Showing papers on "Positronium published in 1997"
TL;DR: In this paper, the positron annihilation rates for the three-body system and the MuPs were evaluated for positronium hydrides, TPs, DPs, and the ion.
Abstract: The bound-state spectra of the positronium hydrides $^{\mathrm{\ensuremath{\infty}}}\mathrm{HPs}$, TPs, DPs, $^{1}\mathrm{HPs}$, and MuPs are considered. The properties of the bound ground S states (L=0) in these systems and the ${\mathrm{Ps}}_{2}$ molecule have been determined by extensive variational calculations. The hyperfine structure of these states is also investigated. The positron annihilation rates have been evaluated for the positronium hydrides, the ${\mathrm{Ps}}_{2}$ molecule, and the ${\mathrm{Ps}}^{\mathrm{\ensuremath{-}}}$ ion and compared. The positron annihilation rates ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{n}\ensuremath{\gamma}}$ (where n\ensuremath{\geqslant}2) in the positronium hydrides are significantly closer to those in the ${\mathrm{Ps}}^{\mathrm{\ensuremath{-}}}$ ion (the three-body system) than in the ${\mathrm{Ps}}_{2}$ molecule.
93 citations
TL;DR: In this paper, the lifetime spectra of positrons annihilating in a porous medium (silicagel) were measured as a function of temperature, and the experimental value of the lifetime for the 1p state was in good agreement with the Tao-Eldrup formula.
69 citations
TL;DR: In this paper, an analysis of the behavior of the elastic scattering and positronium formation cross-sections for s-, p- and d-wave low-energy positron- hydrogen scattering in the vicinity of the PDE threshold is made.
Abstract: An analysis is made of the behaviour of the elastic scattering and positronium formation cross sections for s-, p- and d-wave low-energy positron - hydrogen scattering in the vicinity of the positronium formation threshold. The accurate cross sections, obtained using the Kohn variational method with elaborate trial functions containing many Hylleraas correlation functions, reveal interesting threshold structures which arise from the coupling between the positronium formation and the elastic scattering channels. Wigner's R-matrix method is used to provide a qualitative analysis of these structures. One of the most interesting features of the present results is the presence of a `rounded cusp' in the s-wave elastic scattering cross section at the threshold itself, as predicted by Wigner's threshold theory. Remarkably good fits to the partial-wave cross sections for elastic scattering, positronium formation and Ps - p elastic scattering are obtained using a five-parameter fit for each partial wave.
57 citations
01 Apr 1997
TL;DR: In this paper, the thermalization of positronium (Ps) formed at a few eV in gases was investigated using time-resolved, Doppler broadening measurements of the annihilation photons.
Abstract: The thermalization of positronium (Ps) formed at a few eV in gases is investigated using time-resolved, Doppler broadening measurements of the annihilation photons. Magnetic quenching permits energy measurements about 40 ns after Ps is formed in ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, He, Ne, Ar, isobutane, and neopentane. The thermalization rate is measured by changing the gas density, and a classical elastic scattering cross section and a Ps formation energy are determined. The impact of Ps thermalization on decay rate experiments using gases is also discussed.
53 citations
TL;DR: The first observation of charge exchange involving a positronium target was reported in this article, where the cross section for hydrogen formation has been experimentally determined at proton energies of 11.3, 13.3 and 15.8keV.
Abstract: Hydrogen formation has been observed following proton impact on positronium. This is the first observation of charge exchange involving a positronium target. The cross section for hydrogen formation has been experimentally determined at proton energies of 11.3, 13.3, and 15.8keV. Values of {sigma}{sub H}=26({plus_minus}9), 7.8({plus_minus}2.3), and 7.6({plus_minus}4.4){times}10{sup -16}cm{sup 2} were obtained, in reasonable agreement with recent calculations. {copyright} {ital 1997} {ital The American Physical Society}
49 citations
TL;DR: In this article, the positron scattering by lithium in the energy range 0.5 - 60 eV is studied within a frozen-core model of the atomic target using six different levels of approximation: (i) ; (ii) ) ; (iii) ), (iv) ), (v) ), where a bar denotes a pseudostate.
