Showing papers on "Positronium published in 1973"
TL;DR: In this article, a review of the use of the positron-electron annihilation phenomenon in the physics of condensed matter is presented, where the theory of the annihilation process is outlined, the importance of the two-photon mode of decay is established, and the observable results depend on the initial positron and electron states of the system.
Abstract: This review is concerned with the utilization of the positron-electron annihilation phenomenon in studies of the physics of condensed matter. The theory of the annihilation process is outlined, the importance of the two-photon mode of decay is established, and it is described how the observable results depend on the initial positron and electron states of the system. A brief account of the principal experimental techniques is included. Discussion of the complex nature of positron behavior in molecular substances suggests the particular value of investigations directed at the two photon pick-off mode of decay of the orthopositronium atoms that are formed in these materials. The possibility of positronium formation in ionic and metallic materials is also considered. The many important electron aspects of positron annihilation in metals are dealt with. The independent particle approach to the analysis of two photon angular distributions is discussed and illustrated. A survey of electronic structure investigations includes studies of polycrystalline and single crystal specimens. The relevance of angular correlation measurements in investigations of the Fermi surfaces of metals and alloys is discussed. More recently developed applications to the study of defected and disordered systems are dealt with. A preliminary account of the interpretation of multicomponentmore » lifetime spectra in terms of varying numbers of distinguishable positron states provides the basis for a discussion of studies of positron trapping by defects, voids. and surfaces in ionic and metallic solids, liquids and powders. (auth)« less
563 citations
TL;DR: In this paper, the positron zero-point energy and positron-electron correlation energy were calculated, taking into account the nonlinear response of the electron system to a positron.
Abstract: Positron work functions have been calculated for a number of simple metals and the noble metals. Three separate contributions were estimated: the positron zero-point energy and the positron-electron correlation energy, which together constitute the internal positron chemical potential, and the contribution from the surface-dipole layer. The effect of the ionic lattice was included in the zero-point energy and the surface-dipole barrier. The correlation energy was calculated taking into account the nonlinear response of the electron system to the positron. Of all the metals considered only Au and Cu were calculated to have negative work functions. Positronium work functions were also calculated and in all cases they are smaller than the corresponding positron work function and negative for several metals.
105 citations
TL;DR: In this article, the positron bound states localized at metal surfaces for most metals, which should be manifested in a long-lived positron lifetime component, are discussed. And the behaviour of positronium near metal surfaces is also discussed.
78 citations
01 Jan 1973
TL;DR: In this article, the p-wave positron-hydrogen phase shifts are calculated below the positronium pickup threshold and the results are larger than Armstead's (1968) in all cases near the upper edge of his estimated uncertainty.
Abstract: Rigorous lower-bound p-wave positron-hydrogen phase shifts are calculated below the positronium pickup threshold. The wave function is expanded in terms of the two linearly independent D functions each multiplied by an associated Hilleraas-type radial function with two parameters. Adiabatic and nonadiabatic corrections have been included. The results are found to be larger than Armstead's (1968) in all cases near the upper edge of his estimated uncertainty.
63 citations
TL;DR: In this paper, the combined Zeeman and Stark effect level shifts in the excited states of positronium were calculated by diagonalizing the 16.16 Hamiltonian matrix for crossing electric and magnetic fields, and the unperturbed energy eigenvalues calculated by Fulton and Martin were used as the diagonal elements of the Hamiltonian.
Abstract: Calculations are presented of the combined Zeeman- and Stark-effect level shifts in the $n=2$ excited states of positronium, for positronium moving perpendicular to an applied magnetic field. Lifetimes of the Zeeman- and Stark-mixed $n=2$ states are also calculated. The electric field results from the Lorentz transformation of the magnetic field to the rest frame of the positronium and is perpendicular to the magnetic field. The level shifts and lifetimes are calculated by diagonalizing the 16\ifmmode\times\else\texttimes\fi{}16 Hamiltonian matrix for positronium in crossed electric and magnetic fields. The unperturbed energy eigenvalues calculated by Fulton and Martin are used as the diagonal elements of the Hamiltonian.
