Showing papers on "Positronium published in 1991"
TL;DR: In this article, the gamma-ray spectra resulting from positron annihilation in these components of the interstellar medium are calculated, using two limiting assumptions for the propagation of the positrons.
Abstract: Positronium formation and annihilation are studied in a model for the interstellar medium consisting of cold cloud cores, warm partially ionized cloud envelopes, and hot intercloud gas. The gamma-ray spectra resulting from positron annihilation in these components of the interstellar medium are calculated. The spectra from the individual components are then combined, using two limiting assumptions for the propagation of the positrons, namely, that the positrons propagate freely throughout the interstellar medium, and that the positrons are excluded from the cold cloud cores. In the first case, the bulk of the positrons annihilate in the cloud cores and the annihilation line exhibits broad wings resulting from the annihilation of positronium formed by charge exchange in flight. In the second case, the positrons annihilate mainly in the warm envelopes, and the line wings are suppressed.
92 citations
01 May 1991-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: Gregory et al. as discussed by the authors described modifications to the program CONTIN to correct for source terms in the sample and reference decay curves and for shifts in the position of the zero-time channel of the samples and reference data.
Abstract: We have recently described modifications to the program CONTIN [S.W. Provencher, Comput. Phys. Commun. 27 (1982) 229] for the solution of Fredholm integral equations with convoluted kernels of the type that occur in the analysis of positron annihilation lifetime data [R.B. Gregory and Yongkang Zhu, Nucl. Instr. and Meth. A290 (1990) 172]. In this article, modifications to the program to correct for source terms in the sample and reference decay curves and for shifts in the position of the zero-time channel of the sample and reference data are described. Unwanted source components, expressed as a discrete sum of exponentials, may be removed from both the sample and reference data by modification of the sample data alone, without the need for direct knowledge of the instrument resolution function. Shifts in the position of the zero-time channel of up to half the channel width of the multichannel analyzer can be corrected. Analyses of computer-simulated test data indicate that the quality of the reconstructed annihilation rate probability density functions is improved by employing a reference material with a short lifetime and indicate that reference materials which generate free positrons by quenching positronium formation (i.e. strong oxidizing agents) have lifetimes that are too long (400–450 ps) to provide reliable estimates of the lifetime parameters for the shortlived components with the methods described here. Well-annealed single crystals of metals with lifetimes less than 200 ps, such as molybdenum (123 ps) and aluminum (166 ps) do not introduce significant errors in estimates of the lifetime parameters and are to be preferred as reference materials. The performance of our modified version of CONTIN is illustrated by application to positron annihilation in polytetrafluoroethylene.
66 citations
TL;DR: A detailed description of methods used by the authors for carrying out accurate calculations of low-energy positron scattering by H, He and H 2 using the Kohn variational method and very flexible trial functions containing Hylleraas-type correlation terms is given in this paper.
60 citations
TL;DR: In this paper, the collision of low-energy (below positronium formation threshold) positrons with methane (CH4) and silane (SiH4) molecules is investigated by employing two types of parameter-free model polarization potentials within the density functional description of the target correlation.
Abstract: The collision of low-energy (below positronium formation threshold) positrons with methane (CH4) and silane (SiH4) molecules is investigated by employing two types of parameter-free model polarization potentials within the density functional description of the target correlation. The first potential is based on the correlation energy, in corr, of one positron in a homogeneous electron gas of the target, while the second one is a modified version of the electron correlation potential in which the exchange interaction is absent. In the asymptotic region, both types of polarization potentials have the correct analytic form (- alpha 0/2r4, where alpha 0 is the polarizability of the target). Results for the total and differential cross sections are compared with recent measurements. The authors found that the first polarization potential mentioned above, is better than the second one.
45 citations
TL;DR: In this paper, partial wave elastic and ground state positronium formation cross sections have been calculated for L
Abstract: Positron scattering by atomic hydrogen including positronium formation has been formulated using the R-matrix method and a general computer code written. Partial wave elastic and ground state positronium formation cross sections have been calculated for L
37 citations
TL;DR: The yields and energy spectra of positronium (Ps) emitted from Al(111) surfaces treated by exposure to oxygen gas at low temperatures are measured and it is found that the oxygen induces the emission of Ps with kinetic energies of a few tenths of an eV.
