields, including medicine, materials testing, rocket motors, high power/high energy density storage, gamma-ray lasers, and as an ignition device for initiating nuclear fusion reactions in power plant reactors or hybrid rocket propulsion systems.
TL;DR: In this article, the full-dimensional time-dependent Schrodinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method.
Abstract: The full-dimensional time-dependent Schrodinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method. Our approach combines the finite-difference and Lagrange mesh methods. The method is applied to calculate the quasienergies and ionization probabilities of atomic and molecular systems in intense static and dynamic electric fields. The gauge invariance and accuracy of the method is established. Applications to multiphoton ionization of positronium, the hydrogen atom and the hydrogen molecular ion are presented. At very high laser intensity, above the saturation threshold, we extend the method using a scaling technique to estimate the quasienergies of metastable states of the hydrogen molecular ion. The results are in good agreement with recent experiments.
TL;DR: The full-dimensional time-dependent Schrödinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method that combines the finite-difference and Lagrange mesh methods.
Abstract: The full-dimensional time-dependent Schrodinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method.
Our approach combines the finite-difference and Lagrange mesh methods. The method is applied to calculate the quasienergies and ionization probabilities of atomic and molecular systems in intense static and dynamic electric fields. The gauge invariance and accuracy of the method is established. Applications to multiphoton ionization of positronium and hydrogen atoms and molecules are presented. At very high intensity above saturation threshold, we extend the method using a scaling technique to estimate the quasienergies of metastable states of the hydrogen molecular ion. The results are in good agreement with recent experiments.
TL;DR: In this article, the effects of electron beam and gamma irradiation on microscopic structure of a conducting polystyrene (polystyrene + 15% carbon black) have been studied using positron annihilation lifetime spectroscopy.
TL;DR: In this article, the authors discuss and summarize some aspects concerning the study of the positronium system to probe new physics beyond the Standard Model, and discuss how to apply this knowledge beyond the standard model.
Abstract: We discuss and summarize some aspects concerning the study of the positronium system to probe new physics beyond the Standard Model.
TL;DR: In this paper, a complex equivalent local polarization potential is obtained to describe the rearrangement process and the results for the positronium (n=1) and PS (n = 2) formation cross sections are reported and compared with the experimental measurements and other close-coupling calculations.
Abstract: The optical-model approach has been used to investigate positronium (Ps) formation in e{sup +}-Na collision at low impact energies (0.1-20.0 eV). A complex equivalent local polarization potential is obtained to describe the rearrangement process. The Ps formation cross section is excellently reproduced by this potential. The results for the Ps (n=1) and Ps (n=2) formation cross sections are reported and compared with the experimental measurements and other close-coupling calculations.
TL;DR: In this paper, the authors computed the photon spectrum in orthopositronium to 3 gamma decay using effective field theory methods, which is compatible with Low's theorem on soft photon emission.
Abstract: The photon spectrum in orthopositronium to 3 gamma decay is computed using effective field theory methods. For energetic photons, the spectrum agrees with the Ore-Powell result, but deviates from it when the photon energy is comparable to the positronium binding energy. The decay spectrum in this region depends on a positronium structure function, which is computed in this paper. At still lower energies the photon spectrum is dominated by the parapositronium resonance contribution. Our results are compatible with Low's theorem on soft photon emission.
TL;DR: In this article, the annihilation of ortho-positronium in the α cavities of the hydrated and dehydrated Na-A-type zeolites were analyzed.
01 Jul 2004-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, the low-energy positronium-positronium (Ps-Ps) scattering parameters, s-wave scattering lengths and effective ranges for symmetric/antisymmetric channels, are evaluated on the basis of the semi-phenomenological Ps-Ps effective potentials and of the existence of the Ps molecule.
Abstract: The low-energy positronium–positronium (Ps–Ps) scattering parameters, s-wave scattering lengths and effective ranges for symmetric/antisymmetric channels, are evaluated on the basis of the semi-phenomenological Ps–Ps effective potentials and of the existence of the Ps molecule. The Ps–Ps scattering parameters obtained by recent more elaborate ab initio calculations are reviewed briefly, and shown to be consistent with those by effective potential approach. We also discuss the possible phases of Ps or electron–positron many-body states, and the effects of the Ps–Ps interactions for them. Especially, we estimate the interaction effects in Ps-Bose–Einstein condenstates (Ps-BEC) with the obtained Ps–Ps scattering lengths, and show that Ps-BEC is very weakly-interacting system.
TL;DR: In this paper, the authors discuss and summarize some aspects concerning the study of the positronium system to probe new physics beyond the Standard Model, and discuss how to apply this knowledge beyond the standard model.
