Showing papers on "Positronium published in 1969"
60 citations
TL;DR: In this paper, a variational wave function for the ground state of positronium hydride was presented, which is considerably more accurate than any previously reported, with only kinetic and Coulombic potential energy terms included in the Hamiltonian, the motion of the proton is ignored, and the nucleus-positron distance is excluded from the wave function.
Abstract: A variational wave function for the ground state of positronium hydride is presented. The wave function is considerably more accurate than any previously reported. The only simplifications used are (a) only kinetic and Coulombic potential energy terms are included in the Hamiltonian, (b) the motion of the proton is ignored, and (c) the nucleus-positron distance is excluded from the wave function. The last assumption is the major source of error, but it enables us to evaluate all integrals exactly. Several trial wave functions are examined, the one giving the lowest energy being an open-shell exponential function times a 12-term power series through second-degree terms in the five remaining interparticle coordinates. Positronium hydride is found to be stable with respect to separation into hydrogen and positronium by at least 0.657 eV. The annihilation rate for two-photon events is found to be 2.095 ${\mathrm{nsec}}^{\ensuremath{-}1}$ for the trial wave function giving the lowest energy, and 2.265 ${\mathrm{nsec}}^{\ensuremath{-}1}$ for the trial wave function which most nearly satisfies the positron-electron cusp condition. A search for bound excited states of PsH and a bound state for the system ${e}^{+}\mathrm{He}$ was unsuccessful.
40 citations
TL;DR: In this paper, the non-exponential shoulder in the free-positron component is confirmed and has a total breadth of more than 1300 ns amagat, corresponding to a Ramsauer-Townsend-type minimum in the diffusion cross section Q of about 0031 πa02 at an energy of 17 ev.
Abstract: The diffusion and annihilation of slow positrons have been studied by the method of delayed coincidences for helium at 77 °k and densities of 20-70 amagat. The non-exponential shoulder in the free-positron component is confirmed and has a total breadth of more than 1300 ns amagat, corresponding to a Ramsauer-Townsend-type minimum in the diffusion cross section Q of about 0031 πa02 at an energy of 17 ev. Application of electric fields of up to 33 v cm-1 amagat-1 failed to enhance positronium formation, contrary to the findings of Marder et al., we find Q > 0115 πa02 around 177 ev. Zeff is found to be 3677 ± 0025 at thermal energy, dropping to about 33 near 1 ev, in agreement with recent experiments of Lee and Jones. Drachman's theoretical s-wave phase shifts (quasi-lower bounds) appear to be very reliable and we have set an upper limit on these phase shifts by constructing intermediate values and comparing thermalization times with experiment. Drachman's values of Zeff are a few per cent too low below v = 08 ryd1/2, but probably not anywhere by more than 8%. The variation of the positronium annihilation rate with pressure fits an approximately parabolic form above 40 amagat, resembling similar results of Celitans et al. for oxygen-free argon, but at lower densities in helium this annihilation rate becomes anomalous.
27 citations
TL;DR: In this article, the reaction of positronium was studied in aqueous KMnO4 solutions at various concentrations and pH, and lifetime measurements showed a distinct decrease of τ2 and increase of I2 with increasing temperatures.
Abstract: The reaction of positronium was studied in aqueous KMnO4 solutions at various concentrations and pH. Lifetime measurements show a distinct decrease of τ2 and increase of I2 with increasing temperatures, as expected for chemical reactions in which thermalized positronium is involved. No dependence on the pH of the solution could be observed. The activation energy calculated from the temperature dependence of τ2 is rather small and about 0.1–0.2 eV. The free positron annihilation lifetime was found to be approximately 0.55 nsec, which is in good agreement with the value obtained by using Dirac's formula, 0.5 nsec for water.
27 citations
TL;DR: In this article, a free volume model was developed to describe the experimentally observed I 2 − τ 2 correlation in molecular materials and phase changes were interpreted in terms of this model.
21 citations
TL;DR: In this article, it was shown that a relativistic infinite component wave equation correctly describes the effects in the H atom including the motion of the nucleus, and an exact mass formula for the singlet $l=n\ensuremath{-}1$ levels of positronium was derived.
Abstract: It is shown that a relativistic infinite-component wave equation correctly describes the relativistic effects in the H atom including the motion of the nucleus. An exact mass formula for the singlet $l=n\ensuremath{-}1$ levels of positronium is derived.
19 citations
TL;DR: Positronium formation in positron-helium scattering, including polarizability in potentials appearing in final equations as discussed by the authors, including polarization of the potentials in the final equations.
