Radioactivity is the spontaneous transformation of an unstable atomic nucleus into a different nuclear state or nuclide, accompanied by the emission of particles, electromagnetic radiation or both. The process occurs because the initial nuclear configuration is energetically or quantum mechanically unstable relative to an available final state. Common radioactive processes include alpha decay, beta minus decay, beta plus decay, electron capture and gamma emission from excited nuclei. Radioactive decay is fundamentally probabilistic: the exact decay time of an individual nucleus cannot generally be predicted, but the statistical behaviour of a large population follows well-defined decay laws.
Radioactive decay is commonly described by the exponential law \(N(t)=N_0e^{-\lambda t}\), where \(N\) is the number of undecayed nuclei, \(N_0\) is the initial number, \(t\) is time and \(\lambda\) is the decay constant. The activity is \(A=\lambda N\) and is measured in becquerels, with one becquerel corresponding to one nuclear decay per second. The half-life is the time required for half of a population of radioactive nuclei to decay and is related to the decay constant by \(T_{1/2}=\ln(2)/\lambda\). A half-life is a statistical property of the population and does not imply that every individual nucleus decays after the same interval.
Radioactivity can be classified according to the mechanism and emitted radiation. Alpha decay emits a helium-4 nucleus and reduces the parent mass number by four and atomic number by two. Beta minus decay converts a neutron into a proton while emitting an electron and an electron antineutrino. Beta plus decay converts a proton into a neutron while emitting a positron and an electron neutrino. Electron capture also converts a proton into a neutron by absorbing an orbital electron and emitting a neutrino. Gamma decay generally involves the transition of an excited nucleus to a lower-energy state through photon emission without changing the number of protons or neutrons.
Radioactive decay follows conservation laws involving energy, momentum, angular momentum and electric charge, while the appropriate baryon, lepton and other quantum numbers must also be considered. Neutrinos are essential to the description of beta processes because they carry away energy, momentum, angular momentum and lepton number. The continuous beta spectrum historically played an important role in establishing the existence of the neutrino. Nuclear decay energies are often distributed among multiple products rather than appearing entirely as one emitted particle's kinetic energy. Recoil of the daughter nucleus is also required by momentum conservation, although its energy is often comparatively small.
Natural radioactivity occurs in long-lived primordial radionuclides, radioactive decay products and some continuously produced isotopes. Uranium and thorium decay chains contain sequences of alpha and beta transformations that eventually lead towards stable nuclei. Potassium-40 is another naturally occurring radionuclide with a very long half-life. Cosmic radiation also produces radionuclides in Earth's atmosphere and environment. Human activities can create artificial radionuclides through reactors, accelerators and nuclear reactions, providing isotopes with useful decay properties for medicine, industry and scientific research.
Radioactivity has major medical applications. Diagnostic nuclear medicine uses radioactive tracers whose emissions can be detected to study biological processes, while radiotherapy uses ionising radiation to damage cancer cells. Different radionuclides are selected according to half-life, radiation type, particle energy, biological distribution and chemical behaviour. Technetium-99m, for example, has properties useful for diagnostic imaging, while other radionuclides can deliver therapeutic particle radiation. Safe clinical use requires careful control of administered activity, patient handling, shielding where appropriate and management of radioactive materials.
Radioactive radiation can be harmful because ionising radiation can transfer enough energy to alter molecular structures, including biological molecules and DNA. Risk depends on radiation type, absorbed dose, exposure geometry, dose rate, tissue sensitivity and other factors. Alpha particles have high ionisation density but limited penetration in tissue, whereas gamma rays and other penetrating radiation can travel much farther through matter. Shielding therefore depends on radiation type and energy; materials effective against one radiation may be inappropriate or insufficient for another. Radiation protection principles include justification, optimisation and limiting unnecessary exposure.
Radioactivity is distinct from irradiation and contamination. Irradiation means exposure to radiation and does not necessarily make an object radioactive, whereas radioactive contamination means that radioactive material is physically present where it is not intended to be. A radioactive source can emit radiation without external activation, while an irradiated object may cease receiving radiation immediately after the external source is removed. Radioactivity is also distinct from nuclear fission, which is a particular type of nuclear reaction and is not synonymous with radioactive decay. These distinctions are essential in medicine, industry, environmental monitoring and radiation safety.