Abstract: We have measured the yields and energy spectra of positronium (Ps) emitted from Al(111) surfaces treated by exposure to oxygen gas at low temperatures. We find that the oxygen induces the emission of Ps with kinetic energies of a few tenths of an eV, in agreement with previous work. We also find that Ps is thermally desorbed at low temperatures and has a velocity distribution characterized by a temperature that is the same as that of the sample. The intensity of the thermal Ps component is about 12% of the positrons that reach the surface. The velocity distributions may be interpreted as indicating that the Ps has a sticking coefficient of 1 in the limit of zero velocity, unlike any other system studied to date.
26 citations
TL;DR: In this paper, a time-of-flight technique was used to characterize the components of a positronium (Ps) beam generated by charge exchange reactions of positrons in helium and argon gases.
Abstract: A time-of-flight technique has been used to characterize the components of a positronium (Ps) beam generated by charge-exchange reactions of positrons in helium and argon gases. Ground and excited states of positronium have been resolved and the ratios of the signal rates of Ps to both scattered and incident positrons have been quantified in the Ps energy range 7-41 eV. By observing the pressure dependence of these ratios estimates of the Ps total scattering cross sections have been extracted for both gases at energies in the same range.
25 citations
TL;DR: In this paper, a systematic study has been carried out on the quenching of positronium (Ps) atoms by nitrobenzene in various solvents.
Abstract: A systematic study has been carried out on the quenching of positronium (Ps) atoms by nitrobenzene in various solvents. In order to see the effect of the energy level of Ps, in the localized bubble state, on the quenching capability of nitrobenzene, overall second-order rate constants were plotted against solvent surface tensions. It was found that the rate constant increased with increasing surface tension up to a critical value of ca. 26 dyn cm–1, corresponding to a Ps bubble energy of 1.24 eV. In liquids where the Ps energy is higher than this, the rate constant decreased with increase in solvent viscosity, as is expected for a diffusion-controlled reaction.
24 citations
TL;DR: A formal calculation that takes into full account the spin variables of all the electrons involved in the process as well as the indistinguishability of the electrons is addressed.
Abstract: The annihilation of a positronium atom in matter is often described as occurring through two alternative channels; self-annihilation (involving the electron in the atom itself) and pick-off annihilation (involving an external electron). For many years it has been debated if it is correct to assume the same pick-off annihilation rate for orthopositronium and for parapositronium. We address this problem with a formal calculation that takes into full account the spin variables of all the electrons involved in the process as well as the indistinguishability of the electrons. On the basis of our results, we make clear the conditions leading to a definition of the pick-off rate independent of the spin state of the positronium atom.
23 citations
TL;DR: In this paper, the authors used a similar basis set, in a momentum space formulation of the close-coupled channels approximation, to confirm the existence of a resonance in the positronium formation cross section in positron-hydrogen scattering.
Abstract: A recent calculation by Higgins and Burke (1991), using the R-matrix method, has indicated the existence of a resonance in the positronium formation cross section in positron-hydrogen scattering. This resonance is in the J=0 partial wave at an impact energy of 35.6 eV. The authors have confirmed this observation using a similar basis set, in a momentum space formulation of the close-coupled channels approximation. They have subsequently extended the calculations to allow for some polarization of both the hydrogen and positronium. The results indicate that a resonance in the total J=0 cross section also occurs within the limits of this model, although at a different energy. Further large scale calculations are required to determine whether the resonance persists as the basis is enlarged. However it is unlikely that it could be resolved experimentally due to the presence of the higher partial waves.
21 citations
TL;DR: In this paper, the complex-rotation method was used to calculate doubly excited 1,3P(0) autodetaching resonances in Ps(-, and the wave function is of the Hylleraas type with number of terms up to 1330.
Abstract: The complex-rotation method was used to calculate doubly excited 1,3P(0) autodetaching resonances in Ps(-). The wave function is of the Hylleraas type with number of terms up to 1330, and Feshbach resonances connected with the positronium n = 4, 5, and 6 thresholds are reported. The study has also identified 1P(0) shape resonances connected with the n = 4 and 6 thresholds and a 3P(0) shape resonance connected with the n = 5 threshold.
TL;DR: A parameter-free positron-correlation-polarization (PCOP) potential is employed and it is found that the PCOP model compares reasonably well with measured values.
Abstract: We employ a parameter-free positron-correlation-polarization (PCOP) potential to calculate total (rotationally summed) cross sections for the positron-H{sub 2} (N{sub 2}) collisions below the positronium formation threshold energy. A laboratory frame close-coupling technique, retaining six rotational states ({ital j}=0,2,4,6,8,10), has been used. It is found that the PCOP model compares reasonably well with measured values. Results on rotationally elastic and inelastic cross sections are also reported. We also make a comparison between PCOP and electron-correlation-polarization models for all the cross sections reported here.
