A positron is the antimatter counterpart of the electron, having the same mass but equal and opposite charge. When positrons collide with matter, such as electrons, the two counterparts may annihilate, creating two photons in the process. However, this is only one of several possible processes. Once positrons start to interact with matter they may ecite atoms with some of their energy, free electrons, or even bind with electrons and entire atoms. The Exotic Atom group explores how positrons can bind with electrons to form positronium (Ps), a hydrogen-like atom that is very unstable but uniquely interesting.
Positronium
If a positron has enough energy to ionise electrons within a material it is probable that the electron from the target and the positron form a short-lived, quasi stable hydrogen like exotic atom called positronium. Depending on the alignment of the spins of the constituent particles this atom can be in two states, parallel with a lifetime of 142 ns or antiparallel with 125 ps.
Positron and Positronium physics have several applications in fields ranging from astrophysics [1] to material science [2] and medicine [3]. It is therefore important to obtain detailed information about the interactions between positrons and positronium with matter. In order to facilitate such experiments a positron beamline has been constructed in Vienna.
The positron beamline
In order to use positrons we need a beam with well defined properties. At MBI Vienna positron beam production begins with a β+ emitting sodium-22 radioactive source, whereby a proton turns into a neutron emitting a positron and a neutrino. The positrons from this decay are highly energetic (~300 keV) and therefore need to be slowed before they can be easily manipulated for further experiments.
This is achieved through a solid neon ice moderator, a several micron thick layer of solid gas, in which the positrons quickly lose their energy in ionizing and electronically exciting collisions (Epositron: ~300 keV → ~eV) [4]. Interacting antimatter with matter to cool it down seems counterintuitive, antimatter famously annihilating when it interacts with matter, but it is fairly efficient and easy to do. Once slowed, the positrons can be magnetically guided along the beamline. Any positrons that are still too fast are filtered using deflections coils, while the rest are led into a positron Surko buffer gas trap. This is a form of Penning Malmberg trap [5] which uses a strong solenoid magnetic field to confine the positrons radially, and an electric potential with three distinct regions to trap them axially. Positrons lose their energy in collisions by electronically exciting N2 buffer gas, which is introduced into the trap in small amounts.
A second cooling gas, CO2, is injected into the third stage of the trap to thermalise the positrons to room temperature. Here the positrons mainly lose their energy to vibrational excitations which take place far below the positronium formation threshold. Positrons are ‘stored’ in the third stage of the trap and can be extracted in bunches when necessary.
Positronium Compounds
Numerous theoretical predictions of the properties of positronium compounds have been made [5]. Despite this fact, there has only been one limited experiment that has been able to observe such a state independent of a medium [6]. In this experiment, conducted in 1992, PsH, the simplest of these bound states, was observed in collisions between methane gas and positrons.
Similarl to the 1992 experiment the Exotic Atom group aims to observe molecules containing positronium in collisions with target gases. In such collisions, several outcomes are possible. Some of these outcomes (example: for collisions with methane gas) are listed with their appearance energies in the table below.
| Selected processes of collisions of positrons e+ with methane gas | Onset energies | |
| Direct ionisation | e+ + CH4 → CH4++ e+ + e- | 12.98 eV |
| Positronium formation | e+ + CH4 → CH4++ Ps | 12.98 eV - BPs |
| Dissociation via Ps formation | e+ + CH4 → CH3+ + H + Ps | 7.55 eV |
| PsH formation | e+ + CH4 → CH3++ PsH | 7.55 eV - BPsH |
In this example PsH can be indirectly observed by detecting a CH3+ - ion below the energy threshold of dissociation via positronium formation. Additionally, the binding energy of such a molecule can be determined by comparing the fragment appearance energies for positrons and electrons. While the energies of positrons and electrons will be measured using a retarding field analyser, the ions will be identified using time of flight mass spectrometry. The binding energy can be determined for different target gases, and thus different molecules containing positronium can be investigated.
We have created an animated outreach video that explains our positronium-molecule experiments. You can find the video, an explanation here.
