Radioisotope

A radioisotope (radionuclide, radionuclide, radioactive nuclide or radioactive isotope) is an atom that has excess nuclear energy, making it unstable. This excess energy can be used in three ways: emitted from the core as gamma radiation; transferred to one of its electrons to release it as an internal conversion electron; or used to create and emit a new particle (alpha particle or beta particle) from the nucleus. During these processes, the radioisotope is said to undergo radioactive decay. These emissions are considered ionizing radiation because they are powerful enough to liberate an electron from another atom. Radioactive decay can produce a stable isotope or sometimes produces a new unstable radioisotope that can undergo further decay. Radioactive decay is a random process at the level of individual atoms: it is impossible to predict when any particular atom will decay. However, for a collection of atoms of a single element, the rate of decay, and therefore half life. (t_1/2) for that collection can be calculated from its measured decay constants. The range of half-life of radioactive atoms has no known limits and spans a time range of more than 55 orders of magnitude.

Radioisotopes are produced naturally or artificially in nuclear reactors, cyclotrons, particle accelerators, or radioisotope generators. There are about 730 radioisotopes with half-lives greater than 60 minutes (see list of radioisotopes). Thirty-two of them are primitive radioisotopes that were created before the earth was formed. At least 60 other radioisotopes are detectable in nature, either as children of primordial radioisotopes or as radioisotopes produced through natural production on Earth by cosmic radiation. More than 2,400 radioisotopes have a half-life of less than 60 minutes. Most of them are produced only artificially and have a very short half-life. For comparison, there are about 252 stable isotopes. (In theory, only 146 of them are stable, and the other 106 are thought to decay (alpha decay or beta decay or double beta decay or electron capture or double electron capture)).

All chemical elements can exist as radioisotopes. Even the lightest element, hydrogen, has a well-known radioisotope, tritium. Elements heavier than lead, and the elements technetium and promethium, exist only as radioisotopes. (In theory, elements heavier than dysprosium exist only as radioisotopes, but the half-lives of some of these elements (for example, gold and platinum) are too long to find.)

Unplanned exposure to radioisotopes generally has a deleterious effect on living organisms, including humans, although low levels of exposure occur naturally and harmlessly. The degree of damage will depend on the nature and extent of the radiation produced, the amount and nature of the exposure (close contact, inhalation or ingestion) and the biochemical properties of the element, the most common consequence being an increased risk of cancer.. However, radioisotopes with suitable properties are used in nuclear medicine for both diagnosis and treatment. An image tracer made with radioisotopes is called a radioactive tracer. A pharmaceutical drug made with radioisotopes is called a radiopharmaceutical.

Origin

Natural

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

• Radioisotopes occur in star nucleosynthesis and supernova explosions along with stable isotopes. Most of them disintegrate quickly, but they can still be seen astronomically and can play a role in understanding astronomical processes. Primary radioisotopes, such as uranium and torium, exist today because their average lives are so long (56 billion years) that they have not yet completely disintegrated. Some radioisotopes have such a long average life (many times the age of the universe) that disintegration has only been detected recently, and for most practical purposes they can be considered stable, more notably the bish 209: the detection of this disintegration meant that the bish was no longer considered stable. The disintegration that can be observed in other isotopes that are added to this list of primary radioisotopes.
• Side radioisotopes are radiogenic isotopes derived from the disintegration of primary radioisotopes. They have a shorter half-life than the primary radioisotopes. They emerge in the chain of disintegration of the primary torium isotopes-232, uranium-238 and uranium-235. Examples include natural isotopes of polonium and radio.
• Cosmogenic Isotopes, such as carbon-14, are present because they are continually forming in the atmosphere due to cosmic rays.

Many of these radioisotopes exist in only trace amounts in nature, including all cosmogenic isotopes. Secondary radioisotopes will be produced in proportion to their half-life, so short-lived ones will be very rare. Thus, polonium can be found in uranium ores at about 0.1 mg per metric ton. (1 part in 10¹⁰). More radioisotopes can occur in nature in virtually undetectable quantities as a result of rare events such as spontaneous fission or unusual cosmic ray interactions.

Nuclear Fission

Radioisotopes are produced as the inevitable result of nuclear fission and thermonuclear explosions. The nuclear fission process creates a wide range of nuclear fission products, most of which are radioisotopes. Further radioisotopes can be created from the irradiation of nuclear fuel (creating a range of actinides) and from surrounding structures, producing activation products. This complex mix of radioisotopes with different chemistries and radioactivity makes the management of nuclear waste and the treatment of radioactive fallout particularly problematic.

Synthetic

Isotope artificial americ-241 emitting alpha particles inserted into a fog chamber for display

Synthetic radioisotopes are deliberately synthesized using nuclear reactors, particle accelerators, or radioisotope generators:

• In addition to being extracted from nuclear waste, radioisotopes can be deliberately produced with nuclear reactors, exploiting the high flow of neutrons present. These neutrons activate elements located inside the reactor. A typical product of a nuclear reactor is iridium-192. It is said that the elements that have a great propensity to absorb neutrons in the reactor have a cross section of the high neutron.
• Particle accelerators such as cyclotron accelerate particles to bomb a target and produce radioisotopes. Cyclotrons accelerate protons in a target to produce postitron radioisotopes, for example fluoride-18.
• Radioisotope generators contain a parent radioisotope that breaks down to produce a radioactive daughter. The matrix is usually produced in a nuclear reactor. A typical example is the technology-99m generator used in nuclear medicine. The father produced in the reactor is molybdenum-99.