TL;DR: A method is described of solving Bethe-Salpeter equations for spin-\textonehalf{} particles using a standard quasipotential approximation with a correction series and a three-dimensional, 16-component bound-state equation is obtained whose form is different from the Breit equation.
Abstract: A method is described of solving Bethe-Salpeter equations for spin-\textonehalf{} particles using a standard quasipotential approximation with a correction series. A three-dimensional, 16-component bound-state equation is obtained whose form is different from the Breit equation. The equation has two equivalent first-order forms which show each particle obeying a Dirac equation in the presence of the other. The formalism also reconstitutes the four-dimensional bound-state vertex function from the three-dimensional wave function. The method is tested on positronium and the hydrogen atom, using a single-photon-exchange kernel in the Coulomb gauge. In the bound-state equation, in addition to the Coulomb potential and Breit interaction, the formalism gives a subtracted box potential $(\frac{1}{2E})\frac{{\ensuremath{\alpha}}^{2}}{{r}^{2}}$ where $E$ is the mass of the bound state. With this term included, the energy levels (fine structure and hyperfine structure) are correct to order ${\ensuremath{\alpha}}^{4}$. The reconstituted bound-state vertex function, when substituted in Feynman triangle diagrams, gives the lowest-order atomic dipole transition amplitudes correctly.
TL;DR: In this paper, the positron transmission probabilities of the 1900 AA foil were extracted from the data and fitted to give an effective positron diffusion length of 425-475 AA, and the one-dimensional velocity distribution perpendicular to the foil surface has been estimated to be non-Maxwellian.
Abstract: Emission of positronium (Ps) from thin Ag(100) foils (1500-1900 AA) has been studied in reflection and transmission geometries. Positron transmission probabilities of the 1900 AA foil have been extracted from the data and fitted to give an effective positron diffusion length of 425-475 AA. Furthermore, the one-dimensional velocity distribution perpendicular to the foil surface has been estimated to be non-Maxwellian. The conversion efficiency of incident positrons to Ps in transmission is estimated to be approximately 12% for a 1900 AA foil at 800 K.
TL;DR: It is argued that the surface must be essentially a blackbody for Ps emission, and that this is the first example of a system that fails to exhibity the expected perfect reflection of a very show particle from a cold surface.
Abstract: We have measured the energy spectrum of positronium (Ps) thermally desorbed from Al(111) surfaces. For sample temperatures as low as 84 K and Ps perpendicular energies between 5 and 50 meV the spectrum is a beam Maxwellian, i.e., a simple exponential. We argue that the surface must be essentially a blackbody for Ps emission, and that we thus have the first example of a system that fails to exhibity the expected perfect reflection of a very show particle from a cold surface
TL;DR: The electron density at the positron is a relevant quantity for characterizing a positron-electron correlated pair in matter, since both the intrinsic decay rates in triplet and singlet states and the fine structure interval depend on it as discussed by the authors.
Abstract: The electron density at the positron is a relevant quantity for characterizing a positron-electron correlated pair in matter, since both the intrinsic decay rates in triplet and singlet state and the fine structure interval depend on it. Information about this quantity is generally obtained through magnetic quenching experiments. In the present work we discuss other techniques, based on the lifetime as well as on three-gamma spectroscopy, which do not involve the Zeeman effect in positronium.
TL;DR: Positron lifetime spectra have been measured in C 60 /C 70 and graphite powders at room temperature as mentioned in this paper, and it has been found that the spectrum for C 60/C 70 has only one component of lifetime 393 ps, whereas that for graphite has three components of lifetimes 237 ps, 455 ps, and 88 ns.
Abstract: Positron lifetime spectra have been measured in C 60 /C 70 and graphite powders at room temperature It has been found that the spectrum for C 60 /C 70 has only one component of lifetime 393 ps, whereas that for graphite has three components of lifetimes 237 ps, 455 ps, and 88 ns The single component in C 60 /C 70 does not originate from positronium Considering its lifetime, the positron's ground state in C 60 /C 70 is supposed to be localized in the periodic free space of the C 60 /C 70 crystal structure
TL;DR: It has been shown that, with a suitable choice of the distorting potential, the boundary condition may be satisfied with a proper account of the intermediate continuum states and leads to the same transition amplitude as may be derived using the Vainshtein-Presnyakov-Sobelman approximation.