38 citations
TL;DR: In this paper, the S-wave scattering of positrons by hydrogen atoms was studied using the coupled static approximation with 26 correlation functions for energies up to the excitation threshold of hydrogen.
Abstract: The S-wave scattering of positrons by hydrogen atoms is studied using the coupled static approximation with 26 correlation functions for energies up to the excitation threshold of hydrogen. Up to a certain energy, the formulation satisfies a bound principle. Below the positronium formation threshold, the calculated phase shifts are very close to those of Bhatia et al (1971). Above this threshold, the R matrix elements and the cross sections are very different from those of other earlier calculations. No resonances were detected in a search with energy spacing of about 0.02 eV over the region just below the excitation threshold.
37 citations
TL;DR: The 476 plus or minus 24 keV feature observed from the direction of the galactic center region by Johnson, Hamden, and Haymes may be a positronium-annihilation spectrum as mentioned in this paper.
Abstract: Energetic positrons stopping in gaseous matter annihilate from the positronium (e/sup +/ e/sup -/) state a large fraction of the time. It is shown that this positronium formation in combination with the finite energy resolution of real gamma -ray detectors will yield a redshifted annihilation spectrum. The 476 plus or minus 24 keV feature observed from the direction of the galactic center region by Johnson, Hamden, and Haymes may be a positronium-annihilation spectrum. (auth)
29 citations
TL;DR: In this article, higher-order relativistic contributions to the combined Zeeman and motional Stark effects in positronium were calculated for all the S states and for the 2P states.
Abstract: A calculation of higher-order relativistic contributions to the combined Zeeman and motional Stark effects in positronium is presented. These contributions are necessary for the determination of the fine-structure interval in the ground state from the Zeeman effect and may be important in future experiments on the first excited states of positronium. The contributions to the g/sub j/ factors have been calculated to order alpha /sup 2/ for all the S states and for the 2P states. The energy levels and the higher-order corrections to the motional Stark effect in the first excited state are also presented. Relativistic contributions are obtained from the matrix elements of a Hamiltonian containing the Breit interaction, a Pauli Hamiltonian, and the virtual annihilation interaction. For all nS states the relativistic contributions to the Zeeman effect may be accounted for by the replacement of g/sub e/ by g/sub e/ (l - 5 alpha /sup 2//24n/ sup 2/ T/2mc/sup 2/), where T is the kinetic energy of the atom and g/sub e/ is the gyromagnetic ratio of the free electron. (auth)
29 citations
TL;DR: In this paper, the value of vacuum-polarizsation amplitude is investigated in the region of positronium bound-state poles, where the naive Feynman-graph expansion fails.
Abstract: The value of vacuum-polarizsation amplitude is investigated in the region of positronium bound-state poles, where the naive Feynman-graph expansion fails. The corresponding contribution to positronium-ground-state splitting is explicitly evaluated up to order alpha /sup 6/. The previously given result for the alpha /sup 6/ ln alpha /sup -1/ term co ming from fourth-order vacuum- polarization corrections to the one-photon annihilation diagram is confirmed and put on a sounder basis. The alpha /sup 6/ term is found to receive, besides the obvious contribution from alpha /sup 6/ vacuum-polarization term, an additional alpha /sup 6/-contribution which can be considered as generated by an infinite series of terms of higher nominal order in alpha . (auth)
28 citations
25 citations
TL;DR: In this article, it was shown that the positronium spectrum can easily fall near 476 keV where it was observed, and the actual data on the observed feature look like a positronia feature, because there are very few (statistically zero) photons (as opposed to pulses) with energies greater than about 511 keV, indicating a sudden drop in the spectrum there.
