At the Stefan Meyer Institute (SMI) of the Austrian Academy of Sciences, researchers are building an experiment, which allows us to see how positrons interact with matter.

Created by: Alina Weiser, Christin Schuster, Clara Freytag, Viktoria Kabel

The positrons from the radioactive source are too energetic to be used directly in experiments. To slow them down, scientists are using a very thin layer of neon ice, also known as a "moderator". This requires a very cold surface (250°C colder than a standard freezer). While the positrons fly through the ice, they collide hundreds of times, losing a little bit of energy each time. However, the ice is not perfect but has impurities and holes, in which the positrons are destroyed while their energy is released as gamma rays (light). Only about 1% of the positrons make it through the ice but in return, their energy is about 100.000 times smaller.

As there are still positrons, which are moving too fast, one needs to get rid of those as well. To this end, magnetic fields are used, as these deflect charged particles such as the positrons. When the magnetic field is switched off, all particles are stopped in their flight. Once the magnetic field is turned on, the positrons with low energy make it through the halfpipe while those, which are moving too fast, are not deflected enough and crash against the wall.

In order to prepare the positrons perfectly for the experiment, they need to be collected in a trap. For that purpose, a strong magnetic field and a cascading electric field in the shape of the mountain in the video, are employed. The positrons entering the trap need to lose further energy. This is achieved with the help of a cloud of gas. Through a collision with the cloud, the positrons can pass on their energy to the molecules in the gas cloud - they excite the electrons or cause the molecule to oscillate or rotate.

There is also a unique process, in which the positron steals an electron from the atom. Then, the positron and electron circle each other without a heavy atomic core. This process constitutes the formation of a positronium. Positronium is like hydrogen (1 proton, 1 electron) but instead of the positive proton, it contains a positively charged positron. This atom is not very stable, destroys itself after a short amount of time, and produces light particles in the process.

Finally, if the positron loses energy, it falls down the "mountain" step by step until it is trapped at the lowest level. There, many positrons are collected and stored until they are needed.

The positron has now overcome all obstacles and has the right energy to interact with the molecules. At the Molecule Prom - that is, in the reaction chamber - the positron can interact with CH₄ (or other gases).

In this process, different reactions can take place:

1. CH₄ is ionised, that is, the positron knocks an electron out of the CH₄ molecule, and both of them fly away. A positively charged CH₄⁺ is left behind.

2. The positron steals and electron from the CH₄ and they form a positronium. This bond is destroyed after a short time and converted into light. Again, a CH₄⁺ ion is formed in the process.

3. Just as in the secod case, the positron steals an electron from the molecule and form positronium. However, the CH₄ molecule can also decay further, in which case a hydrogen atom can combine with the positronium, forming PsH. A CH₃⁺ molecule remains.

Many special molecules involving positronium, such as PsH, have been predicted theoretically but haven't been found so far. Our experiment is designed to create and thus confirm the existences of many different such molecules.

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