Uses

Radioisotopes are used in two main ways: either by their radiation alone (irradiation, nuclear batteries) or by a combination of their chemical properties and radiation (tracers, biopharmaceuticals).

• In biology, carbon radioisotopes can serve as radioactive tracers because they are chemically very similar to non-radioactive isotopes, so most chemical, biological and ecological processes treat them in an almost identical way. One can then examine the result with a radiation detector, such as a Geiger counter, to determine where the supplied atoms were incorporated. For example, plants could be cultivated in an environment where carbon dioxide contains radioactive carbon; then the parts of the plant that incorporate atmospheric carbon would be radioactive. Radioisotopes can be used to monitor processes such as DNA replication or amino acid transport.
• In nuclear medicine, radioisotopes are used for diagnosis, treatment and research. Radioactive chemical tracers that emit gamma or positron rays 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: computed monophotonic emission tomography, postitron emission tomography (PET) and Cherenkov luminescence Images. Radioisotopes are also a method of treatment in the hematopoietic forms of tumors; the success of the treatment of solid tumors has been limited. The most powerful gamma rays sterilize syringes and other medical equipment.
• In food conservation, radiation is used to stop root crop outbreak after harvest, to kill parasites and pests, and to control the ripening of stored fruits and vegetables.
• In industry and mining, radioisotopes are used to examine welds, detect leaks, study the rate of wear, erosion and corrosion of metals, and for the analysis of a wide range of minerals and fuels.
• In spacecraft and elsewhere, radioisotopes are used to supply energy and heat, in particular through radioisotopic thermoelectric generators (RTGs).
• In astronomy and cosmology, radioisotopes play a role in understanding the star and planetary process.
• In particle physics, radioisotopes help to discover new physics (physics beyond the Standard Model) by measuring the energy and timing of their beta disintegration products.
• In ecology, radioisotopes are used to track and analyze polluting agents, to study the movement of surface waters and to measure rain and snow runoffs, as well as streams and rivers.
• In geology, archaeology, and paleontology, natural radioisotopes are used to measure the ages of rocks, minerals and fossil materials.

Examples

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

Legend: Z = atomic number; N = neutron number; MD = decay mode; ED = decay energy; EC = electronic capture; FE = spontaneous fission; IC: internal conversion

Smoke detectors in the home

Americ container-241 in a smoke detector.

The americo-241 capsule as found in the smoke detector. The darkest metal circle in the center is americ-241; the surrounding housing is aluminum.

Radioisotopes are present in many homes, as they are used inside the most common household smoke detectors. The radioisotope used is americium-241, which is created by bombarding plutonium with neutrons in a nuclear reactor. It decays by emitting alpha particles and gamma radiation to become neptunium-237. Smoke detectors use a very small amount of ²⁴¹Am (approximately 0.29 micrograms per smoke detector) in the form of americium dioxide. ²⁴¹Am is used for this because it emits alpha particles that ionize the air in the detector's ionization chamber. A small electrical voltage is applied to the ionized air which gives rise to a small electrical current. In the presence of smoke, some of the ions are neutralized, thus decreasing the current, which activates the detector's alarm.

Impact on organisms

Radioisotopes entering the environment can cause harmful effects such as radioactive contamination. They can also cause harm if used excessively during treatment or if exposed to living things in other ways, from radiation poisoning. The potential damage to health from exposure to radioisotopes depends on a number of factors, and "can damage the functions of healthy tissues and organs. Radiation exposure can produce effects ranging from reddening of the skin and hair loss, to radiation burns and acute radiation syndrome. Prolonged exposure can lead to cell damage, which in turn can lead to cancer development. Signs of cancer cells may not show up for years, or even decades, after exposure."

Summary table of the classes of “stable” and radioactive isotopes

The following is a summary table of the total list of isotopes with half-lives greater than one hour. Ninety of these 989 isotopes are theoretically stable, except for proton decay (which has never been observed). About 252 isotopes have never been observed to decay and are considered classically stable.

The remaining tabulated radioisotopes have half-lives greater than 1 hour, and are well characterized (see list of isotopes for full tabulation). They include 30 isotopes with measured half-lives longer than the estimated age of the universe (13.8 billion years), and 4 other isotopes with half-lives long enough (>100 million years) to be radioactive primordial isotopes, and can be detected on Earth, having survived their presence in interstellar dust since before the formation of the solar system, some 4.6 billion years ago. More than 60 other short-lived isotopes can be detected naturally as daughters of longer-lived isotopes or cosmic ray products. The rest of the known isotopes are known only by artificial transmutation.

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

This is a summary table for the 989 isotopes with half-lives greater than one hour (including those that are stable), given in the isotope list.

List of commercially available radioisotopes

This list covers common isotopes, most of which are available in very small amounts to the general public in most countries. Others that are not publicly available are commercially marketed in the industrial, medical, and scientific fields and are subject to government regulation.

Gamma Emitters

Beta emitters

Alpha Emitters

Multiple emitters of radiation