orthopositsioonium

Orthopositronium is an exotic atom formed by an electron and its antiparticle, a positron. It is one of the two possible bound states of electron-positron pairs, the other being parapositronium. In orthopositronium, the spins of the electron and positron are aligned parallel, resulting in a total spin of 1. This parallel spin alignment makes orthopositronium a triplet state, whereas parapositronium is a singlet state with antiparallel spins.

Orthopositronium is significantly more stable than parapositronium. Parapositronium, being a spin-singlet state, can annihilate into two gamma-ray photons, a process that is highly probable and occurs very quickly, with a lifetime of about 1.25 x 10^-10 seconds. Orthopositronium, due to its spin-triplet nature, cannot annihilate into two photons according to conservation laws. Instead, it primarily annihilates into three gamma-ray photons. This three-photon annihilation process is less probable and thus orthopositronium has a much longer lifetime, approximately 1.4 x 10^-7 seconds in vacuum.

The existence and properties of orthopositronium are crucial in various fields of physics, including quantum electrodynamics (QED) and in experimental techniques like positron annihilation spectroscopy. Positron annihilation spectroscopy uses the emitted gamma rays from orthopositronium decay to probe the electronic structure and defect properties of materials. The characteristic signature of orthopositronium in such experiments can indicate the presence of voids or free volume within a material. The precise measurement of orthopositronium decay rates has also served as a stringent test of QED predictions.