Radionuclide

Not to be confused with radionucleotide.

A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom that has excess nuclear energy, making it unstable. This excess energy can either create and emit, from the nucleus, new radiation (gamma radiation) or a new particle (alpha particle or beta particle), or transfer this excess energy to one of its electrons, causing it to be ejected (conversion electron). During this process, the radionuclide is said to undergo radioactive decay. These emissions constitute ionizing radiation. The unstable nucleus is more stable following the emission, but sometimes will undergo further decay. 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 element the decay rate, and thus the half-life (t1/2) for that collection can be calculated from their measured decay constants. The duration of the half-lives of radioactive atoms have no known limits; the time range is over 55 orders of magnitude.

Radionuclides both occur naturally and are artificially made using nuclear reactors, cyclotrons, particle accelerators or radionuclide generators. There are about 650 radionuclides with half-lives longer than 60 minutes (see list of nuclides). Of these, 34 are primordial radionuclides that existed before the creation of the solar system, and there are another 50 radionuclides detectable in nature as daughters of these, or produced naturally on Earth by cosmic radiation. More than 2400 radionuclides have half-lives less than 60 minutes. Most of these are only produced artificially, and have very short half-lives. For comparison, there are about 254 stable nuclides.

All chemical elements have radionuclides. Even the lightest element, hydrogen, has a well-known radionuclide, tritium. Elements heavier than lead, and the elements technetium and promethium, exist only as radionuclides.

Radionuclides can have both beneficial and harmful effects on living organisms. Radionuclides with suitable half-lives are used in nuclear medicine for both diagnosis and treatment. An imaging tracer made with radionuclides is called a radioactive tracer. A pharmaceutical drug made with radionuclides is called a radiopharmaceutical.

Origin

Natural

Naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides. Primordial radionuclides, such as uranium and thorium, originate mainly from the interior of stars, and exist in present time since their half-lives are so long they have not yet completely decayed. Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays.

Synthetic

Synthetic radionuclides are artificially produced by human activity using nuclear reactors, particle accelerators or radionuclide generators:

• Radioisotopes produced with nuclear reactors exploit the high flux of neutrons present. These neutrons activate elements placed within the reactor. A typical product from a nuclear reactor is thallium-201 and iridium-192. The elements that have a large propensity to take up the neutrons in the reactor are said to have a high neutron cross-section.
• 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 contain a parent radionuclide that decays to produce a radioactive daughter. 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 produced as an unavoidable side-effect of nuclear and thermonuclear explosions.

Trace radionuclides are those that occur in tiny amounts in nature either due to inherent rarity or due to half-lives that are significantly shorter than the age of the Earth.

Uses

Radionuclides are used in two major ways: for their chemical properties and as sources of radiation.

• In biology, radionuclides of carbon 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.
• 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. This is used in some forms of tomography: single-photon emission computed tomography and positron emission tomography (PET) 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 biochemistry and genetics, radionuclides label molecules and allow tracing chemical and physiological processes occurring in living organisms, such as DNA replication or amino acid transport.
• 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 industry, and in mining, radionuclides 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 particle physics, radionuclides help discover new physics (physics beyond the Standard Model) 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.

Common examples

Americium-241

File:Americium-241.jpg

Americium-241 container in a smoke detector.

Americium-241 capsule as found in smoke detector. The circle of darker metal in the center is americium-241; the surrounding casing is aluminium.

Most household smoke detectors contain americium produced in nuclear reactors. The radioisotope used is americium-241. The element americium is created by bombarding plutonium with neutrons in a nuclear reactor. Its isotope americium-241 decays by emitting alpha particles and gamma radiation to become neptunium-237. Most common household smoke detectors use a very small quantity of ²⁴¹Am (about 0.29 micrograms per smoke detector) in the form of americium dioxide. Smoke detectors use ²⁴¹Am since the alpha particles it emits collide with oxygen and nitrogen particles in the air. This occurs in the detector's ionization chamber where it produces charged particles or ions. Then, these charged particles are collected by a small electric voltage that will create an electric current that will pass between two electrodes. Then, the ions that are flowing between the electrodes will be neutralized when coming in contact with smoke, thereby decreasing the electric current between the electrodes, which will activate the detector's alarm.

Steps for creating americium-241

The plutonium-241 is formed in any nuclear reactor by neutron capture from uranium-238.

1. 238U + n → 239U
2. 239U → 239Np + e− + ν
   e
3. 239Np → 239Pu + e− + ν
   e
4. 239Pu + n → 240Pu
5. 240Pu + n → 241Pu

This will decay both in the reactor and subsequently to form ²⁴¹Am, which has a half-life of 432.2 years.

Gadolinium-153

The ¹⁵³Gd isotope is used in X-ray fluorescence and osteoporosis screening. It is a gamma-emitter with an 8-month half-life, making it easier to use for medical purposes. In nuclear medicine, it serves to calibrate the equipment needed like single-photon emission computed tomography systems (SPECT) to make x-rays. It ensures that the machines work correctly to produce images of radioisotope distribution inside the patient. This isotope is produced in a nuclear reactor from europium or enriched gadolinium. It can also detect the loss of calcium in the hip and back bones, allowing the ability to diagnose osteoporosis.

Impacts on organisms

Radionuclides that find their way into the environment may cause harmful effects as radioactive contamination. They can also cause damage if they are excessively used during treatment or in other ways exposed to living beings, by radiation poisoning. Potential health damage from exposure to radionuclides depends on a number of factors, and "can damage the functions of healthy tissue/organs. Radiation exposure can produce effects ranging from skin redness and hair loss, to radiation burns and acute radiation syndrome. Prolonged exposure can lead to cells being damaged and in turn lead to cancer. Signs of cancerous cells might not show up until years, or even decades, after exposure."

