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Journal of Physics Conference Series

Spin conversion induced by the spin-orbit interaction in positronium collisions

Haruo Saito, Takashi Nakayama, Toshio Hyodo

Published Nov 1, 2009• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1742-6596/194/7/072013

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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 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.
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 problems involved in providing a detailed interpretation of the processes involved in the ultimate fate of positrons slowed down in a gas are examined by considering the idealized case of a gas of atomic hydrogen. The processes leading to the formation and dissociation of positronium are considered as well as those which lead to the slowing down of a positronium pair in a gas and to the conversion of ortho- into para-positronium. It is shown that the rate of the latter process probably depends strongly on the kinetic energy of the pair. Elastic collisions of a pair, in its ground state, with gas atoms are determined almost completely by electron exchange interaction, but this is so strong that it cannot be taken account of in a weak coupling treatment.
Positronium (Ps) atoms exist in different spin configurations that determine their fundamental properties, such as energy level structure, decay rate and response to a magnetic field. The nature of interactions between Ps atoms is also strongly influenced by their relative spin states; oppositely polarized Ps atoms may exchange particles and scatter into different states, or join together to form molecular positronium. Conversely, interactions between spin aligned Ps atoms cannot lead directly to changes of spin states or molecule formation, but at high densities/low temperatures a collection of such atoms may undergo a phase transition to form a Bose-Einstein condensate. Here I consider some of the physics involved in such interactions and the positron beam parameters required to conduct such experiments.