Showing papers on "Positronium published in 1986"
TL;DR: A renormalization group strategy for the study of bound states in field theory is developed in this paper, which is completely different from conventional analyses, based upon the Bethe-Salpeter equation, and it is far simpler
905 citations
TL;DR: In this paper, the existence of a mirror universe of fermions and forces is shown to be impossible, and the possible existence of such a universe is thereby excluded, as shown in Figure 1.
140 citations
TL;DR: In this article, the authors determined the free-volume hole sizes in amine-cured epoxy polymers by comparing the observed ortho-positronium lifetimes with the known lifetime-free volume correlation for low-molecular-weight systems.
Abstract: Positronium annihilation spectroscopy (PAS) has been used to study the microstructural properties of amine-cured epoxy polymers. We have determined the free-volume “hole” sizes in these polymers by comparing the observed ortho-positronium lifetimes with the known lifetime–free volume correlation for low-molecular-weight systems. The free volumes for four epoxies with different crosslink densities are found to vary significantly over the temperature range between −78° and 250°C. The free-volume holes for these polymers are found to range from 0.025 to 0.220 nm3. Two important transition temperatures were found: one corresponds to the glass transition temperature Tg determined by differential scanning calorimetry (DSC), and the other occurs about 80–130°C below Tg. The sub-Tg transition temperature is interpreted tentatively as being where hole size reaches dimensions adequate for positronium trapping or else the onset temperature for local mode or side-chain motions. These two transition temperatures plus two additional onset temperatures are found to be correlated with crosslink densities calculated from stoichiometry.
100 citations
TL;DR: Measurements of cross sections for positronium formation in collisions of positrons of energies up to 251.4 eV with helium atoms and hydrogen molecules are reported.
Abstract: Measurements of cross sections for positronium formation in collisions of positrons of energies up to 251.4 eV with helium atoms and hydrogen molecules are reported. In both gases the cross sections are more than twice as large as the only other experimental values published to date. At low energies the measured values agree quite well with available calculations.
74 citations
TL;DR: A complex temperature dependence of the positron and positronium lifetimes is obtained and is discussed in terms of temperature-dependent positron trapping in various sites and in termsof hydrogen detrapping from dangling bonds.
Abstract: Angular correlation of positron-electron annihilation radiation (ACAR) experiments and positron-lifetime measurements have been performed in a-Si and a-Si:H films as a function of temperature. Positronium formation in microvoids is observed in a-Si:H, but not in a-Si. From the width of the narrow positronium ACAR components we estimate the average diameter of the microvoids to be \ensuremath{\sim}20 A\r{}. A complex temperature dependence of the positron and positronium lifetimes is obtained and is discussed in terms of temperature-dependent positron trapping in various sites and in terms of hydrogen detrapping from dangling bonds.
60 citations
TL;DR: The two-photon annihilation rate for positronium hydride is calculated with the use of extensive Hylleraas-type wave functions in which all six interparticle coordinates are used.
Abstract: The two-photon annihilation rate for positronium hydride is calculated with the use of extensive Hylleraas-type wave functions in which all six interparticle coordinates are used. The binding energy against dissociation into a positronium atom and a hydrogen atom is improved upon the previous best value by about 3.8%. The two-photon annihilation rate, calculated with a wave function that gives a positron-electron cusp value of -0.492${a}_{0}^{\mathrm{\ensuremath{-}}1}$ (compared with the exact value of -0.5${a}_{0}^{\mathrm{\ensuremath{-}}1}$), is determined as 2.459 ${\mathrm{nsec}}^{\mathrm{\ensuremath{-}}1}$.
51 citations
TL;DR: It is shown that at positron bombarding energies less than 100 eV, the positronium produced from backscattered positrons can be more intense than that produced by the diffusion of thermalized positrons to the surface.
Abstract: The production of positronium with kinetic energy greater than the positronium negative work function is clearly shown by time-of-flight spectroscopy of annihilating triplet positronium produced by the bombardment of low-energy positrons on metals. This higher-energy positronium channel is believed to be a consequence of positrons inelastically backscattered through the metal surface. At positron bombarding energies less than 100 eV, the positronium produced from backscattered positrons can be more intense than that produced by the diffusion of thermalized positrons to the surface.
