Americium-241 (, Am-241) is an isotope of americium. Like all isotopes of americium, it is

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

, with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...

of . is the most common

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

americium Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was n ...

as well as the most prevalent isotope of americium in

nuclear waste Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapons ...

. It is commonly found in ionization type

smoke detector A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors are usually housed in plastic enclosures, typically shaped like a disk about in diameter and thick, but shape and size vary. Smoke can be detecte ...

s and is a potential fuel for long-lifetime

radioisotope thermoelectric generators A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioact ...

(RTGs). Its common parent nuclides are β⁻ from , EC from , and α from . is

fissile In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be t ...

and the

critical mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fi ...

of a bare sphere is and a sphere diameter of . Americium-241 has a specific activity of . It is commonly found in the form of americium-241 dioxide (). This isotope also has one

meta state A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have ...

, , with an excitation energy of and a

of . The presence of americium-241 in

plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exh ...

is determined by the original concentration of plutonium-241 and the sample age. Because of the low penetration of alpha radiation, americium-241 only poses a health risk when ingested or inhaled. Older samples of plutonium containing contain a buildup of . A chemical removal of americium-241 from reworked plutonium (e.g. during reworking of

plutonium pit Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits ...

s) may be required in some cases.

Nucleosynthesis

Americium-241 has been produced in small quantities in

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

s for decades, and many kilograms of have been accumulated by now. Nevertheless, since it was first offered for sale in 1962, its price, about per gram of , remains almost unchanged owing to the very complex separation procedure. Americium-241 is not synthesized directly from uranium – the most common reactor material – but from the plutonium isotope . The latter needs to be produced first, according to the following nuclear process: :

\mathrm

The capture of two neutrons by (a so-called (n,γ) reaction), followed by a β-decay, results in : :

\mathrm

The plutonium present in spent nuclear fuel contains about 12% of . Because it converts to , can be extracted and may be used to generate further . However, this process is rather slow: half of the original amount of decays to after about 14 years, and the amount reaches a maximum after 70 years. The obtained can be used for generating heavier americium isotopes by further neutron capture inside a nuclear reactor. In a

light water reactor The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...

(LWR), 79% of neutron captures on convert to and 10% to its

nuclear isomer A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have ...

: :79%:

\mathrm

Decay

Americium-241 decays mainly via

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

, with a weak

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

byproduct. The α-decay is shown as follows:

\mathrm

The α-decay energies are for of the time (the one which is widely accepted for standard α-decay energy), for 13% of the time, and for the remaining 2%. The γ-ray energy is for the most part, with little amounts of other energies such as , and . The second most common type of decay that americium-241 undergoes is

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakd ...

, with a branching ratio of 3.6×10⁻¹² and happening 1.2 times a second per gram of . It is written as such (the asterisk denotes an excited nucleus):

\mathrm

The least common (rarest) type of decay for americium-241 is

cluster decay Cluster decay, also named heavy particle radioactivity or heavy ion radioactivity, is a rare type of nuclear decay in which an atomic nucleus emits a small "cluster" of neutrons and protons, more than in an alpha particle, but less than a typic ...

, with a branching ratio of less than 7.4×10⁻¹⁶. It is written as follows:

\mathrm

Applications

Ionization-type smoke detector

Americium-241 is the only synthetic isotope to have found its way into the household, where the most common type of

(the ionization-type) uses (americium-241 dioxide) as its source of

ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...

. This isotope is preferred over because it emits 5times more alpha particles and relatively little harmful gamma radiation. With its half-life of , the americium in a smoke detector decreases and includes about 3%

neptunium Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after the planet Uranus, led to it bein ...

after , and about 5% after . The amount of americium in a typical new smoke detector is (about 1/3000 the weight of a small grain of

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...

) with an activity of . Some old industrial smoke detectors (notably from the Pyrotronics Corporation) can contain up to . The amount of declines slowly as it decays into

neptunium-237 Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be sy ...

(), a different transuranic element with a much longer half-life (about ). The radiated

alpha particles Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...

pass through an

ionization chamber The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...

, an air-filled space between two

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...

s, which allows a small, constant

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...

to pass between the capacitor plates due to the radiation ionizing the air space between. Any smoke that enters the chamber blocks/absorbs some of the alpha particles from freely passing through and reduces the ionization and therefore causes a drop in the current. The alarm's circuitry detects this drop in the current and as a result, triggers the piezoelectric buzzer to sound. Compared to the alternative optical smoke detector, the ionization smoke detector is cheaper and can detect particles which are too small to produce significant light scattering. However, it is more prone to

false alarms A false alarm, also called a nuisance alarm, is the deceptive or erroneous report of an emergency, causing unnecessary panic and/or bringing resources (such as emergency services) to a place where they are not needed. False alarms may occur with ...