Abstract: Positronium formation into ground and n=2 levels has been studied in collisions of positrons with atomic hydrogen in the framework of an approximation called the boundary-corrected continuum-intermediate-state (BCCIS) approximation in the energy range of 0.08--2 keV. The conventional continuum-intermediate-state approximation does not satisfy the correct boundary condition. It has been shown that, with a suitable choice of the distorting potential, the boundary condition may be satisfied with a proper account of the intermediate continuum states. It has also been shown that the BCCIS approximation leads to the same transition amplitude as may be derived using the Vainshtein-Presnyakov-Sobelman approximation. The results obtained here are found to be in good agreement when compared with other theoretical results.
01 May 1991-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, a beam-based β+γ technique was proposed to measure the lifetime of a positron together with the energy of one of the annihilation quanta in a triple-coincidence setup.
Abstract: A monoenergetic positron beam (6 × 104 e+ s−1) in the energy range of 0.5 MeV ⩽ E ⩽ 6.5 MeV ( ΔE E ≈ 10 −4 FWHM ) has been installed at the Stuttgart Pelletron accelerator. Besides experiments on e+ e− scattering (Bhabha scattering) and e+ e− annihilation-in-flight experiments, β+ γ positron-lifetime measurements in solids and melts have been performed. Substantially increased count rates and virtually eliminated background are among the advantages of the beam-based β+γ technique over the conventional γγ coincidence technique. Measurements under difficult conditions (e.g., in high-temperature melts) are simplified. Time-resolved information on the evolution of the positron states (e.g., trapping of positrons at defects) and a sensitive detection of positronium formation are obtained by measuring the lifetime of a positron together with the energy of one of the annihilation quanta in a triple-coincidence setup (age-momentum correlation). In contrast to the conventional γγE age-momentum-correlation technique, the beam-based β+γE technique permits high count rates and almost completely eliminates the deterioration of the measured spectra by pileup effects and by random coincidences.
TL;DR: In this article, a change in the nature of the quenching reaction is observed from spin conversion in benzene (Bz), N, N-dimethylacetamide (NDMA), and N-methylformamide (NMFA) to Ps oxidation in meta cresol (m-C); it is attributed to a modification of the electron density distribution in the quench molecule due to hydrogen bonding with them-C molecules.
Abstract: Positron annihilation lifetime spectroscopy (LS) and the Doppler broadening of the annihilation radiation lineshape (DBARL) technique have been used in conjunction to study solvent and temperature effects on both the positronium (Ps) quenching reactions with the free-radical HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) and the annihilation parameters. A change in the nature of the quenching reaction is observed from spin conversion in benzene (Bz),N,N-dimethylacetamide (NDMA), andN-methylformamide (NMFA) to Ps oxidation inmeta cresol (m-C); it is attributed to a modification of the electron density distribution in the quenching molecule due to hydrogen bonding with them-C molecules. The data also indicate that the full-width at half-maximum of the momentum distribution associated to para-positronium (p-Ps) resulting from the spin conversion reaction is smaller by a factor of 2 than that forp-Ps directly formed in the positron spur. The ortho-positronium (o-Ps) pickoff lifetime and momentum distribution are shown to give access to physicochemical and structural properties of the solvents.
TL;DR: Positron annihilation lifetime measurements were carried out for various types of zeolites and the lifetime spectra were resolved into four components as discussed by the authors, which were attributed to the annihilation of o-Ps (Ps = positronium atoms) on the surface and in the cages of the zeolite.
TL;DR: Using a spatially filtered pulsed laser at 486 nm, photoexcited positronium to the 2{ital S} state and detected it via a delayed and spatially separated 532-nm photoionization pulse, indicating that a cw measurement is presently feasible.
Abstract: Using a spatially filtered pulsed laser at 486 nm we have photoexcited positronium to the 2{ital S} state and detected it via a delayed and spatially separated 532-nm photoionization pulse. We have extended our observations down to low 486-nm power to explore the possibility of a cw measurement of the positronium 1{ital S}-2{ital S} interval. A model calculation taking into account the velocity distribution of the 235-K positronium from an oxygenated cold Al(111) target, the parameters of the laser beams, and the nonlinear excitation probability explains our data. The same model indicates that a cw measurement is presently feasible.
TL;DR: In this paper, it was found that an external magnetic field markedly enhances the orthopositronium annihilation rate in certain organic solutions above and beyond that expected for the Zeeman effect.
