Radionuclide

A radionuclide is a nuclide that is unstable and known to undergo radioactive decay into a different nuclide, which may be another radionuclide or be stable. Radiation emitted by radionuclides is almost always ionizing radiation because it is energetic enough to liberate an electron from another atom.
Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single nuclide, the decay rate, and thus the half-life for that nuclide, can be calculated from the measurement of the decay. The range of the half-lives of radioactive atoms has no known limits and spans a time range of over 55 orders of magnitude.
All the chemical elements have radionuclides - even the lightest element, hydrogen, has one well-known radionuclide, tritium. Elements heavier than lead, and the elements technetium and promethium, have only radionuclides and do not exist in stable forms, though bismuth can be treated as stable with the half-life of its natural isotope being over a trillion times longer than the current age of the universe.

Production and effects

Artificial production methods of radionuclides include neutron sources such as nuclear reactors, as well as particle accelerators such as cyclotrons.
Exposure to radionuclides generally has, due to their radiation, a harmful effect on organisms including humans, although low levels of exposure occur naturally. The degree of harm will depend on the nature and extent of the radiation produced, the amount and nature of exposure, and the biochemical properties of the element. Increased risk of cancer is considered unavoidable, and worse cases experience radiation-induced cancer, chronic radiation syndrome or acute radiation syndrome. Radionuclides are weaponized by the fallout effects of nuclear weapons and by radiological weapons.
Radionuclides with suitable properties are used in nuclear medicine for both diagnosis and treatment. An imaging tracer made with radionuclides is called a radioactive tracer. Radionuclide therapy is a form of radiotherapy. A pharmaceutical drug made with radionuclides is called a radiopharmaceutical.

Origins

Overview

Radionuclides occur naturally and are artificially produced in nuclear reactors, cyclotrons, particle accelerators or radionuclide generators. There are 735 known radionuclides with half-lives longer than an hour ; 35 of those are primordial radionuclides whose presence on Earth has persisted from its formation, and another 62 are detectable in nature, continuously produced either as daughter products of primordial radionuclides or by cosmic radiation. More than 2400 radionuclides have half-lives less than 60 minutes. Most of those are only produced artificially, and have very short half-lives. For comparison, there are 251 stable nuclides.

Natural

On Earth, naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides.

Radionuclides are produced in stellar nucleosynthesis and supernova explosions along with stable nuclides. Most decay quickly, but some can be observed astronomically and can play a part in understanding astrophysical processes. Primordial radionuclides, such as uranium and thorium, still exist because their half-lives are so long that the Earth's initial content has not yet completely decayed. Some radionuclides have half-lives so long that decay has only recently been detected, and for most practical purposes they can be considered stable, most notably bismuth-209: detection of this decay meant that bismuth was no longer considered stable. It is possible that decay may be observed in other nuclides now considered stable, adding to the list of primordial radionuclides.
Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. They arise in the decay chain of the primordial isotopes thorium-232, uranium-238, and uranium-235 - such as the natural isotopes of polonium and radium - some are also produced by natural fission and other nucleogenic processes.
Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed on Earth, typically in the atmosphere, due to the action of cosmic rays.

Many of these radionuclides exist only in trace amounts in nature, including all cosmogenic nuclides. Secondary radionuclides in a decay chain will occur in proportion to their half-lives, so short-lived ones will be very rare. For example, polonium can be found in uranium ores at a concentration about 1 part 10¹⁰ of uranium by calculating the ratio of half-lives of polonium-210 to uranium-238, its ultimate parent.

Nuclear fission

Radionuclides are produced as an unavoidable result of nuclear fission and nuclear explosions. The process of nuclear fission creates a wide range of fission products, most of which are radionuclides. Further radionuclides are created from irradiation of the nuclear fuel and of the surrounding structures, yielding activation products. This complex mixture of radionuclides with different chemistries and radioactivity makes handling nuclear waste and dealing with nuclear fallout particularly problematic.

Synthetic

s are created in nuclear reactors or by particle accelerators or as decay products of such:

As well as being extracted from nuclear waste, radioisotopes can be produced deliberately with nuclear reactors, exploiting the high flux of neutrons present. These neutrons activate elements placed within the reactor. A typical product from a nuclear reactor is iridium-192, from activation of iridium targets. The elements that have a large propensity to take up neutrons in the reactor are said to have a high neutron cross-section, but even at low cross-sections this process is generally economical.
Particle accelerators such as cyclotrons accelerate particles to bombard a target to produce radionuclides. Cyclotrons accelerate protons at a target to produce positron-emitting radionuclides, e.g. fluorine-18.
Radionuclide generators, standard for many medical isotopes, contain a parent radionuclide that decays to produce a shorter-lived radioactive daughter. A typical example is the technetium-99m generator, which employs molybdenum-99 produced in a reactor.
Uses

Radionuclides are used in two major ways: either for their radiation alone or for the combination of chemical properties and their radiation. For scientific study they may be used for their chemical properties alone when there is no stable form of that element.

