Positronium

Positronium

Positronium is an exotic atomic system consisting of a positron (the antiparticle of the electron) and an electron bound together into a quasi-stable state. The system is unique because it is composed entirely of leptons of opposite charge, which annihilate each other, producing gamma rays, after an average lifetime that depends on the specific state of the positronium atom. Positronium illustrates quantum mechanical principles and is important in the study of quantum electrodynamics (QED), particularly in the context of matter-antimatter interactions.

Properties

Positronium exists in two main forms, depending on the relative orientations of the spins of the positron and electron. These are singlet state para-positronium (p-Ps), where the spins are antiparallel and form a singlet state, and triplet state ortho-positronium (o-Ps), where the spins are parallel, forming a triplet state. Ortho-positronium has a longer lifetime than para-positronium because the annihilation of the triplet state into two gamma rays is forbidden by conservation laws, requiring a minimum of three gamma rays for annihilation, which is a less probable event.

Lifetime

The average lifetime of para-positronium is approximately 125 picoseconds, and it decays predominantly into two gamma rays. Ortho-positronium, on the other hand, has an average lifetime of about 142 nanoseconds and decays into three gamma rays. These lifetimes and decay modes are subject to slight modifications due to interactions with surrounding matter and external fields, which can be of interest in various research fields, including astrophysics and material science.

Production

Positronium can be produced in several ways, including through the interaction of positrons emitted by nuclear decay or generated by particle accelerators with dense media like metals or insulators. It can also form in less dense media, such as gases, but with lower efficiency. The study of positronium formation and decay provides insights into the electronic structure and defects in materials.

Applications

Positronium has applications in fundamental physics research, such as tests of quantum electrodynamics and measurements of the gravitational interaction of antimatter. In material science, positronium annihilation lifetime spectroscopy (PALS) is a technique used to study the size and distribution of nano-scale voids in materials. In medicine, positronium formation is a principle behind positron emission tomography (PET), a diagnostic imaging technique that uses positron annihilation to detect and image metabolically active tissues, such as tumors.

See Also

• Electron
• Positron
• Quantum electrodynamics
• Antimatter
• Gamma ray

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  Diagram of a positronium atom

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  Image of a positronium beam experiment