50 citations
TL;DR: In this paper, the formation and interaction of positronium with simple systems is discussed and the formation channel is open even at zero positron energy; indeed the reaction is exothermic.
Abstract: Publisher Summary This chapter discusses the formation and interaction of positronium with simple systems. Positronium has the structure of a hydrogenic atom with a reduced mass of half the electron mass. The ground-state energy is therefore -6.8 eV and the dipole polarizability is 36a03. If the ionization potential is less than 6.8 eV, as is the case for the alkali atoms, the positronium formation channel is open even at zero positron energy; indeed the reaction is exothermic. All the positronium formed in the collisions of positrons with the target system, one-quarter is assumed to be parapositronium and three-quarters orthopositronium. The energy interval between the positronium formation threshold and the first excitation threshold of the target atom, E1, is referred to as the Ore gap. Within this energy gap positrons can either be elastically scattered or form positronium. Positronium formation can occur at energies beyond the upper limit of the Ore gap, and it continues to make a significant contribution to the total cross section at positron energies as high as 100 eV, but it then competes with many other inelastic processes and it becomes almost impossible to treat the process in a very precise manner. Within the Ore gap only two channels are open and very accurate results can be obtained, at least for simple target atoms, using rather similar techniques to those employed to calculate the essentially exact values of the scattering parameters for low-energy elastic scattering of positrons by hydrogen and helium.
47 citations
TL;DR: The binding energy of positronium molecules is calculated variationally by using extensive Hylleraas-type wave functions in which all six interparticle coordinates are used, consistent with the value of 0.0005 Ry, obtained by Lee using the Green's-function Monte Carlo method.
Abstract: The binding energy of positronium molecules is calculated variationally by using extensive Hylleraas-type wave functions in which all six interparticle coordinates are used. The binding energy against dissociation into two positronium atoms is determined as 0.0302 Ry. The present result is consistent with the value of 0.0303\ifmmode\pm\else\textpm\fi{}0.0005 Ry, obtained by Lee using the Green's-function Monte Carlo method.
44 citations
TL;DR: In this article, a strong magnetic field is considered and a photon is canalized along the magnetic field by adiabatically transforming into a bound electron-positron pair, which is a stronger effect than the analogous photon capture by transforming into an unbound pair at the edge of the continuum discussed previously by the authors.
Abstract: Positrinium atom is considered in a strong magnetic field
$$B \gg \alpha ^2 B_{cr} = m^2 ce^3 /\hbar ^3 = 2.35 \times 10^9 G$$
in a vector-potential gauged asA
x
=−By. The energy spectrum is obtained including its dependence on the centre-of-mass wave vector across the magnetic field. The pole-like contributions into the photon polarization operator coming from the positronium states are calculated and dispersion curves of joint photon-positronium states are obtained as trajectories of poles of the photon Green function in momentum space. When propagating in a strong magnetic field (B≳0.1B
cr
≅4×1012 G) with curved lines of force, a photon is canalized along the magnetic field by adiabatically transforming into a bound electron-positron pair, which is a stronger effect than the analogous photon capture by transforming into an unbound pair at the edge of the continuum discussed previously by the authors. The effect of bound pair formation byγ-quanta in a strong magnetic field may be important near pulsars,γ-burst sources, powerful X-ray sources in close binary systems and other astronomical objects, recognized as magnetic neutron stars.
42 citations
TL;DR: Using Faddeev's three-body scattering formalism for positron--hydrogen-atom collisions, an amplitude is obtained for the forward scattering angle which shows a cusp due to the effect of positronium formation to the continuum and heretofore has not been predicted.
