Radioactive decay

Characteristics and common modes

Decay is stochastic at the level of a single nucleus but follows precise statistical rules for large numbers of atoms. The rate of disappearance of a radioactive species is described by exponential decay and summarized by a half-life, the characteristic time in which half the nuclei in a sample will have decayed. Typical modes of decay include:

• Alpha decay: emission of a helium nucleus (two protons and two neutrons), reducing atomic number and mass.
• Beta decay: conversion of a neutron to a proton or vice versa with emission of an electron or positron and a neutrino, changing the atomic number.
• Gamma decay: emission of high-energy photons as a nucleus relaxes from an excited state, usually without changing element identity.

History and naming

Natural radioactivity was identified in the late 19th century when emissions from certain minerals were detected. Becquerel pioneered early observations of spontaneous radiation, and the work of Marie Curie and Pierre Curie clarified and popularized the phenomenon. The term "radioactivity" was established around that time, and recognition of these discoveries followed in international awards and in the naming of measurement units.

Uses and examples

Radioactive isotopes and the radiation they emit have many practical applications. In medicine, selected radionuclides are used for diagnostic imaging and for targeted radiotherapy. In industry and science they serve as tracers, process gauges, and sterilization sources. Geochronology and archaeology use decay-based methods to date materials by comparing parent and daughter isotopes. Nuclear reactors and space power systems exploit controlled decay or fission for energy, and small devices like smoke detectors rely on weak radioactive sources for detection.

Safety, regulation and notable distinctions

The radiation produced by decay is ionizing and can damage living tissue; protection strategies include minimizing exposure time, maximizing distance, and using appropriate shielding. Regulatory frameworks control the production, transport and disposal of radioactive materials. Important conceptual distinctions are stability versus instability of isotopes, spontaneous versus induced transformations, and nuclear versus chemical change. Units and terminology in this field reflect its history and the scientists who contributed to its development.