Showing papers on "Positronium published in 2007"
TL;DR: It is found that when intense positron bursts are implanted into a thin film of porous silica, Ps2 is created on the internal pore surfaces much more efficiently than the competing process of spin exchange quenching, which appears to be suppressed in the confined pore geometry.
Abstract: It has been known for many years that an electron and its antiparticle, the positron, may together form a metastable hydrogen-like atom, known as positronium or Ps (ref. 1). In 1946, Wheeler speculated that two Ps atoms may combine to form the di-positronium molecule (Ps2), with a binding energy of 0.4 eV. More recently, this molecule has been studied theoretically; however, because Ps has a short lifetime and it is difficult to obtain low-energy positrons in large numbers, Ps2 has not previously been observed unambiguously. Here we show that when intense positron bursts are implanted into a thin film of porous silica, Ps2 is created on the internal pore surfaces. We found that molecule formation occurs much more efficiently than the competing process of spin exchange quenching, which appears to be suppressed in the confined pore geometry. This result experimentally confirms the existence of the Ps2 molecule and paves the way for further multi-positronium work. Using similar techniques, but with a more intense positron source, we expect to increase the Ps density to the point where many thousands of atoms interact and can undergo a phase transition to form a Bose-Einstein condensate. As a purely leptonic, macroscopic quantum matter-antimatter system this would be of interest in its own right, but it would also represent a milestone on the path to produce an annihilation gamma-ray laser.
223 citations
Journal Article•
TL;DR: A new determination of the decay rate of the negative ion of positronium (Ps-), using a beam-foil method and a stripping-based detection technique, is reported in this paper.
Abstract: A new determination of the decay rate of the negative ion of positronium (Ps-), using a beam-foil method and a stripping-based detection technique, is reported. The measured result of gamma = 2.089(15) ns(-1) is a factor of 6 more precise than the previous experimental value of gamma = 2.09(9) ns(-1), and is in excellent agreement with the theoretical value of gamma = 2.086(6) ns(-1).
89 citations
TL;DR: An outcome of a large-scale variational calculation, this is the first result for second-order corrections to a decay of a three-particle bound state.
Abstract: We present a precise theoretical prediction for the decay width of the bound state of two electrons and a positron (a negative positronium ion), Gamma(Ps-)=2.087 963(12)/ns. We include O(alpha2) effects of hard virtual photons as well as soft corrections to the wave function and the decay amplitude. An outcome of a large-scale variational calculation, this is the first result for second-order corrections to a decay of a three-particle bound state. It will be tested experimentally in the new positronium-ion facility in Garching in Germany.
32 citations
TL;DR: In this paper, the authors measured the lifetime of ortho-positronium in mesoporous silica films and found that the interaction between positronium and the pore surface is weaker in the pores whose surface is covered with CH3 groups.
Abstract: Lifetimes of ortho-positronium in mesoporous silica films were measured before and after surface trimethylsilylation of –OH groups. Variations of positronium lifetimes in the mesopores upon the surface modification indicate that the interaction between positronium and the pore surface is weakened in the pores, whose surface is covered with –CH3 groups, in comparison with those covered with –OH groups. This is consistent with the authors’ previous observation that positronium slowing down is less efficient in the pores covered with –CH3 groups. The present work demonstrates that in the porosimetric application of positron annihilation lifetime spectroscopy, the interaction between positronium and the pore surface has to be properly taken into consideration.
30 citations
TL;DR: In this article, the momentum-space coupled-channel optical method has been applied to positron-helium scattering and the positronium formation cross sections at the energy range from the threshold to $400\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ were reported and compared with available experimental measurements and theoretical calculations.
Abstract: The momentum-space coupled-channel optical method has been applied to positron-helium scattering. The ionization continuum and the positronium formation channels are included via a complex equivalent-local optical potential. The positronium formation cross sections at the energy range from the threshold to $400\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and the total scattering cross sections at the energies from 17 to $500\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ are reported and compared with available experimental measurements and theoretical calculations. The agreements between the present results and the corresponding measurements are satisfactory.
30 citations
TL;DR: It is found that convergent S-wave model cross sections are obtained only if complete expansions are utilized on both the atomic and the positronium centers, and it is suggested that, in the model and full calculations, the separate contributions to the breakup cross section from both centers become equal at threshold.