Abstract: Depending on the distribution of energy between the scattered positron and the ejected electron in the final channel, two competing processes are involved in positron-impact ionization of atoms, namely (a) direct head-on ionization of an electron and (b) positronium formation to the continuum. Using Faddeev's three-body scattering formalism for positron--hydrogen-atom collisions, we get an amplitude which takes account of both these processes. The doubly differential cross section thus obtained for the forward scattering angle shows a cusp when plotted against the emission energy. The cusp is due to the effect of positronium formation to the continuum and heretofore has not been predicted.
TL;DR: Two of the collision parameters, the angle and the relative separation between the escaping particles, used in the Wannier and Klar models are calculated and find good agreement with the values predicted by these theories.
Abstract: Ionization cross sections for the four combinations of electrons and positrons on H and He/sup +/ have been calculated at energies near the threshold for ionization using the classical trajectory Monte Carlo method. In addition, charge-exchange (positronium formation) cross sections have been calculated for the positron-impact collisions. The ionization cross sections compare well with the predictions of the Wannier and Klar models and experimental observations. Our results indicate that the Wannier-type threshold behavior may extend to higher energies than was previously thought. In addition, we have calculated two of the collision parameters, the angle and the relative separation between the escaping particles, used in the Wannier and Klar models, and find good agreement with the values predicted by these theories.
01 Aug 1986-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, a magnetically confined slow positron beam was designed for surface physics and near-surface disorder studies and it operated in ultrahigh vacuum with a base pressure of 3 nPa.
Abstract: We report on the construction and performance of a magnetically confined slow positron beam. The beam is designed for surface physics and near-surface disorder studies and it operates in ultrahigh vacuum with a base pressure of 3 nPa. The beam uses a Co-58 source with a backscattering W(110) single crystal moderator. We get 4.7 × 106 slow positrons per second striking the sample using a 300 mCi source corresponding to a fast-to-slow positron conversion efficiency 0.28 ± 0.02%. The energy of the incident positrons can be varied from 1 eV to 35 keV. Some applications concerning positron re-emission and diffusion as well as positronium desorption from a surface state are shown.
TL;DR: In this paper, the authors suggest that a system containing several electron-positron pairs (an "e+e- droplet") could be highly localised and tightly bound if there exists a many-body force of short range acting between leptons.
Abstract: The authors suggest that a system containing several electron-positron pairs (an 'e+e- droplet') could be highly localised and tightly bound if there exists a many-body force of short range acting between leptons. Such a state could have properties required to explain the results of recent electron-positron coincidence experiments at GSI.
TL;DR: This work analyzes the problem of a positron at a jellium surface assuming that it is a weakly physisorbed positronium.
Abstract: We analyze the problem of a positron at a jellium surface assuming that it is a weakly physisorbed positronium.
TL;DR: The creation of bound electron-positron states, positronium, by photons in the field of an atom is calculated.
Abstract: The creation of bound electron-positron states, positronium, by photons in the field of an atom is calculated.
TL;DR: In this article, the authors examine one-photon transitions in positronium (Ps) as possible tests of the discrete symmetries of C (charge-conjugation) and P (parity) and/or T (time-reversal) invariance.
Abstract: We examine one-photon transitions in positronium (Ps) as possible tests of the discrete symmetries of C (charge-conjugation) and/or P (parity) and/or T (time-reversal) invariance. We discuss two general classes of experiment. The first class consists of direct searches for transitions forbidden by a given symmetry. The second class is composed of experiments that search for an asymmetry in the rate of an allowed transition upon reversal of an externally controllable vector quantity, such as an applied magnetic field B or rf photon spin S/sub ..gamma../. For a given symmetry, we compare limits on symmetry-violating mixings, which can reasonably be expected from these Ps experiments, with limits placed by existing atomic experiments. We conclude from this analysis, that the most promising experiments in Ps are those that search for C violation with no P violation.
TL;DR: In this paper, it was shown that positronium hyperfine splitting and Delbruck scattering impose stringent bounds on the coupling of a light neutral boson to the electron-positron and the photon field.
Abstract: We show that positronium hyperfine splitting and Delbruck scattering impose stringent bounds on the coupling of a light neutral boson to the electron-positron and the photon field. These limits are relevant for the “new-particle interpretation” of coincident electron-positron lines observed at GSI.
