Showing papers on "Positronium published in 1974"
TL;DR: In this article, a new model of positronium (Ps) formation is proposed, which is based on a reaction between a positron and an electron in the positron spur.
Abstract: A new model of positronium (Ps) formation is proposed. Positronium is assumed to be formed by a reaction between a positron and an electron in the positron spur. Ps formation must compete with electron‐ion recombination and electron or positron reactions with solvent molecules and scavenger impurities. It is also influenced by electron and positron solvation. The model correlates the measured Ps formation probabilities with the spur electron properties determined in radiation chemistry. The predictions of the model are shown to be in good agreement with experimental results for liquids and solids.
359 citations
TL;DR: In this article, a beam of slow positrons in the few-eV region is directed at bakable solid targets in a vacuum of (2-4) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ Torr.
Abstract: A beam of slow positrons in the few-eV region is directed at bakable solid targets in a vacuum of (2-4) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ Torr. Measurements of the $\frac{2\ensuremath{\gamma}}{3\ensuremath{\gamma}}$ annihilation-yield ratio indicate the conversion of the incident positrons into positronium. The conversion efficiency depends on the target material and incident positron energy, varying from less than 25% at 300 K to 40-85% at 900 K.
90 citations
TL;DR: In this paper, 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.
66 citations
TL;DR: The temperature dependence of the rates of positronium reaction with diamagnetic organic molecules in liquids, and the observed complicated conversive-chemical interaction of POSRONIUM with many paramagnetic molecules in the liquid and gas phases lead to the formulation of some general concepts about POSRONium interaction with quenchers as mentioned in this paper.
Abstract: The temperature dependence of the rates of positronium reaction with diamagnetic organic molecules in liquids, and the observed complicated conversive-chemical interaction of positronium with many paramagnetic molecules in the liquid and gas phases lead to the formulation of some general concepts about positronium interaction with quenchers.
50 citations
Book•
01 Jan 1974
TL;DR: A review of the positron-electron annihilation phenomenon in the physics of condensed matter can be found in this article, where a brief discussion of the complex nature of positron behaviour 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.
Abstract: This review is concerned with the utilization of the positron-electron annihilation phenomenon in studies of the physics of condensed matter. A preliminary chapter outlines the theory of the annihilation process, establishes the importance of the two photon mode of decay and describes 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. An adequate description of the initial particle states can demand consideration of the chemical, electronic and structural properties of the material. A brief discussion of the complex nature of positron behaviour 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 important many electron aspects of positron annihilation in met...
41 citations
TL;DR: The ground state of positronium hydride has been found to be stable against dissociation into positrons and hydrogen by at least 0.0736 Ryd (1.00 eV) through a variational calculation.
Abstract: The ground state of positronium hydride has been found to be stable against dissociation into positronium and hydrogen by at least 0.0736 Ryd (1.00 eV) through a variational calculation. This dissociation energy is 26% larger than the most recently reported value. The two-photon annihilation rate is calculated to be 2.33 ns-1.
32 citations
TL;DR: In this article, positron lifetime studies of the following binary organic mixtures: (a) dioxane/n-heptane, (b) triethylamine/n -heptANE, (c) pyrrolidine/n −heptanes, and (d) carbon disulphide/ n -hexane were performed.
27 citations
TL;DR: In this article, both the time spectra and the angular distribution of the annihilation gamma rays have been measured for positrons annihilating in Teflon and polyethylene in an attempt to clarify which annihilation mechanisms account for the various lifetimes in these materials.
Abstract: Both the time spectra and the angular distributions of the annihilation gamma rays have been measured for positrons annihilating in Teflon and polyethylene in an attempt to clarify which annihilation mechanisms account for the various lifetimes in these materials. Some authors have suggested that, in addition to the longest lifetime component, one of the shorter lived components also arises from the pickoff annihilation of orthopositronium. If this were the case, substantially more positronium would be formed than is indicated by the intensity of the longest lived component alone, and should be detectable through a measurement of the intensity of the low momentum portion of the momentum distribution obtained from the angular correlation data. In the present work, four lifetimes are apparent in the time spectra although there is considerable uncertainty in resolving the two shortest components. The angular distribution data, in both cases, yielded a low momentum component whose intensity indicates that onl...
