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Physical Review

Annihilation of Positrons in Gases

Milton Heinberg, Lorne A. Page

Published Sep 15, 1957• 43 citations
Publisher

Abstract

The angular correlation of the two-photon annihilation of positrons annihilating in gases at high pressures demonstrates the two modes of decay---(1) from a bound state, positronium, and (2) from an unbound state---in all gases tested that have low-lying energy states. The increase in the narrow component in argon, plus suitable thermalizing gases, due to the application of a dc magnetic field obeys the theoretical increase due to the mixing of the $1^{1}S$ and $1^{3}S$ $m=0$ states. The increase in positronium formation due to the application of a dc electric field is observed. Preliminary evidence for the existence of $n=2$ states of positronium is seen.

DOI
10.1103/physrev.107.1589

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hep-phCross-listedarXiv:2606.25433•Jun 25, 2026
The angular correlation of the two-photon annihilation of positrons annihilating in gases at high pressures demonstrates the two modes of decay---(1) from a bound state, positronium, and (2) from an unbound state---in all gases tested that have low-lying energy states. The increase in the narrow component in argon, plus suitable thermalizing gases, due to the application of a dc magnetic field obeys the theoretical increase due to the mixing of the 11S1^{1}S11S and 13S1^{3}S13S m=0m=0m=0 states. The increase in positronium formation due to the application of a dc electric field is observed. Preliminary evidence for the existence of n=2n=2n=2 states of positronium is seen.
An overview is presented of recent studies of the interaction of low-energy positrons with atoms and molecules using a trap-based positron beam. The beam is tunable from ~100 meV to many electron volts, with a typical energy resolution of 25 meV (FWHM). A variety of scattering processes are investigated using the properties of positron orbits in a spatially varying magnetic field. Recent scattering measurements are reviewed, and new data for electronic excitation, ionization, and positronium formation in noble gases are presented. In a separate set of experiments, positron annihilation below the threshold for positronium formation is studied in molecules as a function of positron energy. The data show large Feshbach resonances in the range of energies of the molecular vibrations. Within the framework of this model, these results provide the first experimental evidence that positrons bind to hydrocarbon molecules.
The positronium molecule (Ps2)({\mathrm{Ps}}_{2})(Ps2​) has not been experimentally observed yet because its tiny (4.5 eV) binding energy cannot be detected when the molecule annihilates by emitting two photons with energy of 0.51 MeV each. It is shown in this paper that the electric dipole transition between the recently found L=1L=1L=1 excited state and the L=0L=0L=0 ground state with its characteristic photon energy of 4.94 eV is a clear signature of the existence of the positronium molecule and the possibility of its experimental observation is realistic. The probability of this transition is about 17% of the total decay rate. Another Coulomb four-body system containing positron HPs (the positronium hydride or hydrogen positride), is also included for comparison.
Magnetised positronium is formed by impacting low energy positrons onto a gas covered target immersed in a magnetic field (B ≥ 1T). The resulting weakly bound positronium atoms subsequently travel some distance in an arrangement of Penning-type traps whereupon they can be field ionised. The remnant positrons are accumulated and then detected by forced annihilation on the target. The production efficiency of the magnetised atoms has been measured for different species of gases, gas layer thickness and the strength of the magnetic field. The positronium loss as a function of the distance travelled has been measured and is shown to be caused by the magnetron drift of the positronium atom.
We show that the annihilation dynamics of excited positronium (Ps) atoms can be controlled using parallel electric and magnetic fields. To achieve this, Ps atoms were optically excited to n=2 sublevels in fields that were adjusted to control the amount of short-lived and long-lived character of the resulting mixed states. Inclusion of the former offers a practical approach to detection via annihilation radiation, whereas the increased lifetimes due to the latter can be exploited to optimize resonance-enhanced two-photon excitation processes (e.g., 1^{3}S→2^{3}P→nS/nD), either by minimizing losses through intermediate state decay, or by making it possible to separate the excitation laser pulses in time. In addition, photoexcitation of mixed states with a 2^{3}S_{1} component represents an efficient route to producing long-lived pure 2^{3}S_{1} atoms via single-photon excitation.
The lifetime of positrons contained in an electrostatic trap is studied when small quantities of alkanes (i.e., hydrocarbon molecules of the form Cn{\mathrm{C}}_{\mathrm{n}}Cn​H2n+2{\mathrm{H}}_{2\mathrm{n}+2}H2n+2​) are added. As n is increased from 4 to 16, the annihilation rate per molecule is found to increase by a factor of 300 and then to level off. These data are interpreted as evidence for the existence of long-lived resonances and bound states of positrons and neutral molecules, and a model of the formation process is discussed.
The positronium ion (Ps−\mathrm{Ps}^-Ps−), a coulombic three-body bound state of two electrons and a positron, predominantly decays via electron-positron annihilation into electromagnetic final states. While its radiative decay channels have been extensively studied, much less attention has been given to weak processes in this system. In this work, we investigate the rare decay Ps−→e−νμνˉμ\mathrm{Ps}^- \to e^- ν_μ\barν_μPs−→e−νμ​νˉμ​, obtained by replacing the photon in Ps−→e−γ\mathrm{Ps}^- \to e^- γPs−→e−γ with a virtual ZZZ boson. Treating the three-body process as an effective two-body transition, Ps−→e−Z∗(→νμνˉμ)\mathrm{Ps}^- \to e^- Z^*\left(\to ν_μ\barν_μ\right)Ps−→e−Z∗(→νμ​νˉμ​), we compute the decay rate by explicitly evaluating all spin configurations of the initial bound state and final particles. The result agrees with that obtained using the standard spin-summation formalism of quantum field theory. We find that the branching ratio is comparable to that of the weak decay of ortho-positronium, o-Ps→γννˉ\mathrm{o\text{-}Ps} \to γν\barνo-Ps→γννˉ.