Abstract: We consider the positron-impact ionization (breakup) of atomic hydrogen utilizing the full and $S$-wave model calculations, concentrating on the near-threshold energy region. Unlike the corresponding electron-impact case, the $S$-wave model does support the Wannier-like threshold law predicted by Ihra et al. [Phys. Rev. Lett. 78, 4027 (1997)]. It is found that convergent $S$-wave model cross sections are obtained only if complete expansions are utilized on both the atomic and the positronium centers. Furthermore, we suggest that, in the model and full calculations, the separate contributions to the breakup cross section from both centers become equal at threshold.
28 citations
TL;DR: In this paper, a general agreement for T = 300 K was found for the RTE model for CPG with different pore sizes (2-70 nm) at different temperatures (50-500 K).
Abstract: In porous media, the vacuum lifetime of ortho-positronium (o-Ps) of τ = 142 ns can be reduced markedly by pick-off annihilation (interaction with electrons of the host material). So the o-Ps lifetime is determined by the pore size which can be extracted by utilising approved models like the Tao-Eldrup model for pore sizes smaller than 1 nm and the Tokyo model or RTE model for larger pore sizes. The RTE model contains an explicit temperature dependence of the o-Ps lifetime. Experiments on controlled pore glasses (CPG) with different pore sizes (2-70 nm) at different temperatures (50-500 K) were performed to verify the RTE model. A general agreement for T = 300 K could be found. The temperature dependence of the lifetime, especially for low temperatures, could not be approved sufficiently. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
27 citations
TL;DR: In this article, a method of analysing sets of several overlapping multichannel resonances, which are extremely difficult to analyse by other means, is presented, exploiting the Lorentzian profiles of eigenvalues of the energy-dependent time-delay matrix, avoided at their crossings.
Abstract: A useful method of analysing sets of several overlapping multichannel resonances, which are extremely difficult to analyse by other means, is presented. It exploits the Lorentzian profiles of eigenvalues {qi(E)} of the energy-dependent time-delay matrix, avoided at their crossings. The contribution to {qi(E)} from eachS-matrix pole stands out clearly and uniquely from the contributions from the other poles. Thus the resonances that can be easily missed by the conventional technique of studying the eigenphase sum δ(E)or its derivative dδ/dE are singled out asdiabatic resonances in a visually unambiguous way. The method is illustrated with the 1 P o continuum states of the helium atom and the positronium negative ion Ps − below the threshold for He + (n = 5) or Ps(n = 5). Many new resonances are found for Ps − . (Some figures in this article are in colour only in the electronic version)
25 citations
TL;DR: In this paper, a positron pulse was used to create positronium atoms in a porous silica sample and this effect was used as a probe to measure the time dependence of the recombination of laser-induced paramagnetic centers.
Abstract: A laser-pump, positron-probe technique has been developed for measuring the dynamics of laser-induced paramagnetic centers in porous materials. The basis of the technique is the fact that the annihilation rate of ortho-positronium may increase in the presence of unpaired spins. By using an intense positron pulse to create positronium atoms in a porous silica sample we have used this effect as a probe to measure the time dependence of the recombination of laser-induced paramagnetic centers. We have observed nonexponential postirradiation decay kinetics extending over ten orders of magnitude that are similar to those previously observed in silica glasses.
25 citations
TL;DR: In this article, the leading-order relativistic corrections to the ground-state energy of the positronium molecule were computed within the framework of perturbation theory, using a nonrelativistic variational expansion in terms of 6000 explicitly correlated Gaussians that yielded the lowest variational upper bound for this system to date.
Abstract: The leading-order relativistic corrections to the ground-state energy of the positronium molecule $({\mathrm{Ps}}_{2})$ have been computed within the framework of perturbation theory. As the zero-order wave function we used a highly accurate nonrelativistic variational expansion in terms of 6000 explicitly correlated Gaussians that yielded the lowest variational upper bound for this system to date. We also report some expectation values representing the properties of ${\mathrm{Ps}}_{2}$.
24 citations
TL;DR: A recent experiment on Ps-Ps spin exchange quenching represents the first time that two Ps atoms have been observed interacting with each other as discussed by the authors, and the implication is that experimentation on various systems containing more than one Ps atom is now possible, including the Ps2 molecule, the Ps Bose-Einstein condensate, the ps atom laser and the annihilation gamma ray laser.
