In order to understand what positronium is, one first needs to understand a little of the history behind how it was discovered. Discovering and predicting this exotic atom was part of the nuclear research being conducted before and after World War II. The American scientist, Carl Andersen, is believed to have first predicted the existence of positronium at CalTech in 1932. The scientist Stjepan Mohovicic of Croatia, however, was the first to publish this prediction in a 1934 paper. In this paper, the atom was referred to as electrum. Positronium was first experimentally discovered at the Massachusettes Institute of Technology in 1951 by Dr. Martin Deusch.
Positronium, known by the symbol Ps, is an exotic atom. Exotic atoms are known for containing at least one particle of anti-matter. Positronium consists of one positron. A positron is an exotic particle of anti-matter with the opposite charge of an electron. Because the two particles have opposite charges, they are attracted to each other, but since one particle is made of matter while the other is made of anti-matter, these two particles will completely destroy each other.
Positronium, like all exotic atoms, is extremely unstable. This is because positronium is made up of both types of matter particles, matter and anti-matter. These two particles will destroy or annhilate each other completely, leaving behind only photons. Photons are packets of light that can behave as either pure energy with or as a particle with a wavelength. In the case of this particular element, its two particles typically annihilate each other within 125 ps. This annihilation produces two gamma ray photons. In experiments performed in vacuum, three gamma ray photons have been produced by this annihilation, but the process takes about 142 ns. Under varying circumstances, up to five gamma ray photons can be produced during annihilation. The difference is explained by the effect that vacuum has on the spin states of the electron and positron.
The energy levels and orbit of these two particles are the same as those of the ground state of positronium, just like hydrogen, has two possible configurations. Which configuration a particular atom of positronium will be found in depends on the spin orientations of the electron and positron relative to each other.
Although positronium can occur naturally in nature, its short life means that it is very rare. This means that most scientists who wish to experiment with this substance must create it. By firing positrons into mineral quartz, a workable amount of positronium can be produced.