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Journal of Physics Conference Series

Magnetised positronium

Dirk Peter van der Werf, C. J. Baker, David C. S. Beddows, P R Watkeys, Christopher Aled Isaac, Sean Kerrigan, M. Charlton, Helmut H. Telle

Published Jan 1, 2010• 4 citations
Publisher

Abstract

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.

DOI
10.1088/1742-6596/199/1/012005

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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.
Weakly bound positron–electron pairs have been created in vacuum following low energy positron bombardment of a surface held at a temperature close to 4 K. The pairs, which behave as magnetized positronium atoms in the strong (>1 T) magnetic fields used in this experiment, were detected following their field ionization using an arrangement of Penning traps. Yields, which at highest are around 5 × 10−6 per incident positron, are presented and compared with previous work. Measurements of the behaviour of the yield as the distance from the production target to the ionization well was varied are presented and discussed, as are results taken for a fixed well at different magnetic fields. Both data sets were found to be consistent with a model in which the positronium moves across the magnetic field lines with a constant drift speed.
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.
A new physical mechanism for positron accumulation is explained and demonstrated. Strongly magnetized Rydberg positronium is formed and then ionized, allowing us to trap equal numbers of either positrons or electrons over a wide range of conditions. Antiprotons are trapped, cooled, and stacked from the new Antiproton Decelerator facility for the first time. Combining positrons and antiprotons, we have demonstrated the first positron cooling of antiprotons. The cooling takes place in a 4.2 K, nested Penning trap where conditions are ideal for the eventual goal of the formation of antihydrogen.
Conclusions and OutlookCollisions involving positron and positronium are contributing to the understanding of general atomic physics phenomena as well as illuminating specific processes and interactions such as annihilation and exchange. It is hoped that the considerable effort presently aimed towards the production of large ensembles of cold positrons86 will result in beams of extremely well defined energy (≤kT) which should enable the detailed study of near-threshold phenomena, resonances, annihilation and positronic compounds.
We present a method for generating cold neutral atoms via charge exchange reactions between trapped ions and Rydberg positronium. The high charge exchange reaction cross section leads to efficient neutralisation of the ions and since the positronium-ion mass ratio is small, the neutrals do not gain appreciable kinetic energy in the process. When the original ions are cold the reaction produces neutrals that can be trapped or further manipulated with electromagnetic fields. Because a wide range of species can be targeted we envisage that our scheme may enable experiments at low temperature that have been hitherto intractable due to a lack of cooling methods. We present an estimate for achievable temperatures, neutral number and density in an experiment where the neutrals are formed at a milli-Kelvin temperature from either directly or sympathetically cooled ions confined on an ion chip. The neutrals may then be confined by their magnetic moment in a co-located magnetic minimum well also formed on the chip. We discuss general experimental requirements.