Abstract
A study of positron capture in a two-(pressure) stage buffer gas accumulation apparatus is presented for a variety of species, including some molecules which are known to be either efficient for positron trapping, or are frequently used to cool the particles when held in these devices. Absolute accumulation efficiencies are reported for all species. A detailed optimisation procedure, which has identified the main processes responsible for positron capture and loss in the trap, has been deployed to explore accumulation efficiency as the gas pressure and the electrostatic well depth in the trap are systematically varied. Accumulation exploiting energy loss via molecular vibrational transitions has been observed for the first time for a number of gases, though at much lower efficiency than achieved using electronic excitation processes.
Highlights
The positron (e+), the antimatter counterpart of the electron (e−), has been probed and employed for a variety of studies, ranging from materials engineering to antihydrogen production
A study of positron capture in a two-(pressure) stage buffer gas accumulation apparatus is presented for a variety of species, including some molecules which are known to be either efficient for positron trapping, or are frequently used to cool the particles when held in these devices
C J Baker et al relatively large energy spread of the beam, the onset of an energy dependent loss mechanism can be observed at Ψ ∼ ≈ 40 eV, which continues to Ψ ∼ ≈ 10 eV and we suggest this is due to positronium formation via reaction 3, following comparison with cross sections collected by Petrovic et al [41], where the cross section for Ps formation peaks at 2–3 × 10−16 cm2 at around 20 eV
Summary
The positron (e+), the antimatter counterpart of the electron (e−), has been probed and employed for a variety of studies, ranging from materials engineering to antihydrogen production. Most of the BGT systems currently in use rely upon a sodium-22-based, rare gas solid-moderated [24, 25], e+ beam, with trapping accomplished by kinetic energy loss via excitation of the a1Π electronic transition in molecular nitrogen gas, resulting in confinement in a Penning–Malmberg trap (an arrangement of hollow cylindrical electrodes, appropriately electrically biased, immersed in a solenoidal magnetic field; see section 2) as developed by Surko and co-workers [26, 27] Such devices have a maximum accumulation efficiency of ∼30% 10%–20% is more typical due to competing optimisations dictated by experimental requirements.
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