Radioactivity is the spontaneous transformation of an unstable atomic nucleus, accompanied by the emission of particles, electromagnetic radiation, or both. An atom undergoing such a transformation becomes a daughter nuclide, which may itself be radioactive or stable. Radioactivity is a nuclear property, whereas activity is the measurable rate of these transformations in a sample. It is distinct from radiation generally: radiation can also originate from sources that contain no radioactive material. (nrc.gov)
Discovery and historical development
In 1896, Henri Becquerel discovered that uranium compounds emitted penetrating radiation capable of darkening photographic plates without prior exposure to sunlight. Marie Curie subsequently introduced the term radioactivity. Working with Pierre Curie, she investigated radioactive minerals and identified polonium and radium in 1898. Their research demonstrated that the phenomenon was not confined to uranium. Becquerel and the Curies shared the 1903 Nobel Prize in Physics for their discoveries and investigations. (nobelprize.org)
In 1934, Irène Joliot-Curie and Frédéric Joliot demonstrated artificial radioactivity by producing radioactive nuclei through particle bombardment. This established that radioactive substances could be manufactured from initially nonradioactive materials, expanding the experimental and practical uses of radionuclides. They received the 1935 Nobel Prize in Chemistry for the synthesis of new radioactive elements. (nobelprize.org)
Nuclei and decay modes
A nucleus contains protons and neutrons. Its proton number determines the chemical element, while nuclei of the same element with different neutron numbers are isotopes. Radioactive nuclides have unstable nuclear configurations and transform through characteristic decay modes. A daughter nucleus may undergo further transformations, producing a decay chain that ultimately reaches a stable nuclide. (nrc.gov)
The principal modes include:
- Alpha decay: the nucleus emits an alpha particle, consisting of two protons and two neutrons. Its proton number decreases by two and its mass number by four. Alpha particles lose energy rapidly in matter and have relatively short ranges.
- Beta decay: in beta-minus decay, a neutron becomes a proton while an electron and an electron antineutrino are emitted. In beta-plus decay, a proton becomes a neutron while a positron and an electron neutrino are emitted. The mass number remains unchanged.
- Gamma emission: an excited nucleus releases excess energy as a high-energy photon. Its proton and neutron numbers remain unchanged. Gamma emission often follows another nuclear transformation. (nrc.gov)
Other modes include electron capture, in which a nucleus captures an atomic electron, and spontaneous fission, in which a heavy nucleus splits into lighter nuclei. The radiation emitted, its energy, and the probabilities of alternative decay pathways help characterize a radionuclide. (gnssn.iaea.org)
Decay law and half-life
The decay time of an individual nucleus is unpredictable, but a large population follows a regular statistical law. For a single radionuclide with constant decay probability and no replenishment, the expected number of undecayed nuclei is
[ N(t)=N_0e^{-\lambda t}, ]
where (N_0) is the initial number and (\lambda) is the decay constant. The activity is
[ A(t)=-\frac{dN}{dt}=\lambda N(t). ]
Thus, activity decreases exponentially as the parent nuclei disappear. (iaea.org)
The half-life, (T_{1/2}), is the time required for the expected population of parent nuclei to decrease by half:
[ T_{1/2}=\frac{\ln 2}{\lambda}. ]
After two half-lives, one-quarter remains; after three, one-eighth remains. This does not mean that the entire material vanishes: daughter atoms remain, and they may contribute additional activity. Samples containing several radionuclides therefore need not exhibit a single exponential decline in total activity. (iaea.org)
Measurement and detection
The SI unit of activity is the becquerel (Bq), equal to one nuclear transformation per second. The older curie (Ci) equals (3.7\times10^{10}) Bq. Activity measures transformation frequency, not the energy absorbed by a person or object. (epa.gov)
Absorbed dose measures deposited energy per unit mass and is expressed in grays: one gray equals one joule per kilogram. Equivalent and effective doses use sieverts and incorporate radiation weighting; effective dose additionally incorporates tissue weighting. Consequently, identical activities can produce different doses depending on radiation type, energy, geometry, shielding, and whether the source is outside or inside the body. (epa.gov)
Detection instruments measure radiation through its interactions with matter. Survey meters, including the Geiger counter, can register count rates or dose rates. Instruments used for radionuclide identification analyze radiation energy spectra. Their capabilities differ: an instrument suitable for detecting gamma radiation may not detect alpha particles effectively. (nrc.gov)
Natural occurrence and applications
Natural radioactivity includes long-lived primordial radionuclides such as uranium-238, thorium-232, and potassium-40, together with their decay products. Other radionuclides are continually generated by cosmic-ray interactions; carbon-14 forms in the atmosphere. Radioactive material occurs naturally in rocks, soil, food, and living organisms. Radon, a radioactive gas produced within natural decay chains, contributes to environmental radiation exposure. (iwaste.epa.gov)
Radiometric dating uses radioactive decay to determine ages from parent and daughter isotope abundances. Different isotope systems suit different materials and timescales. Carbon-14, with a half-life of approximately 5,730 years, is used to date formerly living material; other systems establish ages of rocks and minerals. (pubs.usgs.gov)
Artificially produced radionuclides serve as tracers in scientific research and medical imaging. Radioactive sources are also used in radiation therapy, industrial thickness and density gauges, smoke detectors, and spacecraft power supplies. These applications exploit either detectable emissions, energy deposition, or heat released during decay. (nrc.gov)