TL;DR: In this paper, the authors used the distorted-wave model of Khan and Ghosh to calculate the positronium formation cross sections in positron-lithium scattering, and the results for the total and differential cross section were reported.
Abstract: Positronium- (Ps) formation cross sections in positron-lithium scattering have been calculated by using the distorted-wave model of Khan and Ghosh. The results for the total and differential cross section are reported. The present Ps-formation cross section decreases with the increase of the incident positron energies. At about 100 eV, Ps formation is found to be negligible.
TL;DR: The annihilation rate of Ps is proportional to the square of the wave function at the origin and can be affected by exposure of the atom to a resonant laser.
Abstract: The annihilation rate of Ps is proportional to the square of the wave function at the origin. This can be affected by exposure of the atom to a resonant laser. The result depends upon whether the excited state is an S or a P state and whether it is singlet or triplet. An alteration of the decay rate by as much as a factor of 2 is possible.
TL;DR: It is calculated that the one-photon decay rate of the polyelectron P/sup + + -/ in the ground state to be 8.8 x 10/sup -2/ sec/Sup -1/.
Abstract: We calculate the one-photon decay rate of the polyelectron P/sup + + -/ in the ground state to be 8.8 x 10/sup -2/ sec/sup -1/. Implications for possible explanations of the anomalous peaks observed in heavy-ion collisions are briefly discussed.
TL;DR: The galactic positron line has been shown to be consistent with the line shape expected for annihilation in a neutral medium based on this measurement and on a theoretical prediction of the direct-annihilation linewidth.
Abstract: The fraction of positrons which survive below the positronium (Ps) formation energy in a low-density galactic environment can directly annihilate with bound electrons in the gas. The details of the measurement of the survival (direct-annihilation) fraction using a low-energy laboratory positron beam are presented for H/sub 2/ and He. The fraction is found to be higher in H/sub 2/ than had been predicted by Bussard, Ramaty, and Drachman. The galactic positron line has been shown to be consistent with the line shape expected for annihilation in a neutral medium based on this measurement and on a theoretical prediction of the direct-annihilation linewidth.
TL;DR: In this article, the authors reported vibrationally inelastic integral cross sections for positron-CO scattering below the positronium threshold, in a single-centre approach.
Abstract: Reports vibrationally inelastic ( nu =0 to nu '=1 and 2) integral cross sections for positron-CO scattering below the positronium threshold. The fixed-nuclei approximation is employed in a single-centre approach. As expected, the final cross sections for the nu '=1 and 2 excitations are structureless with magnitudes of about (3-0.3)*10-17 cm2 and (4-0.15)*10-20 cm2 respectively, in the energy range from the vibrational excitation threshold up to the positronium formation threshold. The corresponding first Born results are in qualitative agreement throughout.
TL;DR: In this article, the authors report on low-temperature positron studies for pure, single crystals of the alkali halides KCl, NaCl, and NaF.
Abstract: We report on low-temperature positron studies for pure, single crystals of the alkali halides KCl, NaCl and NaF. Strong temperature dependences are observed in the lifetime and angular correlation measurements in the temperature range 10–300 K. Delocalized para-positronium is observed at low temperatures in the three crystals. The broadening, with rising temperature, of the narrow peak in the angular correlation curve can be accounted for by the self-trapping model of positronium. The results are discussed in the light of this model, in terms of positronium localization at imperfections and in terms of positron interactions with imperfections created in the spur of the positron. The increased positronium formation with temperature is viewed as an enhanced escape probability of the excess electrons from sinks competing with positronium formation.
TL;DR: The lifetime spectra of positron beams have been used to obtain information at positron energies which are not attainable with positron beam technology as mentioned in this paper. But this information is usually displayed in the form of a number of parameters describing both the free positron annihilation rates and the annihilation of the positronium, Ps, formed during a positron slowing-down process in the gases under study.