Abstract: LEVENTHAL1 has noticed that the γ-ray spectrum due to the annihilation of positronium, which consists of two 511 keV photons from the singlet state and three photons from the triplet state, produces a spectral feature (line) with an apparent peak at an energy less than 511 keV when viewed with a γ-ray telescope having a Gaussian energy resolution. With the energy resolution of the telescope2 used to detect the 1.8 × 10−3 cm−2 s−1 galactic feature at 476 ± 24 keV, Leventhal1 calculates that the observed peak will lie at 490 keV. I calculate, moreover, that if the positronium spectrum sits atop a steeply falling continuum due to other sources, as is observed, then the apparent peak can easily fall near 476 keV where it was observed. Furthermore, the actual data2 on the observed feature look like a positronium feature, because there are very few (statistically zero) photons (as opposed to pulses) with energies greater than about 511 keV, indicating a sudden drop in the spectrum there. I therefore take Leventhal's suggestion seriously and show here that explosive nucleosynthesis is a plausible source of the positrons.
TL;DR: In this paper, the positron-alkali atom bound state is viewed as a positronium atom moving in the field of an alkali ion and being very slightly affected by the latter.
Abstract: The model-potential treatment developed in the preceding paper is applied to the study of the annihilation properties of a positron bound to an alkali atom. By using a model wave function for the positron bound state, accounting explicitly for electron-positron correlation, the following quantities are calculated: 1) the electron density at the position of the positron, 2) the spin-averaged two-photon annihilation rate, 3) the angular correlation function of the pair of emitted photons, and 4) the momentum distribution function of the annihilating electronpositron pair. The electronic densities at the positron and the annihilation rates result to be near the corresponding positronium values. This, together with other reported results, allows us to view the positron-alkali-atom bound state as a positronium atom moving in the field of an alkali ion and being very slightly affected by the latter. The presence of a small secondary maximum in the momentum distribution, the previous work on the subject and some limitations of the modelpotential treatment are discussed in detail.
TL;DR: In this paper, the wave function of the bound state of the positron and F-centre is determined in various approximations with and without the electron-positron correlation, and annihilation parameters are evaluated using this function.
Abstract: The trapping of positrons in F-colour centres in KCl is investigated in some model calculations. The wave function of the bound state of the positron and F-centre is determined in various approximations with and without the electron-positron correlation, and annihilation parameters are evaluated using this function. It is suggested that the screening of the electron-positron interaction in the bulk of the crystal has a strong influence on the trapping by promoting the localization of a neutral positronium-like state in the vacant anion site. In this case agreement with experiments is obtained.
TL;DR: In this article, the authors apply the close coupling approximation to positron helium scattering with allowance for virtual positronium formation, and find that the results depend markedly on the choice of the target helium wave function.
Abstract: The close coupling approximation is applied to positron helium scattering with allowance for virtual positronium formation. The results are found to depend markedly on the choice of the target helium wave function. It is suggested that this is due to the violation of the bound principle for the close coupling approximation when inexact target functions are used. (auth)
TL;DR: The contribution of the triplet-singlet splitting of positronium ground state arising from fourth-order vacuum polarization in the annihilation channel is reported in this paper, where it is found to be Δν = α 2 Ry ∞ [−( 1 4 )α 2 ln α − ].
TL;DR: In this paper, an analysis of the Zeeman levels of positronium is carried out for the $n=1$ and $n = 2$ states, including higher-order corrections.
Abstract: This paper constitutes a continuation of previous work on magnetic interactions of one-electron atoms. In earlier papers the magnetic moment of a hydrogenic atom in its ground state was calculated, including radiative and nuclear-mass corrections. In this work the ${g}_{J}$ and ${g}_{I}$ factors are calculated for arbitrary hydrogenic states, including radiative and nuclear-mass corrections. The magnetic moment of a hydrogenic atom is obtained for any state. An extensive analysis of the Zeeman levels of positronium is carried out for the $n=1$ and $n=2$ states, including higher-order corrections. For the $n=1$ state, the results confirm the validity of the Breit---Rabi formula with $g$ factors in agreement with those given in an earlier paper by Grotch and Hegstrom. The annihilation diagram is included in the analysis but does not lead to any additional correction terms (to the accuracy we are working with). The Zeeman corrections in the $n=1$ state are relevant to the precise experimental determination of the positronium hyperfine structure. The analysis of the $n=2$ state Zeeman structure of positronium is similar to that carried out by Brodsky and Parsons for the $n=2$ states of hydrogenic atoms. Although the results in the hydrogenic case were immediately applicable to Lamb-shift measurements, the present results are not yet applicable since at present there is no experimental data on the excited states of positronium.
