Positrons

Positron Physics

Introduction - What is a positron?

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.

If the positron has a higher than thermal energy, it is probable that the electron from a target and 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.

Positrons are produced naturally in β⁺- decays or (at sufficiently high energies) in a pair production process where photons (in the presence of matter) produce electron-positron pairs.

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 is being constructed in Vienna.

The positron beamline

For the positron beam production, a β⁺- emitting sodium-22 source is used as the starting point. 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 neon moderator, in which the positrons quickly lose their energy in ionizing collisions (Epos: ~300 keV → ~eV) [4]. Once slowed, they can then 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 trap. This form of a Penning Malmberg trap [5] uses a strong solenoid magnetic field to confine the e⁺ 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, SF₆, is injected into the third stage of the trap to thermalise the positron bunch. 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.

Experiments

Molecules containing positronium

Numerous theoretical predictions of the properties of molecules containing positronium atoms have been made. [5] Despite this fact, there has only been one limited experiment that has been able to observe such a state [6]. In this experiment, conducted in 1992, PsH, the simplest of these bound states, was observed in collisions between methane gas and positrons.

Similarly, to that 1992 experiment, the Positron 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.

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 - B_Ps
dissociation via Ps formation	e⁺ + CH₄ → CH₃⁺+ H + Ps	7.55 eV
PsH formation	e⁺ + CH₄ → CH₃⁺+ PsH	7.55 eV - B_PsH

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 spectroscopy. The binding energy can be determined for different target gases and thus different molecules containing positronium can be investigated.