27 citations
TL;DR: In this paper, the amount of quenching of the tau 3 component as a function of the magnetic field was measured in polymers and the overlap probability of interacting positron-electron pair in Teflon was investigated.
Abstract: Measurements of magnetic quenching of positronium atoms in polymers are reported. The amount of quenching of the tau 3 component as a function of the magnetic field leads to the conclusion that: (i) in Teflon the overlap probability mod psi (0) mod 2 of the interacting positron-electron pair does not differ from that typical of positronium in free space; (ii) in the other polymers examined mod psi (0) mod 2 is lower than that of free positronium indicating a relaxation of the positronium atom. The relaxation parameter proves to be sensitive to the medium and this fact is discussed. In addition, some information is given on the quenching of the intermediate component.
27 citations
TL;DR: In this article, a variational calculation for the elastic e+-He scattering at energies below 16 eV was performed and the results of the calculation show good agreement with experimental cross section data.
Abstract: A variational calculation has been performed for the elastic e+-He scattering at energies below 16 eV. Partial waves up toL=3 are included. The trial functions contain distortions which take into account virtual positronium formation. The results of the calculation show good agreement with experimental cross section data.
19 citations
Abstract: The progress of research into the behavior of the positronium in molecular substances in the last several years is reviewed. The Ps atom can be treated as a solute in a liquid. The repulsive exchange force is the dominant force which reacts against the van der Waals attractive force. One of the most useful general relationships found is the one between the pick-off quenching rate and the surface tension of the liquid. The pick-off quenching rate is found, in general, to be an additive property of the functional groups of a molecule. The quenching of theo-Ps atom by an active quenching agent in an inert solvent has been found to be primarily diffusion controlled. Some deviations from the simple Arrhenius equation have been found. For gases, more theoretical work on the collision problems of Ps with atoms or molecules is needed. For amorphous molecular solids, it is likely that the property of pick-off quenching ofo-Ps is similar to that in liquids. However, in crystalline molecular solids, the situation is quite different. A few definite cases have been found where a Ps atom does not form or exist. Further work in this area should be very interesting. The foregoing leads us to believe that positronium or positron annihilation can be useful in the study of fundamental properties of molecular liquids and solids.
TL;DR: Using the precision s-and p-wave elastic-scattering wave functions obtained previously, the authors calculated the annihilation rate for positrons colliding with hydrogen atoms below the positronium-formation threshold.
Abstract: Using the precision s- and p-wave elastic-scattering wave functions obtained previously, we have calculated the annihilation rate for positrons colliding with hydrogen atoms below the positronium-formation threshold. The s-wave results agree well with those of Humberston, while the p-wave results, which are new, contribute about 20% of the total at the higher energies.
TL;DR: In this article, the formation of the n = 2 state of positronium in a gas by optical excitation from the n=1 state was studied. But this was based on 2γ annihilation radiation.
TL;DR: Positron lifetime spectra for various concentrations of bromine and oxygen adsorbed on silica gel are investigated in this article, where the energy of Ps annihilating in the pores of silica gels is found to be 0.25±0.10 eV.
Abstract: Positron lifetime spectra for various concentrations of bromine and oxygen adsorbed on silica gel are investigated. Two-photon angular correlation experiments were also performed on some of these samples to study the quenching process of bromine and oxygen. The cross section for bromine quenching of orthopositronium in silica gel is about two orders of magnitude larger than that for oxygen quenching. The process of bromine quenching of positronium is found to be chemical quenching. Oxygen quenching of positronium in silica gel is found to be conversion quenching. The interaction between positronium and bromine or oxygen in silica gel is found to be similar to that found in the gaseous state. The energy of Ps annihilating in the pores of silica gels is found to be 0.25±0.10 eV.