Abstract: Positron scattering by lithium in the energy range 0.5 - 60 eV is studied within a frozen-core model of the atomic target using six different levels of approximation: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; where a bar denotes a pseudostate. The results demonstrate the importance of including the positronium formation channel in the approximation scheme. The best approximation, (vi), which is modelled upon the highly successful treatment of Kernoghan et al for positron scattering by atomic hydrogen, and which uses pseudostates only on the atom centre, is believed to give the main cross sections for positron scattering off ground state lithium to a very good degree of accuracy. This confidence stems from the corroboration provided by the `complementary' approximation (v) which employs pseudostates only on the positronium centre. A new prediction from (vi) is the ionization cross section. This turns out to be relatively small, as had been assumed in earlier calculations. The idea that positron scattering by lithium might be treatable in a simple eigenstate approximation including the 2s and 2p states of lithium and a few positronium eigenstates seems to be largely correct for elastic scattering, excitation of the atom and total scattering. For these cross sections the simple eigenstate approximations (iii) and (iv) agree with the best results (vi) to better than 20%. However, agreement on total positronium formation is not quite so good, the eigenstate approximations (iii) and (iv) predicting a cross section which is generally larger than that given by (vi). It is found that at low energies the total cross section is dominated by elastic scattering, this dominance being directly taken over by excitation with increasing impact energy. Positronium formation is significant at energies less than 20 eV, although it never exceeds elastic scattering in the energy range studied.
49 citations
TL;DR: In this paper, the positron lifetime and Doppler-broadened annihilation radiation (DBAR) measurements were used for positron annihilation studies in acid and cation-neutralized (Li, Na, K, K+, Rb, Rb+, Cs+, UO22+, Ni2+) Nafion membranes.
Abstract: We report a new result on positron annihilation studies in acid- and cation-neutralized (Li+, Na+, K+, Rb+, Cs+, UO22+, Ni2+) Nafion membranes using positron lifetime and Doppler-broadened annihilation radiation (DBAR) measurements The free-volume structure is characterized using a simple quantum mechanical model of positronium (Ps) in a spherical well Our studies indicate that formation and expansion of clusters is always associated with a change in free-volume structure resulting in smaller free-volume holes © 1997 John Wiley & Sons, Inc J Polym Sci B: Polym Phys 35: 771–776, 1997
46 citations
Abstract: Anomalously large annihilation rates of positrons in binary collisions with molecules are interpreted as originating from the formation of virtual positronium accompanied by pick-off annihilation with One of the other molecular electrons. On this basis, a semiempirical ''universal'' formula is obtained which relates the annihilation rate to the molecular ionization energy in a manner similar to that originally discovered by Murphy and Surko [Phys. Rev. Lett. 67, 2954 (1991)]. This approach is found to lead to an explanation of the variation of Z(eff) over several orders of magnitude and to reproduce the correct general trends found among different molecular families.
40 citations
TL;DR: Close-coupling calculations of positron scattering from atomic sodium are performed from threshold to 100 eV in this article, where the alkali atom is represented by a frozen core model based upon the Hartree - Fock approximation.
Abstract: Close-coupling calculations of positron scattering from atomic sodium are performed from threshold to 100 eV. The alkali atom is represented by a frozen-core model based upon the Hartree - Fock approximation. The close-coupling calculations are performed in a model containing multiple sodium (3s, 3p, 4s, 3d, 4p) and positronium states (1s, 2s, 2p, 3s, 3p, 3d). Subsidiary calculations with three and ten positronium states were performed at selected energies to test the convergence of the positronium formation cross section. The predicted total cross sections agree with the experimental data of Kwan et al but there are differences with the data of Kauppila et al. The net positronium formation cross section underestimates the experimental data of Zhou et al.
37 citations
TL;DR: In this article, the positron annihilation lifetime spectra in an epoxy polymer using the electrotechnical laboratory (ETL) slow-positron facility as a function of positron energy from 0.2 to 3.0 keV were measured.
