A radionuclide is an atom with an unstable nucleus. The radionuclide undergoes radioactive decay by emitting a gamma ray(s) and/or subatomic particles.
Radionuclides are often referred to by chemists and biologists as radioactive isotopes or radioisotopes, and play an important part in the technologies that provide us with food, water and good health. Radionuclides may occur naturally, but can also be artificially produced.
Artificially produced radionuclides can be produced by nuclear reactors, particle accelerators or by radionuclide generators.
Radioisotopes produced with nuclear reactors exploit the high flux of neutrons present. The neutrons are used to activate elements placed within the reactor. A typical product from a nuclear reactor is thallium-201.
Particle accelerators such as cyclotrons accelerate particles to bombard a target to produce radionuclides. Cyclotrons are used to accelerate protons at a target to produce positron emitting radioisotopes e.g. flourine-18.
Radionuclide generators contain a parent isotope that decays to produce a radioisotope. The parent is usually produced in a nuclear reactor. A typical example is the technetium-99m generator used in nuclear medicine. The parent produced in the reactor is molybdenum-99.
Radionuclides are used in two major ways: for their chemical properties and as sources of radiation.
Radionuclides originate ultimately from the interiors of stars. Some, such as uranium, were formed directly in stars, and are still present because their half-lives are so long that they have not yet completely decayed. Radiogenic isotopes, such as carbon-14, are present because they are formed by the decay of longer-lived elements (this is how all the helium currently available was formed: although it is not radioactive, it escapes from the Earth easily, so helium is obtained from underground reservoirs).
Trace radionuclides are those that occur in tiny amounts in nature either due to inherent rarity, or to half-lives that are significantly shorter than the age of the Earth. Synthetic isotopes are not naturally occurring on Earth, but they can be created by nuclear reactions.
Radionuclides of familiar elements such as carbon can serve as tracers because they are chemically very similar to the non-radioactive nuclides, so most chemical, biological, and ecological processes treat them in a near identical way. One can then examine the result with a radiation detector, such as a geiger counter, to determine where the atoms one has provided have ended up. For example, one might culture plants in an environment in which the carbon dioxide contained radioactive carbon; then the parts of the plant that had laid down atmospheric carbon would be radioactive.
In medicine, radionuclides are used for diagnosis and research. Radioactive chemical tracers emit gamma rays which provides diagnostic information about a person's anatomy and the functioning of specific organs. Radiotherapy also uses radiation in the treatment of some illnesses, such as cancer. More powerful gamma sources are used to sterilise syringes and other medical equipment. About one in two people in Western countries are likely to experience the benefits of nuclear medicine in their lifetime.
In food preservation, radiation is used to stop the sprouting of root crops after harvesting, to kill parasites and pests, and to control the ripening of stored fruit and vegetables.
In agriculture and animal husbandry, radionuclides also play an important role. They are used to produce high intake of crops, disease and weather resistant varieties of crops, to study how fertilisers and insecticides work, and to improve the production and health of domestic animals.
Industrially, and in mining, radionuclides are used to examine welds, to detect leaks, to study the rate of wear of metals, and for on-stream analysis of a wide range of minerals and fuels.
Most household smoke detectors contain the radionuclide americium formed in nuclear reactors, saving many lives.
Environmentally, radionuclides are used to trace and analyse pollutants, to study the movement of surface water, and to measure water runoffs from rain and snow, as well as the flow rates of streams and rivers.
Natural radionuclides can be used in archaeology and in paleontology to measure ages. When radioactive carbon, for example, is in the atmosphere, it rapidly becomes separated from its decay products. Once it is bound up in a solid, such as wood or paper, its decay products must remain in place. So by measuring how much of these decay products has accumulated, one can estimate the time when the carbon was captured into solid form.
If radionuclides are released into the environment, through accident, poor disposal, or other means, they can constitute real or perceived dangers from radioactive contamination.