In biology, radionuclides can serve as radioactive tracers because they are chemically very similar to the nonradioactive nuclides, so most chemical, biological, and ecological processes treat them in a nearly identical way. One can then examine the result with a radiation detector, such as a Geiger counter, to determine where the provided atoms were incorporated. For example, one might culture plants in an environment in which the carbon dioxide contained radioactive carbon; then the parts of the plant that incorporate atmospheric carbon would be radioactive. Radionuclides can be used to monitor processes such as DNA replication or amino acid transport.
in physics and biology radionuclide X-ray fluorescence is used to determine chemical composition of the compound. Radiation from a radionuclide source hits the sample and excites characteristic X-rays in the sample. This radiation is registered and the chemical composition of the sample can be determined from the analysis of the measured spectrum. By measuring the energy of the characteristic radiation lines, it is possible to determine the proton number of the chemical element that emits the radiation, and by measuring the number of emitted photons, it is possible to determine the concentration of individual chemical elements.
In nuclear medicine, radioisotopes are used for diagnosis, treatment, and research. Radioactive chemical tracers emitting gamma rays or positrons can provide diagnostic information about internal anatomy and the functioning of specific organs, including the human brain. This is used in some forms of tomography: single-photon emission computed tomography and positron emission tomography scanning and Cherenkov luminescence imaging. Radioisotopes are also a method of treatment in hemopoietic forms of tumors; the success for treatment of solid tumors has been limited. More powerful gamma sources sterilise syringes and other medical equipment.
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. Food irradiation usually uses strong gamma emitters like cobalt-60 or caesium-137.
In industry, and in mining, radiation from radionuclides may be used to examine welds, to detect leaks, to study the rate of wear, erosion and corrosion of metals, and for on-stream analysis of a wide range of minerals and fuels.
In spacecraft, radionuclides are used to provide power and heat, notably through radioisotope thermoelectric generators and radioisotope heater units.
In particle physics, radionuclides help discover new physics by measuring the energy and momentum of their beta decay products.
In ecology, radionuclides are used to trace and analyze 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.
In geology, archaeology, and paleontology, natural radionuclides are used to measure ages of rocks, minerals, and fossil materials. This is called radiometric dating.
Examples

The following table lists properties of selected radionuclides illustrating the range of properties and uses.

Isotope	Z	N	half-life	DM	DE keV	Mode of formation	Comments	-
Tritium	1	2	12.3 y	β	19	Cosmogenic	lightest radionuclide, used in artificial nuclear fusion, also used for radioluminescence and as oceanic transient tracer. Synthesized from neutron bombardment of lithium-6 or deuterium	-
Beryllium-10	4	6	1,387,000 y	β	556	Cosmogenic	used to examine soil erosion, soil formation from regolith, and the age of ice cores	-
Carbon-14	6	8	5,700 y	β	156	Cosmogenic	used for radiocarbon dating	-
Fluorine-18	9	9	110 min	β, EC	633/1655	Cosmogenic	positron source, synthesised for use as a medical radiotracer in PET scans.	-
Aluminium-26	13	13	717,000 y	β, EC	4004	Cosmogenic	exposure dating of rocks, sediment	-
Chlorine-36	17	19	301,000 y	β, EC	709	Cosmogenic	exposure dating of rocks, groundwater tracer	-
Potassium-40	19	21	1.24 y	β, EC	1330 /1505	Primordial	used for potassium–argon dating, source of atmospheric argon, source of radiogenic heat, largest source of natural radioactivity	-
Calcium-41	20	21	99,400 y	EC		Cosmogenic	exposure dating of carbonate rocks	-
Cobalt-60	27	33	5.3 y	β	2824	Synthetic	produces high energy gamma rays, used for radiotherapy, equipment sterilisation, food irradiation	-
Krypton-81	36	45	229,000 y	β		Cosmogenic	groundwater dating	-
Strontium-90	38	52	28.8 y	β	546	Fission product	medium-lived fission product; probably most dangerous component of nuclear fallout	-
Technetium-99	43	56	210,000 y	β	294	Fission product	most common isotope of the lightest unstable element, most significant of long-lived fission products	-
Technetium-99m	43	56	6 hr	γ,IC	141	Synthetic	most commonly used medical radioisotope, used as a radioactive tracer	-
Iodine-129	53	76	15,700,000 y	β	194	Cosmogenic	longest lived fission product; groundwater tracer	-
Iodine-131	53	78	8 d	β	971	Fission product	most significant short-term health hazard from nuclear fission, used in nuclear medicine, industrial tracer	-
Xenon-135	54	81	9.1 h	β	1160	Fission product	strongest known "nuclear poison", with a major effect on nuclear reactor operation.	-
Caesium-137	55	82	30.2 y	β	1176	Fission product	other major medium-lived fission product of concern	-
Gadolinium-153	64	89	240 d	EC		Synthetic	calibrating nuclear equipment, bone density screening	-
Bismuth-209	83	126	2.01y	α	3137	Primordial	long considered stable, decay only detected in 2003	-
Polonium-210	84	126	138 d	α	5307	Decay product	highly toxic, used in poisoning of Alexander Litvinenko	-
Radon-222	86	136	3.8 d	α	5590	Decay product	gas, responsible for the majority of public exposure to ionizing radiation, second most frequent cause of lung cancer	-
Thorium-232	90	142	1.4 y	α	4083	Primordial	basis of thorium fuel cycle	-
Uranium-235	92	143	7y	α	4679	Primordial	fissile, main nuclear fuel	-
Uranium-238	92	146	4.5 y	α	4267	Primordial	main uranium isotope	-
Plutonium-238	94	144	87.7 y	α	5593	Synthetic	used in radioisotope thermoelectric generators and radioisotope heater units as an energy source for spacecraft	-
Plutonium-239	94	145	24,110 y	α	5245	Synthetic	used for most modern nuclear weapons	-
Americium-241	95	146	432 y	α	5486	Synthetic	used in household smoke detectors as an ionising agent	-
Californium-252	98	154	2.64 y	α/SF	6217	Synthetic	undergoes spontaneous fission, making it a powerful neutron source, used as a reactor initiator and for detection devices	-
Lutetium-177	71	106	6.6443 d	β⁻	497, 384, 176	Synthetic	used predominantly in targeted radionuclide therapy against somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors

Key: Z = atomic number; N = neutron number; DM = decay mode; DE = decay energy; EC = electron capture