24 citations
TL;DR: In this paper, the single photon annihilation contributions for the positronium ground state hyperfine splitting were calculated analytically to order (m, e, √ √ 6) using nonrelativistic QED.
Abstract: The single photon annihilation contributions for the positronium ground state hyperfine splitting are calculated analytically to order ${m}_{e}{\ensuremath{\alpha}}^{6}$ using nonrelativistic QED. Based on intuitive physical arguments the same result can also be determined using results from previous calculations. Our result completes the hyperfine splitting calculation to order ${m}_{e}{\ensuremath{\alpha}}^{6}$. We compare the theoretical prediction with the most recent experimental measurement.
TL;DR: In this article, positron lifetime spectroscopy and Doppler broadening were applied to SiO 2 glass to investigate the crystallization process of isothermal and isochronal.
Abstract: Crystallization of SiO 2 glass was studied by positron lifetime spectroscopy and Doppler broadening. Heat treatments were performed in the temperature range between 700 and 1600°C to investigate the crystallization process isothermal and isochronal. Lifetime spectra were analyzed by two lifetime components. The decrease of the short lifetime (300 ps) was attributed to the increasing volume fraction of the crystalline phase. The long lifetime (1500 ps) was related with the pick-off annihilation of the ortho-positronium state in the free volume. Long lifetime increase, with the onset of crystallization, has been explained by expanding cavities at the interface between the crystalline phase and amorphous matrix. Positronium formation decreases with devitrification, which is in agreement with measurements of Doppler broadening. Positron annihilation was shown to be especially suited to investigate surface crystallization.
TL;DR: The ground bound state in the positronium hydride molecule (HPs) was determined from extensive variational four-body calculations with the James-Coolidge four body variational expansion as mentioned in this paper.
Abstract: The ground bound state in the positronium hydride molecule (HPs) is determined from extensive variational four-body calculations with the James-Coolidge four-body variational expansion in the relative coordinates r{sub 12},r{sub 13},r{sub 23},r{sub 14},r{sub 24}, and r{sub 34}. For the positronium hydride with the infinitely heavy nucleus ({sup {infinity}}HPs) the total energy found, E={minus}0.7891369 a.u., is one of the lowest variational values published to date. A number of bound-state properties have been calculated also, including the {delta}-function expectation values, two-body cusps, and the two-photon annihilation rate. {copyright} {ital 1997} {ital The American Physical Society}
TL;DR: In this article, a three-state target elastic positronium close-coupling approximation (CCA) is employed to investigate Ps-He scattering in the energy range 0-200 eV with and without electron exchange.
Abstract: A three-state target elastic positronium close-coupling approximation (CCA) is employed to investigate Ps-He scattering in the energy range 0-200 eV with and without electron exchange. Low-lying phase shifts below the first excitation threshold and the total integrated cross sections using both the models are reported. Estimation of integrated excitation cross sections for and using CCA are presented for the first time. The present total cross sections are in good agreement with the measured data in the incident Ps energy range 20-30 eV.
TL;DR: In this article, the authors examined angular-distribution measurements of positrons elastically scattered from noble-gas targets at energies near the positronium (Ps) threshold and found evidence for the dominance of channel coupling near the Ps threshold between the (incoming, outgoing) orbital angular momentum channels (1,0) and (2,1), respectively.
Abstract: We examine angular-distribution measurements of Kauppila et al and others of positrons elastically scattered from noble-gas targets at energies near the positronium (Ps) threshold. We use an R-matrix theory approach in which the R-matrix elements are used as fitting parameters. An attempt is made to find a set of parameters also consistent with the energy dependence of the (angle-integrated) elastic and Ps-formation cross sections. We show that close to threshold, both below and above, small differences between the experimental angular distributions and calculations by McEachran et al of differential cross sections, which neglect the Ps channel, are due to coupling to the Ps channel. In accordance with calculations of McAlinden and Walters, we find evidence for the dominance of channel coupling near the Ps threshold between the (incoming, outgoing) orbital angular momentum channels (1,0) and (2,1), respectively.
