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Showing papers on "Positronium published in 1968"


Book ChapterDOI
TL;DR: In this paper, a positron and electron interactions with atomic systems are discussed. But the authors point out that the absence of symmetries can make calculations more difficult, and positron interactions are not symmetric.
Abstract: Publisher Summary There are significant physical differences between positron and electron interactions with atomic systems Effects of the identity of the projectile with components of the target, exchange effects, do not arise in positron problems Although this would apparently lead to simplification, in fact the concomitant absence of symmetries can make calculations more difficult If its kinetic energy is high enough, a positron may pick up an electron from the target, forming the very light and large positronium atom; or it may, as do electrons, excite or ionize the target At sufficiently low energies, a positron will scatter only elastically Time to pulse-height converters, whose outputs are fed to multichannel pulse-height analyzers, are used in such work

85 citations



Journal ArticleDOI
TL;DR: In this paper, the authors measured the parameters of triplet positronium annihilation in 193 organic liquids and found a correlation between annihilation quenching cross sections and electron polarizabilities as measured from indices of refraction.
Abstract: The parameters of triplet positronium annihilation in 193 organic liquids have been measured. The annihilation quenching cross sections, 〈συ〉A v = (1 / τ2) (M / ρ), is a linear function of the length of the molecule in homologous series of compounds. Partial quenching cross sections, calculated for 27 structural functional groups, are used to calculate annihilation cross sections. The importance of steric effects in the annihilation mechanism is demonstrated in the study of several isomeric systems. Triplet positronium intensities depend upon specific atoms or groups in the molecule, rather than any property of the molecule as a whole. A correlation is also shown between annihilation quenching cross sections and electron polarizabilities as measured from indices of refraction. Several systems which quenched the triplet positronium were examined, e.g., free radicals, quinones, and paramagnetic species.

68 citations



Journal ArticleDOI
TL;DR: In this article, the van der Waal wave function for the orthopositronium-helium atom system was computed in the static exchange approximation and in an approximation that allows for the long-range van derWaal's forces acting between the neutral atoms.
Abstract: Measurements of the quenching rate of orthopositronium in helium determine the parameter 1Zeff, the effective number of electrons per atom in a singlet state relative to the positron. This number may be calculated from the wave function for the orthopositronium-helium atom system. In this paper, this wave function is computed in the static-exchange approximation and in an approximation that allows for the long-range van der Waal's forces acting between the neutral atoms; the calculated values of 1Zeff at thermal energies are 1Zeff = 0.037 and 1Zeff = 0.048 respectively. These results are considerably smaller than the most recent measured values 1Zeff = 0.1-0.25 and the possible reasons for the discrepancy are discussed.

53 citations




Journal ArticleDOI
R. R. Sharma1
TL;DR: In this paper, a suitable variational wave function was used to obtain the binding energy of the positronium molecule, a stable complex consisting of two electrons and two positrons.
Abstract: A suitable variational wave function has been used to obtain the binding energy of the positronium molecule, a stable complex consisting of two electrons and two positrons. We find the binding energy of the complex to be 0.948 eV. This shows that the previous calculations estimated the binding energy of the complex an order of magnitude too low.

30 citations


Journal ArticleDOI
TL;DR: In this paper, the authors studied the time evolution of the energy distribution of the positrons below the ionization threshold in an external electrostatic field, and found that the field induces diffusion out of the Ore gap, which decreases positronium formation.
Abstract: Positronium (Ps) can be formed whenever positrons, on slowing down in a substance, pass through an Ore gap, i.e., a range of energies, just below the ionization threshold of the substance, of width comparable to the Ps binding energy. We have studied the time evolution of the energy distribution of the positrons below the ionization threshold in an external electrostatic field, and find: (1) The field induces diffusion out of the Ore gap, which decreases Ps formation. (2) The over-all heating effect of the field on the positrons increases Ps formation. Depending on the magnitude of the relevant cross sections, (1) can outweigh (2), at small fields, and cause a significant decrease in the Ps formation. At field strengths larger than a characteristic value, (2) becomes the dominant effect. In this way measurements of the field dependence of Ps formation can give access to the cross sections for Ps formation and for positron-energy loss and scattering in matter.

