What Are Positrons and Positronium?

Positron

A positron is the antimatter counterpart of the electron. It has the same mass as an electron but carries an equal and opposite (positive) electric charge. Because of this, positrons interact with matter in a unique way:

When a positron encounters an electron, the two particles annihilate, converting their mass into energy in the form of 511 keV gamma-ray photons, typically emitted in opposite directions.

Positrons are produced naturally in β⁺ radioactive decay, and they can also be generated artificially in particle accelerators or positron beam systems.

Positrons play an essential role in various scientific fields:

Medical imaging (PET scans)

Materials science—especially in detecting atomic-scale defects, free volume, and vacancy structures

Fundamental physics research on antimatter, quantum electrodynamics (QED), and particle interactions

At Marmara Positron Laboratory (MARPOS), positrons are primarily used to probe the structural defects and free-volume holes inside materials using PALS and DBS techniques.

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Positronium

Positronium (Ps) is an exotic, hydrogen-like atom formed when a positron temporarily binds with an electron. Instead of an electron orbiting a proton (as in hydrogen), the electron and positron orbit their common center of mass.

This system is extremely short-lived because the two particles eventually annihilate, producing gamma photons. Its lifetime depends on its spin state:

Para-positronium (p-Ps)

Spins antiparallel (singlet state)

Short lifetime (~125 picoseconds in vacuum)

Annihilates mostly into two gamma photons

Ortho-positronium (o-Ps)

Spins parallel (triplet state)

Longer lifetime (~142 nanoseconds in vacuum)

Annihilates mostly into three gamma photons

In materials, its lifetime is reduced due to interactions with surrounding electrons

Because positronium is extremely sensitive to its environment, it becomes an excellent microscopic probe of nanometer- and angstrom-scale free volumes in materials. In polymers, membranes, porous materials, ceramics, and thin films, the o-Ps lifetime reveals the size and distribution of free-volume cavities—a unique capability not provided by any other technique.

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Why Positrons and Positronium Matter at MARPOS

At the Marmara Positron Laboratory, positrons and positronium are used as atomic-scale probes to reveal:

Vacancy defects

Free-volume holes (down to ~1 Å – 10 Å)

Structural relaxation

Dopant-induced microstructural changes

Radiation-induced damage

Depth-dependent defect profiles in thin films and multilayers

MARPOS can map defect structures in both bulk materials and thin-film systems using Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler Broadening Spectroscopy (DBS). These techniques give information that other common methods like SEM, XRD, and TEM can't.

These methods make MARPOS one of the few laboratories in Türkiye capable of performing such high-sensitivity defect characterization.