Abstract: Positronium formation in positron-helium scattering, including polarizability in potentials appearing in final equations
11 citations
01 Jan 1969
TL;DR: In this article, a correlation was observed between the lifetime and two-photon annihilation intensity by pick-off process of ortho-positronium in molecular materials, and an empirical model, in terms of free volume, was developed to explain this correlation.
Abstract: A correlation is observed between the lifetime and two-photon annihilation intensity by pick-off process of ortho-positronium in molecular materials. An empirical model, in terms of free volume, is developed to explain this correlation. The variation in lifetime and intensity with change of temperature and pressure, and due to melting of crystals, and glass-transition in polymers is considered, on the basis of this model.
9 citations
TL;DR: In this article, changes in the amounts of positronium formed in polyethylene and nylon 66 in the regions of phase transitions were studied by the two gamma angular correlation method.
4 citations
TL;DR: In this article, the cross section for positronium formation by positron collisions in hydrogen is calculated from equations of three-particle scattering, and they find σ=1.45πa 0 2 at the peak, about 25% higher than the Born approximation value, but at high energies their value is lower.
Abstract: The cross section for positronium formation by positron collisions in hydrogen is calculated from equations of three-particle scattering. We find σ=1.45πa 0 2 at the peak, about 25% higher than the Born approximation value, but at high energies our value is lower. Comparison with other calculations is also reported.
4 citations
TL;DR: In this paper, measurements have been made to determine the long-lived component of triplet positronium and its intensity in normal and degassed silicone fluids of different viscosity grades.
Abstract: Measurements have been made to determine the long-lived component,τ
2, and its intensity,I
2, of positrons decaying in normal and degassed silicone fluids of different viscosity grades. The results show that the lifetime,τ
2, of the triplet positronium decreases as the length of the chain of the silicone fluids increases, though the change is quite small. No dependence of the intensity,I
2, of the triplet positronium on molecular structure has been observed. No effect of the dissolved oxygen on triplet-positronium intensity has been observed. The large variation inτ
2 obtained by Grayet al. for normal aliphatic hydrocarbons having different chain lengths can be interpreted as arising mostly from density changes.
TL;DR: In this article, the lifetime of positronium and the probability of its formation in the reaction system were measured by applying this method to the radiation-induced reaction in organic crystalline state, polymerization of several monomers such as acrylonitrile and N-vinyl-carbazole.
Abstract: It is well known that “positron e+” interacts with matter and forms a positronium (e+ - e−) with the electrons of matter. Both the probability of the positronium formation and the lifetime of positronium are profoundly affected not only by the physical state of molecular substances, such as amount of defects, crystallinity, the size and other properties of crystals, but also by the presence of ions, radicals and excited species, produced in the system. Therefore, one can follow solid state reactions by the measurement of lifetime of positronium and the probability of the formation in the reaction system without any degradative analysis. By applying this method to the radiation-induced reaction in organic crystalline state, polymerization of several monomers such as acrylonitrile, methacrylonitrile, acetoaldehyde and N-vinyl-carbazole, in the crystalline state has been studied in our laboratory. In this paper, the radiation-induced solid state polymerizations N-vinyl-carbazole are reported.
TL;DR: In this article, an interesting correlation between the position long lifetime τ2 and the intermolecular forces in polymers is reported, and the present results favor the positronium polarization concept of Bisi et al.
TL;DR: In this article, a new approach to the problem of positronium formation in an electron gas is discussed by considering the competing processes of thermalization of the incident positron, capture of an electron by the positron and disintegration of the disintegrated positron.
Abstract: A new approach to the problem of positronium formation in an electron gas is discussed by considering the competing processes of thermalization of the incident positron, capture of an electron by the positron and disintegration of the positronium so formed. We calculate, using the methods of quantum field theory of the electron gas: 1) the probability of finding the 1s and 2s states of positronium (plus a hole) in a positron beam passing through the electron gas, 2) the probability of disintegration of the positronium states into electrons and positrons as a result of many-body interactions and 3) the cross-section for thermalization of the incident positron. These calculations show that although the positronium formation cross-section is comparable to the rest of the competing processes for most incident-positron energies, there is, nevertheless, a very narrow band of energies where this cross-section dominates the rest. Therefore, in spite of thermalization of the incident positron and disintegration of the positronium formed in the metal, a large fraction of the incident beam will be converted to positronium when equilibrium is reached during its passage through the electron gas.
01 Jan 1969
TL;DR: In this article, the cross sections for electron capture by positrons from molecular hydrogen forming positronium are calculated in the Born approximation for different incident energies, compared with those obtained using a simple peaking approximation.
Abstract: The cross sections for electron capture by positrons from molecular hydrogen forming positronium are calculated in the Born approximation for different incident energies. The exact numerical results are compared with those obtained using a simple peaking approximation.