Abstract: We discuss and summarize some aspects concerning the study of the positronium system to probe new physics beyond the Standard Model.
TL;DR: These ratios provide evidence that relates to the kinetic energy dependence of ortho-Ps interactions with an aluminum surface, the Ps formation potential at this surface, and the fact that Ps is being formed with inner orbital electrons for CO2 and O2.
Abstract: Positronium (Ps) produced by 4 to 40 eV positrons colliding with Ne, Ar, Kr, CO2, and O2 is investigated by measuring the ratio of signals of two gamma rays in coincidence resulting from (a) three gamma annihilation of ortho-Ps and (b) two annihilation gamma rays due to para-Ps decay and destruction of ortho-Ps at an aluminum scattering cell surface. These ratios provide evidence that relates to the kinetic energy dependence of ortho-Ps interactions with an aluminum surface, the Ps formation potential at this surface, and the fact that Ps is being formed with inner orbital electrons for CO2 and O2.
TL;DR: In this paper, Positronium (Ps) is simulated using Path Integral Monte Carlo (PIMC) to reproduce the results of previous simple theories in which a single quantum particle was used to represent Ps within an idealized pore.
Abstract: Positronium (Ps) is simulated using Path Integral Monte Carlo (PIMC). This method can reproduce the results of previous simple theories in which a single quantum particle is used to represent Ps within an idealized pore. In addition, the calculations treat the e{sup -} and e{sup +} of Ps exactly and realistically model interactions with solid atoms, thereby correcting and extending the simpler theory. They study the pick-off lifetime of o-Ps and the internal contact density, {kappa}, which controls the self-annihilation behavior, for Ps in model voids (spherical pores), defects in a solid (argon), and microporous solids (zeolites).
12 Jul 2004
TL;DR: In this paper, the authors applied the coincidence Doppler broadening (CDB) and positron annihilation lifetime (PAL) techniques to study low-density polyethylene (LDPE), high-dimensional polyethylenes (HDPE) and polypropylene (PP), and antioxidant containing polypropylon (PPA), and found that a significant enhancement of the carbonyl group concentration in PP was found by means of CDB technique, while the enhancement in HDPE was very slight.
Abstract: The coincidence Doppler broadening (CDB) and positron annihilation lifetime (PAL) techniques were applied to study low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), and antioxidant containing polypropylene (PPA). It was demonstrated that the Doppler-broadened lines for LDPE and PPA could be decomposed into two components a narrow component due to positronium (Ps) annihilation and another broader one due to free positron annihilation. The determined Ps-component in the Doppler-broadened line was used to calculate the Ps-corrected CDB ratios for HDPE and PP to LDPE. Different ways to compare CDB ratios in polymers were discussed. During a long-term (∼500 h) positron-irradiation experiment at a low temperature (50 K), a significant enhancement of the carbonyl group concentration in PP was found by means of CDB technique, while the enhancement in HDPE was very slight.
01 Jul 2004-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, the authors measured both upper and lower limits on positronium formation cross-sections for 0.5-25 eV positron scattering for a beam-transmission technique with a heated scattering cell.
Abstract: Total scattering cross-sections have been measured for 1–60 eV positrons and 6–200 eV electrons scattered by Cs atoms using a beam-transmission technique with a heated scattering cell. For 0.5–25 eV positron scattering we have measured both upper and lower limits on positronium formation cross-sections.
TL;DR: In this article, a quantum dynamical treatment of the vibrationally elastic scattering off tetrafluorocarbon (CF4) molecules by very low energy positrons is presented, based on the fixed-nuclei approximation and the interaction potential between the positron and the molecular target.
Abstract: We present a quantum dynamical treatment of the vibrationally elastic scattering off tetrafluorocarbon (CF4) molecules by very low energy positrons. The present calculation is based on the fixed-nuclei approximation and the interaction potential between the positron and the molecular target includes the ab initio electrostatic and the target correlation-polarization effects. The latter contribution is considered by means of a parameter-free model potential based on the density functional theory. The cross sections for this system are obtained by solving multichannel close-coupling equations for the wavefunction of the scattered positron, while no positronium formation channel is taken into account by the present calculation. The elastic cross section, the scattering length (A0) and the location of a virtual state for the total system are obtained and discussed in detail.
TL;DR: In this paper, the metastability of the four-body system is discussed from the point of view of radiative rearrangement collisions of the hydrogen and anti-hydrogen.
Abstract: The metastability of the four-body system is discussed from the point of view of radiative rearrangement collisions of the hydrogen and antihydrogen. Such collisions lead to the formation of an intermediate molecular state which further decays into positronium and protonium or is destroyed via annihilation. We present calculations of the lifetime of the system and the branching ratios for Coulombic decay and annihilation.