TL;DR: In this article, the authors improved the bubble model by modifying the idealised picture of a sharp bubble radius and thereby a formalism is developed which is not only more realistic but also provides elegant analytic solutions.
Abstract: Positronium self-trapping in liquids is frequently described through the bubble model in order to account for the observed decay characteristics, namely, the enhanced pick-off lifetime and the details of the angular correlation of the decay gammas. This model is improved upon in the present paper by modifying the idealised picture of a sharp bubble radius and thereby a formalism is developed which is not only more realistic but also provides elegant analytic solutions.
TL;DR: Theoretical calculations on positronium in crossed fields predict the existence of long-lived states in which the average positron-electron separation is several thousand angstroms as mentioned in this paper.
Abstract: We show that crossed electric and magnetic fields provide a unique way for stabilizing simple matter-antimatter systems. Theoretical calculations on positronium in crossed fields predict the existence of long-lived states in which the average positron-electron separation is several thousand angstroms. These delocalized states are due to the existence of an outer well in the potential for certain values of the pseudomomentum and field strength. The near zero probability for positron-electron overlap suppresses direct annihilation processes. Transition moments between the ground state in the outer well and the Coulomb states are also extremely small, resulting in lifetimes up to the order of one year.
21 May 1997-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this paper, a low energy positron storage ring was proposed for the generation of intense anti-hydrogen and positronium streams using a positron focusing system and electron cooling of the positrons.
Abstract: A method for the generation of intense antihydrogen and positronium streams using a low energy positron storage ring is considered. A longitudinal magnetic field for the focusing system and electron cooling of the positrons are essential features of the ring. The general limitations of the storage ring parameters are determined by the relation between scattering and cooling processes. The positron beam energy is determined by physics processes which are to be studied; it lies in the range from 5 up to 30 keV. The positron lifetime is about 100 s at a vacuum pressure of 100 pTorr. The generation of slow antihydrogen atoms by using electrostatic deceleration of positrons and antiprotons is described. The lowest antihydrogen energy in such a scheme is about a few eV and the generation rate is about 1 atom per second. The positron storage ring design is described. The general parameters of the positron storage ring, which is under design in JINR, are presented. The main purpose of the proposed machine construction is the performance of experimental studies of intense positron beam storing, electron cooling and deceleration in a straight section.
TL;DR: In this paper, high-intensity positron beam at Brookhaven National Laboratory (BNL) was used for positron-impact ionization, which is in good agreement with other experimental and most theoretical results.
Abstract: New data on positron collisions with atomic hydrogen leading to ionization were taken by using the high-intensity positron beam at Brookhaven National Laboratory (BNL). The measured cross section values for positron-impact ionization, which supersede earlier ones, are in good agreement with other experimental and most theoretical results, while the new data for positronium formation, formerly normalized to impact-ionization data, now disagree with recent theoretical predictions.
TL;DR: In this article, the sensitivity of positronium to detect inner surfaces of porous material has been utilized to study model catalysts of Pt, Co, and Ni on γ-alumina.
Abstract: The sensitivity of positronium to detect inner surfaces of porous material has been utilized to study model catalysts of Pt, Co, and Ni on γ-alumina. The results indicate that a carrier material wi...
TL;DR: In this article, the positronium energy levels due to photon exchanges were calculated in the effective Hamiltonian approach and the quoted results were valid for all $S$ states and arbitrary mass ratios.
Abstract: Corrections to the positronium energy levels of order ${\ensuremath{\alpha}}^{6}$ due to photon exchanges are calculated in the effective Hamiltonian approach. The quoted results are valid for all $S$ states and arbitrary mass ratios. We further present implications on the comparison of theory and experiment.
TL;DR: In this paper, the electron capture process with the formation of positronium atoms in the ground state in collisions of high-energy positrons impacting on hydrogenic ions also in ground state is studied theoretically.
Abstract: The electron capture process with the formation of positronium atoms in the ground state in collisions of high-energy positrons impacting on hydrogenic ions also in the ground state is studied theoretically. Contributions from double-step mechanisms in the collision process at high impact energies are discussed. Scaling laws for the theoretical differential and total cross sections valid at sufficiently high impact energies and nuclear charge of the target are obtained.
