Positsioonium

Positsioonium is a bound state of an electron and its antiparticle, the positron. It is a short-lived exotic atom, often referred to as an exciton. Unlike ordinary atoms composed of protons and electrons, positsioonium consists of matter and antimatter. Upon formation, the electron and positron orbit each other, similar to how an electron orbits the nucleus in a hydrogen atom. The existence of this exotic atom was first predicted theoretically by physicist Stephen Hawking and others.

Positsioonium can exist in several states, analogous to the energy levels of atomic hydrogen. The most common states are singlet (para-positsioonium) and triplet (ortho-positsioonium). Para-positsioonium has a spin of 0, while ortho-positsioonium has a spin of 1. These different spin states have significantly different lifetimes. Para-positsioonium annihilates into two gamma-ray photons with a half-life of approximately 0.125 nanoseconds. Ortho-positsioonium, on the other hand, annihilates into three gamma-ray photons and has a much longer half-life of about 140 nanoseconds.

The study of positsioonium is important for testing fundamental physics theories, including quantum electrodynamics (QED). The precise measurement of its properties, such as its energy levels and decay modes, provides stringent tests of these theories. It is also used as a tool in material science to study defects and free volume in solid materials, as positrons injected into a material can form positsioonium with electrons within the material. The annihilation characteristics of this positsioonium then reveal information about the material's structure.