Positron

The positron, also referred to as the antielectron, is the antimatter counterpart of the electron. It possesses a positive electric charge of one elementary unit, a spin of ½, and a rest mass identical to that of the electron. When a positron encounters an electron, the two particles annihilate, typically producing two or more photons if the interaction occurs at low energies. Positrons can be formed through several processes, including positron emission during radioactive decay, pair production from high-energy photons interacting with matter, and energetic atmospheric or cosmic phenomena.

Theoretical Foundations

The theoretical basis for the positron emerged from developments in quantum mechanics and relativistic physics during the late 1920s. In 1928, Paul Dirac introduced an equation uniting quantum mechanics, special relativity and electron spin to explain phenomena such as the Zeeman effect. The Dirac equation admitted both positive and negative energy solutions, creating a conceptual difficulty since quantum theory did not allow negative energy states to be ignored. Dirac proposed that all negative energy states were filled in what became known as the Dirac sea, preventing transitions that would otherwise destabilise ordinary matter.
Initially, Dirac speculated that the proton might correspond to the negative-energy solution, but criticisms from contemporaries—including Robert Oppenheimer, who argued that this would make the hydrogen atom unstable, and Hermann Weyl, who demonstrated that the antiparticle must share the electron’s mass—rendered this interpretation untenable. By 1931, Dirac concluded that the equation predicted a new particle: an antielectron with the same mass as an electron but an opposite charge, capable of mutual annihilation with the electron.
Subsequent theoretical developments explored the implications of these solutions in new ways. Ernst Stückelberg and later Richard Feynman interpreted the positron as an electron travelling backward in time, a valid reformulation of the mathematical structure of quantum field theory. John Archibald Wheeler extended this idea to suggest that all electrons might be manifestations of a single particle tracing a convoluted worldline through time. Yoichiro Nambu applied similar reasoning to particle–antiparticle creation and annihilation processes, arguing that these events could be viewed as changes in the temporal direction of particle trajectories. Although conceptually striking, this interpretation remains a mathematical equivalence rather than a description of macroscopic cause and effect.

Experimental Clues and Discovery

Early particle detectors such as Wilson cloud chambers played a central role in the identification of the positron. Several researchers in the 1920s observed anomalous tracks resembling those of electrons but curving in the opposite direction under magnetic fields. Dmitri Skobeltsyn introduced the use of magnetic fields in cloud chambers in 1925 and noted charged particle cosmic rays, laying groundwork for later discoveries. Although some claims suggest he may have seen positron-like tracks, Skobeltsyn himself disputed such interpretations.
In 1929, Chung-Yao Chao at the California Institute of Technology recorded unusual behaviour in photon scattering experiments consistent with positively charged electron-like particles, but the results were not pursued further. Decades later, Carl Anderson acknowledged that Chao’s findings influenced his own thinking.
The positron was conclusively identified by Anderson on 2 August 1932. Using a cloud chamber combined with a lead plate and a magnetic field, he observed tracks with curvature corresponding to the electron’s mass-to-charge ratio but bending in the direction expected for a positive charge. He published the discovery later that year and was awarded the Nobel Prize in Physics in 1936. Although Anderson did not propose the term positron, he accepted it following a suggestion from the editor of Physical Review.
At the time of Anderson’s publication, Frédéric and Irène Joliot-Curie had already photographed similar tracks in Paris but had interpreted them as protons. Patrick Blackett and Giuseppe Occhialini in Cambridge also observed positrons in 1932 but delayed publication while gathering further evidence.

Natural Production Mechanisms

Positrons arise naturally in several contexts. In radioactive decay, isotopes undergoing β⁺ decay emit positrons along with neutrinos. An example is potassium-40, a long-lived primordial isotope present in trace amounts in the human body and responsible for a small but continuous flux of positron emission. Cosmic rays also generate many types of antiparticles, including positrons, through interactions with atmospheric nuclei.
Studies have shown that lightning-related gamma-ray flashes in the upper atmosphere can generate positrons, formed when electrons accelerated by strong electric fields produce high-energy photons that interact with air molecules. Measurements by specialised instruments have also detected antiprotons and other antiparticles in Earth’s magnetosphere.
In extremely high-temperature environments, matter and antimatter can form spontaneously once the mean particle energy exceeds the threshold for pair production. In the early universe, during the epoch of baryogenesis, matter and antimatter were created and annihilated continuously. The survival of matter and absence of free antimatter—referred to as baryon asymmetry—is attributed to violations of CP symmetry, though the precise mechanism remains unresolved.

Artificial and Mixed Sources

Positron production from β⁺ decay may be considered both natural and artificial, depending on whether the parent radioisotope is naturally occurring or synthesised. Potassium-40 remains the most prominent naturally occurring isotope that emits positrons. Although it comprises only a minute fraction of total potassium, it is the most abundant internal radioisotope in the human body. A person weighing around seventy kilograms experiences thousands of potassium-40 decays per second, although only a tiny proportion result in positron emission.
Positrons are also produced in numerous laboratory and technological processes. High-energy photon beams can induce pair production when incident on dense materials. Positron emission tomography (PET), a widely used medical imaging technique, exploits positron-emitting radioisotopes to map metabolic activity within the body.