Abstract: It was recently found that an external magnetic field markedly enhances the orthopositronium annihilation rate in certain organic solutions above and beyond that expected for the Zeeman effect. Several solutions have been studied, but the effect has been found only for nitrobenzene in n‐hexane. A mechanism for this is proposed and tested numerically with a kinetics scheme modified to include nonhomogeneous spur effects. The mechanism features the quenching of orthopositronium by a short‐lived triplet solute species, the concentration of which is moderated by the applied magnetic field.
TL;DR: In this article, positron annihilation data in some organic polycrystalline scintillators, obtained through lifetime and three gamma spectroscopies, as well as magnetic quenching measurements, were discovered.
Abstract: We present positron annihilation data in some organic polycrystalline scintillators, obtained through lifetime and three gamma spectroscopies, as well as magnetic quenching measurements. Strong magnetic quenching effects at low fields were discovered, which cannot be explained in terms of simply “swollen” Positronium (qPs): a possible explanation in terms of interactions between the qPs and the magnetic moments of the spur products is proposed.
TL;DR: In this paper, the lineshapes of Doppler-broadened positron annihilation radiation were determined at different acidity content of a silica-alumina catalyst, and the results together with those obtained from temperature programmed desorption and infrared spectra demonstrated that positronium atoms are selectively oxidized by surface protons and do not react with Lewis acid sites.
TL;DR: In this article, features of positrons annihilated in benzene were investigated by the three gamma coincidences technique, together with lifetime spectroscopy and a magnetic quenching experiment.
Abstract: Features of positrons annihilated in benzene were investigated by the three gamma coincidences technique, together with lifetime spectroscopy and a magnetic quenching experiment A high three-quantum yield was found; the result reveals the presence of two positron bound states a 'relaxed' positronium system and a more complex one
TL;DR: The n=2 excited state capture cross sections in e+-H2 scattering have been calculated using the first Born approximation in this paper, in the energy range 30-1000 eV.
Abstract: The n=2 excited state capture cross sections in e+-H2 scattering have been calculated using the first Born approximation. The integrated and differential capture cross sections are predicted in the energy range 30-1000 eV. The total Ps formation cross sections ( sigma TPs= sigma rgPs+ Sigma 2sPs+ sigma 2pPs) are found to be in good agreement with the measured values in the energy range 50-150 eV.
TL;DR: In this article, the existence of magnetically bound positronium states (superpositronium) was investigated starting from Barut's relativistic two-body equations, and quasi-bound energy states were found in the form of resonances of the cross-section calculated with the use of the phase of the scattering states.
Abstract: The possibility of the existence of magnetically bound positronium states (superpositronium) is investigated starting from Barut’s relativistic two-body equations. For the total angular momentum j=0 quasi-bound energy states are found in the form of resonances of the cross-section calculated with the use of the phase of the scattering states. The energies are in agreement with the observed kinetic energy of the disintegration electrons or positrons in the heavy-ion collision experiments. From the width of the resonances the lifetime of the quasi-bound states can be estimated. Really bound superpositronium states could not be found.
TL;DR: It is found that treating the atomic nuclei as if they were isotopic spin projections (Straton and Girardeau, Phys.
Abstract: Cross sections for positronium formation by capture from the negative hydrogen ion are given. Orthogonalization corrections to the Coulomb (first-order) Born approximation (CBA) differential and total cross sections are calculated using approximate H{sup {minus}} wave functions of both Loewdin (Phys. Rev. 90, 123 (1953)) and Chandrasekhar (Astrophys. J. 100, 176 (1944)). The present calculation of the CBA cross sections using the post interaction for Loewdin's wave function (LCBAPS) disagrees with the calculation of Choudhury, Mukherjee, and Sural (CMS) (Phys. Rev. A 33, 2358 (1986)), whereas our results using the prior interaction agree. Thus, where CMS found an order of magnitude post-prior discrepancy in the differential cross sections except at forward angles, and a markedly different shape to the minima, the present post and prior results differ by 1--10 % at 100 eV, and the minima have the same shape and occur within one degree of each other. Chandrasekhar's open-shell'' wave function, which is superior to Loewdin's in bound-state problems since it gives a negative binding energy, gives post and prior cross sections that are almost indistinguishable at this energy and are 1/2 to 2/3 as large as the LCBA. Various methods of orthogonalizing the unbound projectile to the possible boundmore » states are considered. It is found that treating the atomic nuclei as if they were isotopic spin projections (Straton and Girardeau, Phys. Rev. A 40, 2991 (1989)) of a single type of nucleon'' gives cross sections that are an improvement over the CBA.« less