Abstract: Publisher Summary Positron physics has experienced a period of rapid expansion resulting from the application of positron beams of well-defined energies to an increasing range of experiments. The development of positron beam technology has been crucially dependent upon the discovery and design of efficient moderators to produce the low-energy (about 1eV) positrons. The cross sections for annihilation are only comparable to those for atomic interactions at very low velocities and it has to be emphasized that lifetime spectra, therefore, provide information at positron energies which are not attainable with positron beams. This information is usually displayed in the form of a number of parameters describing both the free positron annihilation rates and the annihilation of the positronium, Ps , formed during the positron slowing-down process in the gases under study. Another important lifetime parameter is the positronium formation fraction F , which is defined as the ratio of the total number of positronium events (ortho plus para) to the total number of gas events in a given lifetime spectrum.
TL;DR: In this article, the photodetachment cross-section of Ps- is estimated to have a maximum of about 68 10-18 cm2 at a wave-length of between 1.8 104 and 2.1 104 A and shows a significant peak just below the threshold for exciting Ps (n = 2).
Abstract: Accurate variational calculations of the photodetachment cross-section of Ps- are reported. Both length and velocity forms are reported. The sum rule S-1 is satisfied to within 2%. The cross-section has a maximum of about 68 10-18 cm2 at a wave-length of between 1.8 104 and 2.1 104 A and shows a significant peak just below the threshold for exciting Ps (n = 2).
TL;DR: In this article, the total cross sections of e+-CO vibrationally elastic scattering in the energy range from 0.25 eV up to the positronium threshold were calculated.
Abstract: The author presents the first calculations on the total cross sections of e+-CO vibrationally elastic scattering in the energy range from 0.25 eV up to the positronium threshold. In the adiabatic-nuclei approximation, a single-centre expansion approach is employed to determine the fully converged scattering parameters. The e+-CO interaction energy is composed of an accurate repulsive static term and a parameter-free attractive correlation-polarisation potential. The results for the total cross sections are compared with recent measurements; the agreement is fair except below 2 eV, where theory underestimates the experimental data by about 50%. Results on the differential, momentum transfer and rotational excitation are also reported in the same energy region.
TL;DR: In this paper, the authors reported the first calculations on the total (elastic plus inelastic) cross sections ( sigma t) of positron-SiH4 scattering in a wide energy range (1-400 eV).
Abstract: The author reports the first calculations on the total (elastic plus inelastic) cross sections ( sigma t) of positron-SiH4 scattering in a wide energy range (1-400 eV). A spherical complex optical potential (SCOP) is generated between the target and the projectile and treated exactly in a partial-wave analysis to extract complex phaseshifts. The real part of the optical potential consists of an accurate repulsive static term calculated from one-centre near-Hartree-Fock silane wavefunctions plus a polarisation potential of the correct asymptotic form (- alpha 0/2r4, where alpha 0 is the dipole polarisability of the molecule) smoothly joined with the target correlation energy at short distances. For the imaginary part of the SCOP, he employs a recently semi-empirically derived form, which is based on the electron absorption potential. The final sigma t results compare fairly well with the only available measurements of Sueoka and Mori. In particular, the present model reproduces the sharp rise of sigma t near the positronium formation threshold and a bell-shaped feature around 20 eV. Below the positronium threshold, his elastic cross sections agree to within 2-3% with the more rigorous close-coupling results of Jain et al. (1986).
TL;DR: Theoretical calculations of momentum distributions of positronium (Ps) atoms ejected from clean metal surfaces are presented and compared with recent experimental results in this article, where the authors find that the momentum dependence of the Ps-forming interaction significantly affects the shape of the spectra.
Abstract: Theoretical calculations of momentum distributions of positronium (Ps) atoms ejected from clean metal surfaces are presented and compared with recent experimental results. The authors find that the momentum dependence of the Ps-forming interaction significantly affects the shape of the spectra. They also show that, within the model, the energy distribution of Ps atoms does not vary directly with the density of electronic states just outside the surface.
TL;DR: In this article, the results of Monte Carlo calculations of the positronium, Ps, fraction versus the initial positron energy in low-density helium, argon, and hydrogen are discussed.