Many-body calculations of positron scattering and annihilation from noble-gas atoms

Abstract

Abstract. We have applied many-body theory methods to study the interaction of low-energy positrons with noble-gas atoms. The positron-atom correlation potential includes explicitly the contribution from the target polarization by the positron and that from the virtual positronium (Ps) formation. It is demonstrated that the correlations and Ps formation (or tunnelling of electrons from the atom to the positron) create virtual levels in the positron-atom system. The existence of the virtual levels strongly influences the scattering and increases the positron annihilation rate by up to 400 times. The calculated elastic cross sections are in good agreement with experimental data. The inclusion of virtual Ps formation greatly improves the agreement with experimental annihilation rates with respect to calculations taking only the polarization into account. Our calculations have highlighted the difference between the calculation of positronatom scattering and the calculation of corresponding annihilation rates. The annihilation rate is very sensitive to the behaviour of the wavefunction at small positron-electron separations. A simple approximate formula based on the Sommerfeld factor is suggested to account for the effect of electron-positron Coulomb attraction on the annihilation rates.

Positron-Atom Complexes as Quantum Halo States

“…The mean value of r 2 is taken to be that of the positron for e Be and e Mg. For e He 3 S e , e Li, and e Na; hr 2 i is taken to be the mean of the electron and positron expectation values. the annihilation rate [34,35]. These positron-molecule bound states are likely to be similar in character to e Be, with postulated energies ranging from 5 10 ÿ4 to 5 10 ÿ3 hartree [34].…”

Positron scattering and annihilation from hydrogenlike ions

“…Research on atomic systems [9,20,[29][30][31] has shown that there is a dynamical connection between the size of the lowenergy phase shifts and the annihilation parameter. An attractive interaction leads to the amplitude of the wave function being larger close to the origin, and therefore can be expected to increase the annihilation parameter.…”

“…This effect will be called the clustering effect and is well known in condensed-matter systems [33,34], positron-atom scattering systems [20,29,30,36], and positron-atom bound states [19,37].…”

Variational calculation of low-energy positron life time in Xe atoms

“…One-electron HartreeFock with random phase approximations [12] was employed to calculate the scattering cross section of positrons on noble gas and alkali atoms. The many-body theory methods [13] are applied to study the interaction of low-energy positrons with Xe atoms which described by means of a non-local energy dependent-correlation potential. The polarized or- * Electronic address: mabdelmageed@hotmail.com bital method [14] is applied to calculate the elastic scattering cross section and annihilation rates of low energy positrons on xenon atoms below the positronuim formation threshold.…”