A
radionuclide, or a
radioactive nuclide, is an
atom with an unstable
nucleus,
characterized by excess energy available to be imparted either to a
newly created radiation particle within the nucleus or via
internal conversion. During this process, the radionuclide is said to undergo
radioactive decay, resulting in the emission of
gamma ray(s) and/or
subatomic particles such as
alpha or
beta particles.
[1] These emissions constitute
ionizing radiation. Radionuclides occur naturally, or can be produced artificially.
Radionuclides are often referred to by chemists and physicists as
radioactive isotopes or
radioisotopes. Radioisotopes with suitable
half-lives play an important part in a number of technologies (for example,
nuclear medicine). Radionuclides can also present both real and perceived dangers to health.
The number of radionuclides is uncertain because the number of very
short-lived
radionuclides that have yet to be characterized is extremely large and
potentially unquantifiable. Even the number of long-lived radionuclides
is uncertain (to a lesser degree), because many "stable" nuclides are
calculated to have half-lives so long that their decay has not been
experimentally measured. The total
list of nuclides contains 90 nuclides that are, in theory, stable, and 254 total
stable nuclides
that have not been observed to decay. In addition, however, there exist
34 primordial radionuclides that exist from the creation of the solar
system, plus another 50 radionuclides detectable in nature as daughters
of these, plus a few produced by cosmic radiation. Including artificial
radionuclides, about 650 radionuclides that have been experimentally
observed to decay, with half-lives longer than 60 minutes (see
list of nuclides
for this list). Of all these, less than a hundred natural radionuclides
are known (they have been observed on Earth, but not as a consequence
of human activity).
Including artificially produced nuclides, more than 3300 nuclides are
known (including ~3000 radionuclides), many of which (> ~2400) with
decay half-lives shorter than 60 minutes. This list expands as new
radionuclides with very short half-lives are characterized.
All
elements
form a number of radionuclides, although the half-lives of many are too
short for them to be observed in nature. Even the lightest element,
hydrogen, has a well-known radioisotope,
tritium. The heaviest elements (heavier than
bismuth)
exist only as radionuclides. For every chemical element, many
radioisotopes that do not occur in nature (due to short half lives or
the lack of an ongoing natural production mechanism), have been produced
artificially.
Origin
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 interiors of
stars and are still present as 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.
[2]
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. 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. Synthetic
isotopes are inherently not naturally occurring on Earth, but can be
created by
nuclear reactions.
Uses
Radionuclides are used in two major ways: for their chemical properties and as sources of
radiation. Radionuclides of familiar elements such as
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 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
nuclear medicine,
radioisotopes are used for diagnosis, treatment, and research.
Radioactive chemical tracers emitting gamma rays or positrons can
provide diagnostic information about a person's 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 scanning and
Cerenkov 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.
Radionuclides are also 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. Natural
radionuclides are used in
geology,
archaeology, and
paleontology to measure ages of rocks, minerals, and fossil materials.
Common examples
Americium-241
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, Am-241 decays by emitting
alpha particles and
gamma radiation to become
neptunium-237.
Most common household smoke detectors use a very small quantity of
Am-241 (about 0.29 micrograms per smoke detector) in the form of
americium dioxide.
Smoke detectors use Am-241 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.
[3][4]
Steps for creating americium-241
The
plutonium-241 is formed in any nuclear reactor by neutron capture from
uranium-238.
- 238U + n → 239U
- 239U → 239Np + e− + ν
e
- 239Np → 239Pu + e− + ν
e
- 239Pu + n → 240Pu
- 240Pu + n → 241Pu
This will decay both in the reactor and subsequently to form Am-241, which has a half-life of 432.2 years.
[5][6]
Gadolinium-153
The
Gd-153 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.
[7] It can also detect the loss of
calcium in the hip and back bones, allowing the ability to diagnose osteoporosis.
[8]
Dangers
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."
[9]
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
[10]), 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
[11] for the 905 nuclides with half-lives longer than one hour (including those that are stable), given in
list of nuclides.
| 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 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 |
Bioremediation
Lichens can accumulate radionuclides to a high level.
[12]
See also
Notes
- R.H. Petrucci, W.S. Harwood and F.G. Herring, General Chemistry (8th ed., Prentice-Hall 2002), p.1025-26
- Eisenbud, Merril; Gesell, Thomas F (1997-02-25). Environmental Radioactivity: From Natural, Industrial, and Military Sources. p. 134. ISBN 9780122351549.
- Smoke Detectors and Americium
- Office of Radiation Protection – Am 241 Fact Sheet – Washington State Department of Health
- "Smoke Detectors: Uses of Radioactive Isotopes". Chemistry Tutorial : Radioisotopes in Smoke Detectors. AUS-e-TUTE n.d. Retrieved March 30, 2011.
- Reaction in a Smoke Detector
- PNNL: Isotope Sciences Program – Gadolinium-153
- "Gadolinium". BCIT Chemistry Resource Center. British Columbia Institute of Technology. Retrieved 30 March 2011.
- . World Health Organization http://www.who.int/mediacentre/factsheets/fs371/en/. Retrieved January 27, 2014.
- "Cosmic Detectives". The European Space Agency (ESA). 2013-04-02. Retrieved 2013-04-15.
- Table data is derived by counting members of the list; see WP:CALC. References for the list data itself are given below in the reference section in list of nuclides
- Gadd, G. M. (2009). "Metals, minerals and microbes: Geomicrobiology and bioremediation". Microbiology 156 (3): 609. doi:10.1099/mic.0.037143-0.
References
External links