Summary table for classes of nuclides, "stable" and radioactive

Following is a summary table for the total list of nuclides with half-lives greater than one hour. Ninety of these 905 nuclides are theoretically stable, except to proton-decay (which has never been observed). About 254 nuclides have never been observed to decay, and are classically considered stable.

The remaining 650 radionuclides have half-lives longer than 1 hour, and are well-characterized (see list of nuclides for a complete tabulation). They include 28 nuclides with measured half-lives longer than the estimated age of the universe (13.8 billion years), and another 6 nuclides with half-lives long enough (> 80 million years) that they are radioactive primordial nuclides, and may be detected on Earth, having survived from their presence in interstellar dust since before the formation of the solar system, about 4.6 billion years ago. Another ~51 short-lived nuclides can be detected naturally as daughters of longer-lived nuclides or cosmic-ray products. The remaining known nuclides are known solely from artificial nuclear transmutation.

Numbers are not exact, and may change slightly in the future, as "stable nuclides" are observed to be radioactive with very long half-lives.

This is a summary table for the 905 nuclides with half-lives longer than one hour (including those that are stable), given in list of nuclides.

Stability class	Number of nuclides	Running total	Notes on running total
Theoretically stable to all but proton decay	90	90	Includes first 40 elements. Proton decay yet to be observed.
Energetically unstable to one or more known decay modes, but no decay yet seen. Spontaneous fission possible for "stable" nuclides ≥ niobium-93; other mechanisms possible for heavier nuclides. All considered "stable" until decay detected.	164	254	Total of classically stable nuclides.
Radioactive primordial nuclides.	34	288	Total primordial elements include uranium, thorium, bismuth, rubidium-87, potassium-40 plus all stable nuclides.
Radioactive nonprimordial, but naturally occurring on Earth.	~ 51	~ 339	Carbon-14 (and other isotopes generated by cosmic rays) and daughters of radioactive primordial elements, such as radium, polonium, etc.
Radioactive synthetic (half-life ≥ 1.0 hour). Includes most useful radiotracers.	556	905	These 905 nuclides are listed in the article List of nuclides.
Radioactive synthetic (half-life < 1.0 hour).	>2400	>3300	Includes all well-characterized synthetic nuclides.

List of commercially available radionuclides

This list covers common isotopes, most of which are available in very small quantities to the general public in most countries. Others that are not publicly accessible are traded commercially in industrial, medical, and scientific fields and are subject to government regulation. For a complete list of all known isotopes for every element (minus activity data), see List of nuclides and Isotope lists. For a table, see Table of nuclides.

Gamma emission only

Isotope	Activity	Half-life	Energies (keV)
Barium-133	9694 TBq/kg (262 Ci/g)	10.7 years	81.0, 356.0
Cadmium-109	96200 TBq/kg (2600 Ci/g)	453 days	88.0
Cobalt-57	312280 TBq/kg (8440 Ci/g)	270 days	122.1
Cobalt-60	40700 TBq/kg (1100 Ci/g)	5.27 years	1173.2, 1332.5
Europium-152	6660 TBq/kg (180 Ci/g)	13.5 years	121.8, 344.3, 1408.0
Manganese-54	287120 TBq/kg (7760 Ci/g)	312 days	834.8
Sodium-22	237540 Tbq/kg (6240 Ci/g)	2.6 years	511.0, 1274.5
Zinc-65	304510 TBq/kg (8230 Ci/g)	244 days	511.0, 1115.5
Technetium-99m	1.95×104 TBq/g (5.27 × 107 Ci/g)	6 hours	140

Beta emission only

Isotope	Activity	Half-life	Energies (keV)
Strontium-90	5180 TBq/kg (140 Ci/g)	28.5 years	546.0
Thallium-204	17057 TBq/kg (461 Ci/g)	3.78 years	763.4
Carbon-14	166.5 TBq/kg (4.5 Ci/g)	5730 years	49.5 (average)
Tritium (Hydrogen-3)	357050 TBq/kg (9650 Ci/g)	12.32 years	5.7 (average)

Alpha emission only

Isotope	Activity	Half-life	Energies (keV)
Polonium-210	166500 TBq/kg (4500 Ci/g)	138.376 days	5304.5
Uranium-238	12580 KBq/kg (0.00000034 Ci/g)	4.468 billion years	4267

Multiple radiation emitters

Isotope	Activity	Half-life	Radiation types	Energies (keV)
Caesium-137	3256 TBq/kg (88 Ci/g)	30.1 years	Gamma & beta	G: 32, 661.6 B: 511.6, 1173.2
Americium-241	129.5 TBq/kg (3.5 Ci/g)	432.2 years	Gamma & alpha	G: 59.5, 26.3, 13.9 A: 5485, 5443

See also

• List of nuclides shows all radionuclides with half-life > 1 hour
• Hyperaccumulators table – 3
• Radioactivity in biology
• Radiometric dating
• Radionuclide cisternogram
• Uses of radioactivity in oil and gas wells

External links

• EPA – Radionuclides – EPA's Radiation Protection Program: Information.
• FDA – Radionuclides – FDA's Radiation Protection Program: Information.
• Interactive Chart of Nuclides – A chart of all nuclides
• National Isotope Development Center – U.S. Government source of radionuclides – production, research, development, distribution, and information
• The Live Chart of Nuclides – IAEA