TL;DR: In this paper, it was shown that any system of four unit charges (m1+,m2+,m3−,m4−) forms a stable molecule provided two like-sign charges have the same mass, for instance m3 = m4.
Abstract: Computational evidence is presented to support the conjecture that any system of four unit charges (m1+,m2+,m3−,m4−) forms a stable molecule provided two like-sign charges have the same mass, for instance m3 = m4. Examples are the positronium molecule (e+,e+,e−,e−), the hydrogen molecule (p,p,e−,e−), the positronium hydride (p,e+,e−,e−), the exotic di-muonic system (p,e+,μ−,μ−), or excitonic molecules in semiconductors. The result is established using two independent and reliable variational methods.
TL;DR: In contrast to the findings of a recent analysis of the experimental results for positronium formation, that the p-wave contribution is dominant, the present results show that the d-Wave contribution is the largest throughout most of the Ore gap as mentioned in this paper.
Abstract: Calculations are made of the s-, p- and d-wave contributions to the positronium formation cross section for positron - helium scattering in the Ore gap. The Kohn variational method is used with very flexible trial functions, and the results obtained are believed to have converged to within 20% of their exact values. In contrast to the findings of a recent analysis of the experimental results for positronium formation, that the p-wave contribution is dominant, the present results show that the d-wave contribution is the largest throughout most of the Ore gap. Nevertheless, the total positronium formation cross sections, calculated from the sum of the variational s-, p- and d-wave contributions and the results of the Born approximation for all higher partial waves, display a very similar energy dependence to that of the experimental results, but are approximately 30% smaller.
TL;DR: The effect of dynamically coupling the motion of the impinging positron with that of the molecular nuclei during the scattering process is examined in this paper, where it is shown that the effects are increasing the size of the total integral cross sections but are different at different energies.
Abstract: The effect of dynamically coupling the motion of the impinging positron with that of the molecular nuclei during the scattering process is examined. At the collision energies below the threshold of positronium formation one finds that the effects are increasing the size of the total integral cross sections but are different at different energies. Comparison with experiments is carried out for the target and shows rather good agreement between calculations and the available measurements. The presence of a threshold peak in the inelastic channel is also discussed in analogy with the case of electron scattering from the same molecule.
19 May 1997
TL;DR: In this paper, the authors developed a versatile, pulsed source of cold (0.018 eV FWHM), low-energy positrons (E/spl sim/0-9 eV).
Abstract: Summary form only given. We have developed a versatile, pulsed source of cold (0.018 eV FWHM), low-energy positrons (E/spl sim/0-9 eV). Ten microsecond duration pulses of 10/sup 5/ positrons are extracted from a thermalized, room temperature positron plasma stored in a Penning trap. The frequency, duration, and amplitude of the pulses can be varied over a wide range. This technique allows for generation of electron pulses by simply inverting the polarity of all electrode potentials. Using this technique, we have also generated quasi steady state electron and positron beams with comparable energy resolution. There are numerous potential applications of such a source of intense, cold positrons. Examples include material surface characterization, such as depth profiling, positron and positronium gas scattering, and annihilation studies.
TL;DR: In this article, the authors compute the next-to-leading contribution to the hyperfine splitting in positronium within the framework of nonrelativistic QED (NRQED).
Abstract: We compute the next-to-leading $O(m{\ensuremath{\alpha}}^{5})$ contribution to the hyperfine splitting in positronium within the framework of nonrelativistic QED (NRQED). When applied to the ground state, our calculation reproduces the known result, providing a further test of NRQED techniques. In addition to providing a very simple method of calculation of this standard result, we also obtain new expressions for excited states of positronium with negligible additional effort. Our calculation requires the complete next-to-leading matching of the lowest-dimension NRQED four-Fermi couplings.
TL;DR: In this paper, the positron re-emission data from a SiC(0001) surface indicate that this surface has a negative positron workfunction, which is consistent with a positron surface state on SiC.