The scattering and pick-off quenching of orthopositronium in helium

Fraser, P A

doi:10.1088/0022-3700/1/5/135

Cited by 75 publications

(65 citation statements)

References 5 publications

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“…where r 0 is the classical radius of the electron, c is the speed of light, and Z eff is the effective number of electrons that participate in the annihilation process [4,45]. In general the parameter Z eff is different from the number of electrons in the target atom, Z.…”

Section: B Scattering Phase Shiftsmentioning

confidence: 99%

Positron scattering and annihilation on noble-gas atoms

2014

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Positron scattering and annihilation on noble gas atoms is studied ab initio using many-body theory methods for positron energies below the positronium formation threshold. We show that in this energy range the many-body theory yields accurate numerical results and provides a nearcomplete understanding of the positron-noble-gas-atom system. It accounts for positron-atom and electron-positron correlations, including the polarization of the atom by the positron and the nonperturbative process of virtual positronium formation. These correlations have a large effect on the scattering dynamics and result in a strong enhancement of the annihilation rates compared to the independent-particle mean-field description. Computed elastic scattering cross sections are found to be in good agreement with recent experimental results and Kohn variational and convergent close-coupling calculations. The calculated values of the annihilation rate parameter Z eff (effective number of electrons participating in annihilation) rise steeply along the sequence of noble gas atoms due to the increasing strength of the correlation effects, and agree well with experimental data.

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Section: B Scattering Phase Shiftsmentioning

confidence: 99%

Positron scattering and annihilation on noble-gas atoms

2014

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“…Besides obtaining the phase shifts in the low-energy region, it is also possible to determine the annihilation parameter, Z eff [40][41][42]. The fundamental idea is to compare exact and model potential calculations of Z eff and so fix the enhancement factor, G [39,43,44].…”

Section: Positron Annihilationmentioning

confidence: 99%

Elastic positron-cadmium scattering at low energies

Bromley

Mitroy

2010

Phys. Rev. A

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The elastic and annihilation cross sections for positron-cadmium scattering are reported up to the positroniumformation threshold (at 2.2 eV). The low-energy phase shifts for the elastic scattering of positrons from cadmium were derived from the bound and pseudostate energies of a very large basis configuration-interaction calculation of the e + -Cd system. The s-wave binding energy is estimated to be 126 ± 42 meV, with a scattering length of A scat = (14.2 ± 2.1)a 0 , while the threshold annihilation parameter, Z eff , was 93.9 ± 26.5. The p-wave phase shift exhibits a weak shape resonance that results in a peak Z eff of 91 ± 17 at a collision energy of about 490 ± 50 meV. Calculations over the past decade have demonstrated that positrons can form bound states with a wide range of neutral atoms [1,2]. Apart from the intrinsic interest in the quantum mechanics of these systems, the strong evidence for positron binding to atoms has provided support for the hypothesis that positron-molecule bound states associated with vibrationally excited states are predominantly responsible for the massive positron annihilation rates observed in many molecular-gas experiments [3]. The experimental realization and identification of positronic atoms and molecules is very difficult, so evidence for their existence is best sought indirectly by means of scattering experiments.The first-principles calculation of positron-atom interactions is a challenging proposition due to the tendency for the atomic electrons to localize around the positron, forming a composite structure somewhat akin to the positronium (Ps) atom [1,4]. The configuration-interaction (CI) method has been used to determine the energies of the positronic magnesium (e + Mg) and positronic zinc (e + Zn) ground states. In addition, their low-energy elastic and annihilation cross sections have been extracted from the energies of physical and low-energy pseudostates [5,6]. The presence of a Ps-like cluster dramatically slows down the convergence of the CI expansion with respect to the partial waves included in the orbital basis [1,4]. For example, the prediction of the 2 P o excited state of e + Ca was performed with a CI basis of dimension 900 000 [7,8]. Even then, the prediction of binding was reliant on an extrapolation to the → ∞ limit.The structures that are present in electron-atom scattering experiments [10]. Other calculations on the positron-copper and positron-zinc systems (which both support a 2 S e bound state) revealed a structure in their low-energy annihilation spectrum due to a weak resonance in their 2 P o scattering channels [6,11].The reason for the strong structure in the e + -Mg system, and the weaker structures in the e + -Cu and e + -Zn systems, is the attractive polarization interaction between the atom and the positron. The magnesium atom has a dipole polarizability, α d , of 71.35 a.u. [12][13][14][15] (an atomic unit corresponds to one a 3 0 ), while that of copper is 41.7 ± 3.4 a.u. [16] (there are other estimates that lie toward the upper side ...

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“…Another phenomenon is the ionisation and fragmentation of molecules following annihilation. It takes place when the positron energy is below the Ps-formation threshold 11 . The first study of this kind, Ref.…”

Section: Annihilation Spectra and Fragmentationmentioning

confidence: 99%

“…The low-energy positron annihilation rate in a gas, λ, is usually written in terms of a dimensionless parameter Z eff [11], defined by…”

Section: Introductionmentioning

confidence: 99%

Positron annihilation in large polyatomic molecules. The role of vibrational Feshbach resonances and binding

Gribakin

Lee

2008

Eur. Phys. J. D

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Abstract. We analyse the process of rapid positron annihilation in large polyatomic molecules due to positron capture into vibrational Feshbach resonances. Resonant annihilation occurs in molecules which can bind positrons, and we analyse positron binding to alkanes using zero-range potentials. Related questions of spectra of annihilation gamma quanta and molecular fragmentation following annihilation, are discussed briefly.

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The scattering and pick-off quenching of orthopositronium in helium

Cited by 75 publications

References 5 publications

Positron scattering and annihilation on noble-gas atoms

Positron scattering and annihilation on noble-gas atoms

Elastic positron-cadmium scattering at low energies

Positron annihilation in large polyatomic molecules. The role of vibrational Feshbach resonances and binding

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