1936 Nobel Prize in Physics(2)
Reason for Award
for the discovery of the positron (Phys. Rev.:43(1933) 491-498; Phys. Rev.:44(1933) 406-423)
Laureates
United States of America
Explanation
The electron has a positively charged twin called the "positron." Mr. Anderson filled a box with mist, surrounded it with a magnet, and watched cosmic rays leave tracks. He noticed a track bending the opposite way from an electron; that was the positron. When a positron meets an electron they disappear in a flash of light. The discovery showed that every bit of matter can have a mirror-image twin.
Related Keywords
positron
The positron is a lepton with the same mass as the electron but a positive charge. Its existence was theoretically predicted by Dirac’s relativistic quantum mechanics. Anderson confirmed it by observing a track bending opposite to an electron in a cloud chamber with a magnetic field. When a positron annihilates with an electron, two 511 keV gamma rays are emitted; this principle underlies PET medical imaging. Verification of antimatter spurred research into why the universe shows a matter–antimatter imbalance.
cloud chamber
A cloud chamber visualizes charged-particle tracks as droplets condense in a supersaturated vapor. Invented by C.T.R. Wilson in 1901, it became a mainstay of early particle physics. Applying a magnetic field allows determination of momentum and charge sign from track curvature. Anderson used cosmic rays in such a chamber to detect an unknown positively charged particle. Cloud chambers are still used in education and radiation demonstrations, preserving their historical significance.
antimatter
Antimatter consists of particles that have the same mass as ordinary matter but opposite charges and quantum numbers. When matter and antimatter meet they annihilate, converting their mass into energy. Big-bang models predict nearly equal amounts of matter and antimatter, yet the present universe is dominated by matter. This asymmetry is closely tied to CP-violation and baryogenesis mechanisms. Discovery of the positron launched antimatter research, leading to the creation and trapping of antiprotons and antihydrogen.
beta decay
Beta decay is a process in which a nucleus transforms into another by emitting an electron (β–) or a positron (β+) together with a neutrino. In β+ decay a proton turns into a neutron, producing a positron and serving as a natural source of positrons. Mediated by the weak interaction, beta decay is crucial in stellar energy transport and cosmic nucleosynthesis. The continuous beta spectrum hinted at the neutrino, hypothesized by Pauli in 1930. Positron-detection techniques form the basis of PET imaging and tracer studies that use β+-emitting isotopes.
pair production
Pair production is a process in which a high-energy photon converts into an electron–positron pair near an atomic nucleus; conservation laws require energies above 2m_ec² = 1.022 MeV. The Bethe–Heitler theory calculates its cross-section within quantum electrodynamics (QED), showing dependence on photon energy and nuclear charge. Pair production is crucial in the early stages of cosmic-ray showers and in shaping the spectra of γ-ray astronomical sources. Anderson’s observation of the positron indirectly supported the existence of the pair-production mechanism. Modern high-energy γ-ray telescopes employ converter-tracker designs that rely on pair production for detection.