Radioactivity

Radioactivity is the spontaneous decay of unstable atomic nuclei, accompanied by emission of ionizing radiation in the form of particles or photons. It was discovered in 1896 by Henri Becquerel and rapidly studied by Marie and Pierre Curie. Radioactive decay is probabilistic and is described by the half-life, the time for half the atoms in a sample to decay. Decay chains can produce a series of daughter nuclides until a stable nucleus is formed. The emitted radiation includes alpha particles (helium nuclei), beta particles (electrons or positrons), and gamma rays (high-energy photons); alpha and beta particles are charged and can be stopped by material shielding, while gamma rays require denser shielding.

Natural sources provide most exposure: primordial radionuclides in rocks, cosmic rays, and indoor radon gas. Human-made sources include nuclear reactors, particle accelerators, medical isotopes, and radiographic devices.

Measurements use activity in becquerels (decays per second) or curies; absorbed dose in grays; and equivalent or effective dose in sieverts, which account for biological impact. Common instruments include Geiger–Muller counters, scintillation detectors, and dosimeters.

Applications span energy, medicine, and science. Nuclear energy relies on fission of heavy isotopes; medicine uses radiopharmaceuticals for imaging and therapy (for example, PET with positron emitters); radiography and material testing employ X-rays and gamma rays; radiocarbon dating uses carbon-14; archaeology and geology apply various isotopes for dating and tracing material flows.

Health and safety emphasize minimizing dose: shielding, distance, and time, and adherence to radiation protection principles and regulations under ALARA.