19 citations


Journal ArticleDOI
TL;DR: In this paper, a new conceptual basis for chemical reactions between positronium and ionic or neutral species in liquids is outlined, and a simple mathematical analysis is given to predict the effects on the experimental position annihilation lifetime data which depend on ortho-para conversion, pick-off, and compound formation.
Abstract: A new conceptual basis is outlined for chemical reactions between positronium and ionic or neutral species in liquids. It is first shown that the positronium formation fraction in pure liquids depends not only on the dissociation energy of the medium (Ore-gap theory) but also on the threshold, IA–IP, where IA and IP are the ionisation potentials of the liquid and positronium. The energy regions in which positronium reactions may occur are defined in terms of ionisation potentials and bond strengths of the constituents of the liquid and of positronium. Special features of positronium reactions are described, and general conditions and examples are given of oxidation of Ps by electron transfer, compound formation, and double decomposition, and of reduction. ‘Anomalous’ features of earlier experimental data and recent experiments are satisfactorily explained in terms of the new concepts. These concepts are then applied to describe the conditions for reactions in mixtures. The effects are additive except when complexes are formed.A simple mathematical analysis is given to predict the effects on the experimental position annihilation lifetime data which depend on ortho–para conversion, pick-off, and compound formation. Experimental data are used, when available, to check the predictions.

18 citations


Journal ArticleDOI
TL;DR: The theoretical development is applied to estimating the thermalization time of positronium in rare gas moderators, and is shown to agree with the limited experimental results presently available.
Abstract: The energy decay of particles moving through a moderating medium is discussed for the case in which only elastic collisions occur between these incoming particles and the moderator atoms; the random thermal motion of the moderator atoms is taken into account. It is shown that if the cross section is independent of particle velocity the equation for the energy decay emerges in a rather simple form involving the hyperbolic cotangent. Finally, the theoretical development is applied to estimating the thermalization time of positronium in rare gas moderators, and is shown to agree with the limited experimental results presently available.

Journal ArticleDOI
TL;DR: In this paper, two-photon angular correlation and lifetime studies have been performed for positron-electron pairs annihilating in liquid (−65°C) and solid (−105°C), respectively.
Abstract: Two‐photon angular correlation and lifetime studies have been performed for positron–electron pairs annihilating in liquid (− 65°C) and solid (− 105°C) ammonia. The angular distributions were the same for both the liquid and solid samples. A low‐momentum component indicated an amount of positronium formation that is consistent with the lifetime results. The momentum distribution determined from the data is compared with a calculated distribution based on a cell model, using SCF molecular orbitals for ammonia and realistic positron wavefunctions. The lifetime results are discussed in the light of the free‐volume model.

Journal ArticleDOI
TL;DR: For the positronium-free atom in the ground state, the hyperfine structure energy splitting is ΔW0, 0, 8.34, 1.5, 12.5 kG.
Abstract: In order to investigate the behavior of positronium in water, magnetic quenching measurements have been performed at various temperatures from 0 to 45 °C and with applied fields up to 12.5 kG. From these measurements it has been possible to evaluate the effect of the water. In fact, while for the positronium-free atom in the ground state the hyperfine structure energy splitting is ΔW0 = 8.34 × 10−4 eV, for positronium in water we find two consistent values, ΔW = (6.32 ± 0.20) × 10−4 eV and ΔW = (7.30 ± 0.80) × 10−4 eV, obtained by considering the magnetic-field effect on the positronium component which decays by pickoff and on its decay rate.

Journal ArticleDOI
TL;DR: In this paper, a quasi-linear dependence on the percentage of volume which in matter is external to the ionic spheres is deduced by starting from considerations on the '' Ore gap ''.
Abstract: Positron annihilation has been studied in some ionic compounds such as hydroxides and sulfates by two-quantum angular measurements. It is concluded that a fraction of the positrons, those not forming positronium, annihilate almost exclusively in the neighbourhood of the negative ion, oxygen, while the remaining fraction forms positronium atoms. This fraction has been derived for each sample and plotted against the percentage of volume which in matter is external to the ionic spheres: a quasi-linear dependence is seen and is deduced by starting from considerations on the « Ore gap ».

Journal ArticleDOI
TL;DR: Experiments in support of the two-photon nature of the 10 -8 sec lifetime and the presence of a 1.4 × 10 -7 sec lifetime corresponding to the three-photons decay in oxides are presented in this paper.

Journal ArticleDOI
T. Roy1, J. Das1
TL;DR: In this article, the cross-section for positronium formation in positron-hydrogen collisions has been calculated in the lowest-order approximation from the viewpoint of field theory for bound states in quantum electrodynamics.
Abstract: The cross-section for positronium formation in positron-hydrogen collisions has been calculated in the lowest-order approximation from the viewpoint of field theory for bound states in quantum electrodynamics. The result is then compared with that of Massey and Mohr and seems to be better as regards the position of the peak value.

Journal ArticleDOI
TL;DR: In this article, a new suggestion was made to explain the behaviour of the positronium annihilating in ice, which was later confirmed to be the case in the case of the PDE.