Abstract: Our recent experiment on Ps-Ps spin exchange quenching represents the first time that two Ps atoms have been observed interacting with each other. The implication is that experimentation on various systems containing more than one Ps atom is now possible, including the Ps2 molecule, the Ps Bose-Einstein condensate, the Ps atom laser and the annihilation gamma ray laser. Here we describe various aspects of such research, including the development of new techniques, work currently underway and some possible future experiments. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, a large number of regular and some singular bound state properties of the ground 11S(L = 0)-states of the positronium and hydrogen negative ions were determined to a very high numerical accuracy.
Abstract: A large number of regular and some singular bound state properties of the ground 11S(L = 0)-states of the positronium Ps? and hydrogen ?H? negative ions are determined to a very high numerical accuracy. The highly accurate variational wavefunctions are constructed with the use of exponential basis functions written in the three-body perimetric coordinates. In particular, the total energy of the ground state of the Ps? ion determined in our calculations is E = ?0.262?005?070?232?980?107?770?3745 au, while the analogous ground state energy for the ?H? ion is E = ?0.527?751?016?544?377?196?589?733 au. These values are the best-to-date variational energies obtained for these systems.
TL;DR: In this paper, a reduction in the amount of positronium emitted from an atomically clean Al(111) surface that depends on the incident positron beam density was observed.
Abstract: We have observed a reduction in the amount of positronium emitted from an atomically clean Al(111) surface that depends on the incident positron beam density. We interpret this as evidence for the formation of molecular positronium, created following interactions between two pseudopositronium atoms trapped in a surface state. We find that this process is highly sensitive to the condition of the surface and is easily suppressed by changes thereupon. The implications of our data for planned spectroscopic studies of molecular positronium are discussed, as well as improvements to the experimental procedure that will allow more detailed measurements of the thermodynamics of the formation of this molecule from metal surfaces.
TL;DR: A first-principles density-functional calculation of the key parameters determining the interaction potential between Ps and an alpha-quartz surface shows that there is indeed a bound state with an energy of 0.14 eV, a value which agrees very well with the experimental estimate of approximately 0.15 eV.
Abstract: Whether positronium (Ps) can be physisorbed on a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain a Ps2 molecule on a material host. Experiments in the past have produced evidence for physisorbed Ps on a quartz surface, but firm theoretical support for such a conclusion was lacking. We present a first-principles density-functional calculation of the key parameters determining the interaction potential between Ps and an alpha-quartz surface. We show that there is indeed a bound state with an energy of 0.14 eV, a value which agrees very well with the experimental estimate of approximately 0.15 eV. Further, a brief energy analysis invoking the Langmuir-Hinshelwood mechanism for the reaction of physisorbed atoms shows that the formation and desorption of a Ps2 molecule in that picture is consistent with the above results.
TL;DR: In this paper, the authors measured the o-Ps lifetimes in mesoporous materials in vacuum and in the presence of air, and the measured lifetimes were examined in terms of various available models for positron porosimetry.
Abstract: The o-Ps lifetimes in mesoporous materials in vacuum and in the presence of air have been measured. The measured lifetimes are examined in terms of various available models for positron porosimetry. The o-Ps lifetimes measured in vacuum agree well with those predicted from the extended Tao–Eldrup model. In the presence of air, the o-Ps lifetimes are seen to reduce. The enhancement in the annihilation rate of positronium caused by air is seen to increase with decrease in the pore size. The positronium lifetimes measured in the presence of air in the present experiment and the literature data are fitted to a model based on the statistical probability of positronium interacting with the pore wall. This describes the increase in the quenching rate with decrease in pore size by taking into account the reactivity of air as well as the reduction in the average pore size due to adsorption of air on the pore surface. This simple model can be used in positron porosimetry for measurements carried out in the presence of air.
TL;DR: In this article, a multichannel three-body theory for scattering states in the presence of an imaginary absorption potential is developed in order to compute the direct e+e− annihilation amplitude.