Abstract: Some s-wave scattering parameters are calculated using a close-coupling wave function similar to that of Drachman's but with the explicit addition of the positronium channel and its polarization. The elastic phase shifts are in close agreement with those of Bhatia et al. and the inelastic R-matrix diagonal elements show a better phase-shift bound than previous calculations. (auth)
TL;DR: The angular correlation of positron annihilation radiation from the ϵ phase of plutonium metal at 535°C indicates that approximately 1% of the positrons are annihilating directly from the singlet state of positronium.
TL;DR: In this paper, a quadruple coincidence system was used to examine the detailed shape of three-photon positron lifetime spectra in argon, and the density dependence of the annihilation of ortho-positronium in a simple monatomic gas was found to depend linearly on density.
Abstract: Marked deviations from simple exponential decay are known to characterize the two-photon annihilation of positrons in the noble gases. In this experiment, a quadruple coincidence system was used to examine the detailed shape of three-photon positron lifetime spectra in argon. No deviation from a simple exponential was observed over the density range 2.57 to 33 amagat, except when a high electric field was applied. The ability to distinguish three-photon ortho-positronium annihilation from other modes of positron annihilation permitted, for the first time, a detailed investigation of the density dependence of the annihilation of ortho-positronium in a simple monatomic gas. Over the whole density range, the ortho-positronium annihilation rate was found to depend linearly on density, with a value of 7.15 ± 0.10 μs−1 for the vacuum rate of ortho-positronium annihilation and 0.250 ± 015 μs−1 amagat−1 for the quenching rate of positronium in argon. With a field of 125 V cm−1 amagat−1 (at 2.57 amagat), a deviati...
TL;DR: The positron annihilation rate in the Wigner crystal has been shown to approach the positronium value continuously from above as r s becomes large as discussed by the authors, which is roughly in agreement with what is expected from the knowledge of the rate in low-density metals.
Abstract: The positron annihilation rate is calculated in the Wigner crystal and shown to approach the positronium value continuously from above as r s becomes large. For r s ≈15, the numerical value is roughly in agreement with what is expected from the knowledge of the rate in low-density metals.
TL;DR: In this article, the Kohn variational method is applied to the s-wave elastic scattering of positrons by hydrogen atoms, and the trial wave function is taken to include a function allowing for the effect of virtual positronium as well as correlation functions.
01 Jan 1973
TL;DR: In this paper, an examination was made of positronium formation in a gas and the 511 keV red shift spectrum produced by the formation, and the results showed that the red-shift spectrum was a function of the density of the gas.
Abstract: An examination was made of positronium formation in a gas and the 511 keV red shift spectrum produced by the formation
01 Jan 1973
TL;DR: For example, the elastic scattering of slow electrons by atoms is determined largely by the combined effect of (a) the mean static field of the unperturbed atom (b) the dipole distortion of this field by the incoming electron and (c) electron exchange.
Abstract: The study of the rates of the various collision processes associated with positrons with energy ranging from thermal up to a few tens of eV is of interest, not only for its own sake but because of the more stringent tests which it imposes on theoretical approximations than does the corresponding study of electron collisions. For example, the elastic scattering of slow electrons by atoms is determined largely by the combined effect of (a) the mean static field of the unperturbed atom (b) the dipole distortion of this field by the incoming electron and (c) electron exchange. Of these the first two always give rise to attractive forces. On the other hand for positrons (a) is always repulsive and is opposed by the dipole distortion (b) which is attractive as for electrons. The accurate evaluation of the net effect of (a) and (b) is often more difficult when they act in opposite sense. For positron collisions, exchange is replaced by virtual positronium formation which tends to provide additional attraction but is difficult to calculate accurately because it depends strongly on electron-positron correlation.