TL;DR: Positron lifetime measurements were performed on eight different amino acids, τ1, τ2 and I2 values were observed at around 0.35 nsec, 0.7-1.2 nsec and 20-30% respectively as discussed by the authors.
TL;DR: In this paper, the authors measured the transmission of a positron beam through helium, neon and argon gases from 4 eV up to the positronium formation thresholds, where a variable longitudinal magnetic field affects the transmission and allows some degree of phase shift separation.
Abstract: The authors have measured the transmission of a positron beam through helium, neon and argon gases from 4 eV up to the positronium formation thresholds. A variable longitudinal magnetic field affects the transmission and allows some degree of phase shift separation, analogously to the performance of an angular distribution experiment. The results for neon and argon are presented.
TL;DR: In this paper, the Bethe-Salpeter equation for a fermion-antifermion system coupled by photons is considered in the Feynman gauge, and the kernel is that resulting from exchange of a single photon.
TL;DR: In this paper, the temperature effect on positronium quenching rates for p-benzoquinone and iodine in several solvents was determined, and it was shown that the reaction energy of all the reactions studied are either negligible or less than 0.1 eV.
Abstract: The temperature effect on positronium quenching rates for p-benzoquinone and iodine in several solvents was determined. The quenching rate constantk1 follows ak1∝1/η relationship better than thek1∝T/η one, predicted by ordinary theory. A simple theoretical treatment based on diffusion theory shows that the temperatureT of the solute, positronium, may not be the same as the temperature of the solvent if the positronium is not fully thermalized. This temperature is found to be about twice the room temperature for the solvents investigated. The findings further support the theory that positronium reactions are “diffusion controlled” in liquid solutions. The activation energies of all the positronium reactions studied are either negligible or less than 0.1 eV.
TL;DR: In this paper, a new approach was proposed to obtain narrow and tunable spectral lines of positronium annihilation radiation (H ω λ = 0511 MeV ) based on the conversion of ortho-positronium atoms with a certain velocity of motion to the para-positionion under the effect of laser radiation and magnetic field.
TL;DR: Theoretical Hartree-Fock calculation for the [ X = e + ] systems with X = O, S, Se, and Te which established the stability of bound states for the system was summarized in this paper.
TL;DR: In this paper, the rate constants for the reactions between thermal Ps atoms and a series of iron (III) compounds including some complexes which can be considered as model compounds for biologically active systems have been determined.
Abstract: The rate constants for the reactions between thermal Ps atoms and a series of iron (III) compounds including some complexes which can be considered as model compounds for biologically active systems have been determined.
TL;DR: An improved wave function for positronium hydride is reported in this paper, which yields a total energy of −0.7792 a.u., the lowest and hence most accurate value yet reported.
Abstract: An improved wave function for positronium hydride is reported. The new wave function, which is of the same form as previously reported [Lebeda, C. F., Schrader, D. M.: Phys. Rev.178, 24 (1969)] yields a total energy of −0.7792 a.u., the lowest and hence most accurate value yet reported. This total energy indicates that positronium hydride is more stable than separated positronium and hydrogen atoms by 0.794 eV. The calculated annihilation rate, 2.22 nsec−1, is compared with available experimental evidence.
TL;DR: In this paper, the shape of the 2-photon angular correlation in ice has been obtained for several temperatures between −40 and 0°C, and the narrow component was analyzed in terms of 2 parts, one caused by delocalized and the other by localized Ps.
Abstract: The detailed shape of the Ps momentum distribution in ice, as given by the 2-photon angular correlation, has been obtained for several temperatures between −40 and 0°C. The narrow component was analyzed in terms of 2 parts, one caused by delocalized and the other by localized Ps. The latter one is interpreted in terms of Ps trapped by thermally excited defects with a formation energy of about 0.4 eV.
Journal Article•