Abstract: A problem to account for the direct electron–positron annihilation in positron–hydrogen scattering above the positronium formation threshold has been resolved within the time-independent formalism. The generalization of the optical theorem is derived for the case when an absorption potential is present in the Hamiltonian. With this theorem, the annihilation cross section is fully determined by scattering amplitudes. This allows us to separate out analytically the contribution of the positronium formation effect from the overall annihilation cross section. The rest is determined as the direct annihilation cross section. It is done uniformly below as well as above the positronium formation threshold. The multichannel three-body theory for scattering states in the presence of an imaginary absorption potential is developed in order to compute the direct e+e− annihilation amplitude. Special attention has been paid to an accurate definition of the coordinate part of the absorption potential as the properly constructed zero-range potential, which corresponds to the delta function originating from the first-order perturbation theory. The calculated direct annihilation cross section below the positronium formation threshold is in good agreement with the results of other authors. The direct annihilation cross section computed with the formalism of the paper shows nonsingular behaviour at the positronium formation threshold. A number of e+e− direct annihilation cross sections and positronium formation cross sections in the energy gap between Ps(1s) and H(n = 2) thresholds are reported. A sharp increase in the calculated direct annihilation cross section across the resonant energy is found for all first S- and P-wave Feshbach resonances.
TL;DR: In this article, it was shown that positron annihilation acquired acceptance for structural investigation of solids but results in porous media, where positron lifetime spectroscopy (LT) reveals substantial positron formation.
TL;DR: Positronium formation on trapped electrons as a function of positron irradiation time by high-energy positrons and temperature were investigated in the case of n-heptadecane, as a representative of odd-numbered alkanes as discussed by the authors.
TL;DR: In this paper, the positron annihilation technique has been proposed for determining the size distribution of free-volume holes in solids, in particular, polymers, which is an important physicochemical problem.
Abstract: Determination of the size distribution of free-volume holes in solids, in particular, polymers, is an important physicochemical problem. The positron annihilation technique has been proposed for this purpose. The central point in this technique is the quantitative interpretation of data, especially, for substances with a high specific surface area. A developed free-volume system in open-pore membrane materials, such as poly(trimethylsilylpropyne) PTMSP and the spirocyclically bound benzodioxane polymer PIM-1, and polymeric sorbents (hypercrosslinked polystyrenes) makes it possible for the first time to compare the sorption characteristics and positron annihilation data on the character of size distribution of nanopores in these polymers. In combination with the results of mathematical simulation of the structure and radiothermoluminescence measurements, the array of data indicate the structural inhomogeneity of the test amorphous materials. It was shown that this inhomogeneity in relation to the positron annihilation technique is expressed in the insufficiency of the representation of the orthopositronium decay curve by one component that takes into account the Gaussian lifetime distribution (symmetrical pore size distribution) and in the necessity of use of several decay components. The feasibility of revealing a nonrandom character of pore size distribution gives the positron annihilation technique an advantage over other approaches (inverse gas chromatography, 129Xe NMR) to investigation of nanopores in polymers.
TL;DR: 1,1-difluoroethylene molecules have been studied for the first time experimentally and theoretically by electron and positron impact and Resonance peaks observed have been shown to be mainly due to the vibrational and elastic channels, and assigned to the B2, B1, and A1 symmetries.
Abstract: 1,1-difluoroethylene (1,1-C2H2F2) molecules have been studied for the first time experimentally and theoretically by electron and positron impact. 0.4-1000 eV electron and 0.2-1000 eV positron impact total cross sections (TCSs) were measured using a retarding potential time-of-flight apparatus. In order to probe the resonances observed in the electron TCSs, a crossed-beam method was used to investigate vibrational excitation cross sections over the energy range of 1.3-49 eV and scattering angles 90 degrees and 120 degrees for the two loss energies 0.115 and 0.381 eV corresponding to the dominant C-H (nu2 and nu9) stretching and the combined C-F (nu3) stretching and CH2 (nu11) rocking vibrations, respectively. Electron impact elastic integral cross sections are also reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range from 0.5 to 50 eV in the static-exchange approximation and from 0.5 to 20 eV in the static-exchange plus polarization approximation. Resonance peaks observed centered at about 2.3, 6.5, and 16 eV in the TCSs have been shown to be mainly due to the vibrational and elastic channels, and assigned to the B2, B1, and A1 symmetries, respectively. The pi* resonance peak at 1.8 eV in C2H4 is observed shifted to 2.3 eV in 1,1-C2H2F2 and to 2.5 eV in C2F4; a phenomenon attributed to the decreasing C=C bond length from C2H4 to C2F4. For positron impact a conspicuous peak is observed below the positronium formation threshold at about 1 eV, and other less pronounced ones centered at about 5 and 20 eV.
