Positronium (Ps) is an unstable system consisting of an electron and its anti-particle. These two are together called an ‘exotic atom’. Being unstable, after an average lifetime of 125 ps (Pico seconds) they start to annihilate each other and produce two gamma ray photons. Let’s compare the system to a hydrogen atom. It’s similar in the way of energy levels but has a smaller mass, resulting in the frequencies associated with the spectral lines being less than half of those in the hydrogen atom. What’s genuinely interesting about this is, generally speaking, when a particle and its antiparticle get close, they will annihilate each other.This obviously isn’t the case with positronium. Really, there’s only a chance at any given time of a particle and its corresponding antiparticle annihilating one another.
Scientist Stjepan Mohorovii of Croatia is responsible for predicting the existence of positronium in 1934. He wrote a paper that was published in Astronomische Nachrichten. In the paper he referred to Positronium ‘electrum’. It’s been debated that Carl Anderson predicted the existence in 1932. The actual discovery is accredited to Martin Deutsch at MIT in 1951 it was then that the system was names positronium.
Positronium has 3 possible states, ground, singlet and triplet. In the ground state the system could be configured in two different ways, depending on the orientations of the spin of the two atoms within. The second state is singlet. Positronium in this stage is also called para-positronium and has antiparallel spins. The lifetime averages around 125 ps. It decays preferentially into two gamma quanta with energy of 511 keV each. Para-positronium can decay into any even number of photons but likelihood decreases as the number increases in size. The triplet state or ortho-positronium has parallel spins. In a vacuum it has a mean lifetime of 142.05±0.02 ns. The leading mode of decay is three gamma quanta. Other modes of decay are insignificant. For example, the five photons mode has a branching ratio of ~1.0×10−6.
The similarities between hydrogen allows for a rough estimate in the way of energy levels. The precise answer is obtained using the Bethe–Salpeter equation. The Bethe–Salpeter equation was first published in 1950. It described the bound states of a two-body quantum field theoretical system in a relativistically covariant formalism. This equation is used in many aspects of physics and comes in many different forms. This is used in determining the energy levels in positronium because positronium is the bound state of an electron–positron pair.
There are currently many interesting studied involving positronium, including making experiments with a high density positronium gas.