Rydberg Positronium-electron scattering

In our last experiment, we showed how a curved quadrupole guide can be used to transport Rydberg Positronium (Ps*) around a 45 degree bend (blue path in picture below) into a region away from the positron beam line (yellow path below), which can then be implemented into a scattering region (PRA 95 053409). In addition, this new off-axis setup eliminated detection difficulties that were encountered in our previous straight guide experiments.

The scattering chamber setup is shown in the right hand side picture below. Guided Ps* atoms emerging from the end of the quadrupole are introduced into a scattering region where they collide with electrons that are thermoinically ejected (orange path) from a Molybdenum filament . This region is surrounded by gamma-ray detectors coupled to photomultiplier tubes (PMT) which record annihilation events inside the chamber. However, these PMTs in themselves are not sufficient to conclude what kind of interaction has taken place, we can only conclude that a Ps atom annihilated if an event is detected, but not if there were intermediate steps (such as charge exchange between the free electrons and the valence electron) before annihilation. To overcome this, we used a micro-channel plate (MCP) which has the benefit of being able to detect only certain charged particles (i.e. differentiating between electrons and positrons) depending on the electric fields applied in the detection region.

When the Ps*collides with free electrons a few different things can happen; the electron could break up the Ps* into electron and positron (ionisation: e^{-} + Ps^{*} \rightarrow e^{-} + e^{+} + e^{-} ) or, the positron from the Ps* will be stolen away by the incoming free electron, resulting in a new short lived Ps which annihilates before hitting the MCP, thus giving a diminished signal (charge exchange: e^{-} + Ps^{*} \rightarrow Ps + e^{-}). Additionally, Positronium and an electron can form a negative Ps ion (Ps¯ ), which is the bound state of a Ps atom and an orbiting electron, this ions have been shown to be quite unstable, having a lifetime of <1ns.

Above you can see some experimental Time of Flight (TOF) data showing the possibility of having detected a charge exchange process. The blue curve on the left panel is a TOF signal of guided Positronium atoms after hitting the MCP detector. The Ps* atoms have a mean flight time of around 10\mus. When we let the electron beam into the scattering region  we observe a suppression of the TOF spectra (green curve), this is due to the electric fields in front of the MCP detector, which should only allow positrons to be detected. The PMT signal for both cases (shown in the right panel) are the same indicating that this drop in MCP signal is in fact electron related, not as a result of less Positronium being guided. All that remains to do now is to determine the nature of this signal but nonetheless, it’s intriguing how readily Ps* interacts with electrons (maybe other particles too), especially when considering that a direct method of producing anti-Hydrogen is via charge exchange collisions between Ps* and antiproton ( Ps^{*} + \overline{p} \rightarrow \overline{H}+ e^{-} ).

Optimizing Positronium Rydberg manipulation

In recent experiments we showed how we successfully manipulated the trajectories of Rydberg Positronium atoms using electric field inside a curved quadrupole guide, which allows us to perform velocity selection on the Positronium atoms that enter the quadrupole field.

These curved guide experiments were performed by exciting Positronium to n = 14, which is a state that has a relatively low mean lifetime of ~8 \mus, which is comparable to the flight time of our experiment, meaning that many of the n = 14 atoms undergo radiative decay before they can be detected. However, when judging what the most efficient n for guiding is, we need to have into consideration a few other factors. The lifetime of different n states goes up as n^3, but the field-ionisation rate goes up as ~n^{-4}, and the dipole moment as n^2. Combining all of these factors and implementing them into trajectory-simulations based on the electric fields generated by the curved guide, we were able to determine that the most efficient n for curved guiding is n = 14, as it can be seen from the graph below.

These simulations agree with our data and were instrumental in determining the optimum parameters of the experiment.

We are currently working on implementing this off-axis curved guide into a scattering experiment using helium ions, below you can see at the bottom of the image the new scattering chamber that has been coupled to the Positronium Rydberg beam.