TL;DR: In this article, the positronium intensity rise in long-chain alkanes is studied and two kinds of electron traps are shown to be discharged at low temperature (≈200 K) and remain populated up to the transition point to the rotator phase.
TL;DR: In this paper, the positronic charge in expressions for the potential energy is partitioned between the electronic and positronic Hamiltonians, which assures an acceptable electron density distribution around the positron.
Abstract: The adiabatic separation of electronic and positronic degrees of freedom is proposed. The positronic charge in expressions for the potential energy is partitioned between the electronic and positronic Hamiltonians. This partitioning assures an acceptable electron density distribution around the positron. Our adiabatic method gives good values of binding energies and annihilation rates for model systems, in which the positron is weakly bound. On the other hand, it does not work well if the wave function contains a significant contribution of the positronium cluster.
TL;DR: In this paper, samples of polyamide 6 (PA6) of different morphology were studied with use of WAXS, SAXS and PALS technique and determined ortho-positronium characteristics reflect variations in morphology and particularly the presence of defects at the interface between the crystalline and amorphous region as well as the interaction between the amorphized phases existing in the polyamide samples.
TL;DR: In this paper, a theoretical model of the positron annihilation lifetime spectrum including the mechanisms of slow positronium (Ps) localisation and delayed Ps formation from a positron and a trapped electron was developed.
TL;DR: The first study of a high-density collection of positronium (Cassidy et al., 2005) revealed Ps-Ps spin exchange interactions in a porous silica sample as discussed by the authors.
TL;DR: In this article, the S (t) parameter curves measured with trapped electrons and no trapped electrons in polyethylene are interpreted by introducing possible positron reactions, which is quite natural to believe that the delayed Ps formation competes with the positron localization.
Abstract: Positronium (Ps) can be formed by a reaction of trapped electrons and free positrons at low temperatures in molecular solids. While Ps formation by Ore and spur process must be fast, Ps formation by trapped electrons and free positrons is possible even at later positron age. The evidence of this delayed Ps formation was obtained by age-momentum correlation measurement of electron-positron pair annihilation γ-rays. In this work the S (t) parameter curves measured with trapped electrons and no trapped electrons in polyethylene are interpreted by introducing possible positron reactions. It is quite natural to believe that the delayed Ps formation competes with the positron localization. We have successfully obtained a good simultaneous fit to the observed time dependent S-parameter curves, S (t), with and without the trapped electrons in the sample. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, the AMOC was used to investigate the influences of functional groups in polymer on positron and positronium annihilation process, and the results showed that the S-parameters relating to ortho-positronium were higher than those relating to free annihilation in all samples and the annihilation process was affected by functional group.
TL;DR: In this article, the decay rate of the negative positronium (Ps) ion was determined using a beam-foil method and a stripping-based detection technique, which was a factor of 6 times more precise than the previous experimental result and there was excellent agreement both with the latter and with the theoretical value.
Abstract: The negative positronium (Ps–) ion consisting of two electrons and a positron (e+e–e–), represents the simplest three-body system with a bound state. Its constituents are stable, point-like particles, and it is essentially free from perturbations by strong interaction effects. Together with the rather unique mass ratio, these properties make the positronium ion an interesting object for studying the quantum-mechanical three-body problem. We present a new determination of the decay rate of Ps–, using a beam-foil method and a stripping-based detection technique. The measured value of Γ = 2.089(15) ns–1 is a factor of six times more precise than the previous experimental result, and there is excellent agreement both with the latter and with the theoretical value. With the new high-intensity positron source NEPOMUC at the FRM-II research reactor in Munich being available, a further improvement in precision seems possible. Moreover, the high flux of low-energy positrons at this facility brings other properties...
TL;DR: Positronium annihilation lifetime spectroscopy (PALS) is a unique porosimetry technique with broad applicability in characterizing nanoporous materials, especially insulators.
TL;DR: In this article, the positron-helium system was studied with the momentum-space coupled-channel optical method, where positron ionization continuum and positronium formation channels were included via a complex equivalent-local optical potential in the present calculations.
Abstract: The positron--helium system is studied with the momentum-space coupled-channel optical method. The positronium formation cross sections from the positronium formation threshold to 400 eV and total cross sections from 17 to 3000 eV are calculated. Ionization continuum and positronium formation channels are included via a complex equivalent-local optical potential in the present calculations. The results are compared with the available experimental and theoretical data.