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Journal of Physics Conference Series

Conversion of ortho-Ps into para-Ps in collisions with atoms

Amanda Allen, G. F. Gribakin

Published Nov 1, 2009• 0 citations
Publisher

Abstract

The cross section for positronium spin-conversion by the spin-orbit interaction in collisions with atoms has been calculated. Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)] predicted that this effect is responsible for the dearth of ortho-Ps in heavier noble gases, Kr and Xe. Our cross sections are broadly in agreement with their estimates and the measurements of Saito and Hyodo [Phys. Rev. Lett. 97, 253402 (2006)].

DOI
10.1088/1742-6596/194/7/072021

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The cross section for positronium spin-conversion by the spin-orbit interaction in collisions with atoms has been calculated. Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)] predicted that this effect is responsible for the dearth of ortho-Ps in heavier noble gases, Kr and Xe. Our cross sections are broadly in agreement with their estimates and the measurements of Saito and Hyodo [Phys. Rev. Lett. 97, 253402 (2006)].
Spin-orbit interaction of positronium (Ps) with the surrounding atoms, predicted by Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)], has been detected experimentally. We have found that the lifetime of the magnetically unperturbed ortho-Ps in Xe gas of 1 atm decreases significantly when a magnetic field of 1.0 T is applied. This decrease is attributed to the Ps spin conversion caused by spin-orbit interaction during Ps-Xe collision. The annihilation cross section of ortho-Ps due to this interaction has been determined to be 3 times as large as that expected by Mitroy and Novikov.
When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is $\mathrm{o}\mathrm{r}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{P}\mathrm{s}\ensuremath{\rightarrow}\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{\text{\ensuremath{-}}}\mathrm{P}\mathrm{s}$ conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.
The positronium spin conversion reaction induced by spin-orbit interaction is investigated. We obtain the reaction rates during positronium-Xe and positronium-Kr collisions by using the Zeeman mixing of positronium states. At thermal energies corresponding to room temperature, the reaction rate for spin conversion due to spin-orbit interaction is found to be almost twice that for the positronium pick-off reaction. We also study the energy dependence of the reaction rate. The mean energy of positronium is controlled by changing the gas temperature and using positronium in thermal equilibrium. We found that the reaction rate increases with the collision energy.
A pseudopotential for positronium-atom interaction, based on electron-atom and positron-atom phase shifts, is constructed, and the phase shifts for Ps-Kr and Ps-Ar scattering are calculated. This approach allows us to extend the Ps-atom cross sections, obtained previously in the impulse approximation [I. I. Fabrikant and G. F. Gribakin, Phys. Rev. Lett. 112, 243201 (2014)], to energies below the Ps ionization threshold. Although experimental data are not available in this low-energy region, our results describe well the tendency of the measured cross sections to drop with decreasing velocity at v<1v<1v<1 a.u. Our results show that the effect of the Ps-atom van der Waals interaction is weak compared to the polarization interaction in electron-atom and positron-atom scattering. As a result, the Ps scattering length for both Ar and Kr is positive, and the Ramsauer-Townsend minimum is not observed for Ps scattering from these targets. This makes Ps scattering quite different from electron scattering in the low-energy region, in contrast to the intermediate energy range from the Ps ionization threshold up to $v\ensuremath{\sim}2$ a.u., where the two are similar.
The positronium spin conversion reaction induced by spin-orbit interaction is investigated. We obtain the reaction rates during positronium-Xe and positronium-Kr collisions by using the Zeeman mixing of positronium states. At thermal energies corresponding to room temperature, the reaction rate for spin conversion due to spin-orbit interaction is found to be almost twice that for the positronium pick-off reaction. We also study the energy dependence of the reaction rate. The mean energy of positronium is controlled by changing the gas temperature and using positronium in thermal equilibrium. We found that the reaction rate increases with the collision energy.
The total elastic scattering and ortho-para conversion cross sections of low-energy ortho-positronium with hydrogen atoms have been calculated in the static exchange approximation including the l=0, 1 and 2 partial waves with the incident positronium wavenumber k ranging from 0 to 3.0 (in atomic units) for l=0 and from 0 to 1.2 for l=1 and 2. The scattering lengths are a (+) =7.275 a 0 and a (-) =2.476 a 0 for the space-symmetric and anti-symmetric cases respectively. A resonance is found in the anti-symmetric s-wave scattering at k approximately=2.07, but its real existence is of course questionable because of neglected effects.