Manufacturing process

The process for making the americium used in the buttons on ionization-type smoke detectors begins with americium dioxide. The is thoroughly mixed with gold, shaped into a briquette, and fused by pressure and heat at over . A backing of silver and a front covering of gold (or an alloy of gold or

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself ...

) are applied to the briquette and sealed by hot forging. The briquette is then processed through several stages of cold rolling to achieve the desired thickness and levels of radiation emission. The final thickness is about , with the gold cover representing about one percent of the thickness. The resulting foil strip, which is about wide, is cut into sections long. The sources are punched out of the foil strip. Each disc, about in diameter, is mounted in a metal holder, usually made of aluminium. The holder is the housing, which is the majority of what is seen on the button. The thin rim on the holder is rolled over to completely seal the cut edge around the disc.

RTG (radioisotope thermoelectric generator) power generation

As has a ''roughly'' similar half-life to (432.2 years vs. 87 years), it has been proposed as an active

radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioa ...

s, for use in spacecraft. Even though americium-241 produces less heat and electricity than

plutonium-238 Plutonium-238 (238Pu or Pu-238) is a fissile, radioactive isotope of plutonium that has a half-life of 87.7 years. Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suit ...

(the power yield is for ''vs.'' for ) and its radiation poses a greater threat to humans owing to gamma and neutron emission, it has advantages for long duration missions with its significantly longer half-life. The

European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (120 ...

is working on RTGs based on americium-241 for its space probes as a result of the global shortage of plutonium-238 and easy access to americium-241 in Europe from nuclear waste reprocessing. Its shielding requirements in an RTG are the second lowest of all possible isotopes: only requires less. An advantage over is that it is produced as nuclear waste and is nearly isotopically pure. Prototype designs of RTGs expect 2–2.2 W_e/kg for 5–50 W_e RTGs design, putting RTGs at parity with RTGs within that power range, as the vast majority of the mass of an RTG is not the isotopes, but the thermoelectrics, radiators, and isotope containment mass.

Neutron source

Oxides of pressed with

beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to for ...

can be very efficient neutron sources, since they emit

during

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

: :

\mathrm

Here americium acts as the alpha source, and beryllium produces neutrons owing to its large cross-section for the (α,n) nuclear reaction: :

\mathrm

The most widespread use of neutron sources is a

neutron probe A neutron probe is a device used to measure the quantity of water present in soil. A typical neutron probe contains a pellet of americium-241 and beryllium. The alpha particles emitted by the decay of the americium collide with the light berylliu ...

– a device used to measure the quantity of water present in soil, as well as moisture/density for quality control in highway construction. neutron sources are also used in well logging applications, as well as in neutron radiography, tomography, and other radiochemical investigations.

Production of other elements

Americium-241 is sometimes used as a starting material for the production of other transuranic elements and

transactinide Superheavy elements, also known as transactinide elements, transactinides, or super-heavy elements, are the chemical elements with atomic number greater than 103. The superheavy elements are those beyond the actinides in the periodic table; the l ...

s – for example, neutron bombardment of yields :

\mathrm

From there, 82.7% of decays to and 17.3% to : 82.7% →

\mathrm

17.3%→

\mathrm

In the nuclear reactor, is also up-converted by neutron capture to and , which transforms by β-decay to : :

\mathrm

Irradiation of by or ions yields the isotopes (

einsteinium Einsteinium is a synthetic element with the symbol Es and atomic number 99. Einsteinium is a member of the actinide series and it is the seventh transuranium element. It was named in honor of Albert Einstein. Einsteinium was discovered as a com ...

) or ( dubnium), respectively. Furthermore, the element

berkelium Berkelium is a transuranic radioactive chemical element with the symbol Bk and atomic number 97. It is a member of the actinide and transuranium element series. It is named after the city of Berkeley, California, the location of the Lawrence B ...

( isotope) had been first intentionally produced and identified by bombarding with alpha particles, in 1949, by the same Berkeley group, using the same cyclotron that had been used for many previous experiments. Similarly,

nobelium Nobelium is a synthetic chemical element with the symbol No and atomic number 102. It is named in honor of Alfred Nobel, the inventor of dynamite and benefactor of science. A radioactive metal, it is the tenth transuranic element and is the penul ...

was produced at the

Joint Institute for Nuclear Research The Joint Institute for Nuclear Research (JINR, russian: Объединённый институт ядерных исследований, ОИЯИ), in Dubna, Moscow Oblast (110 km north of Moscow), Russia, is an international research c ...

Dubna Dubna ( rus, Дубна́, p=dʊbˈna) is a town in Moscow Oblast, Russia. It has a status of ''naukograd'' (i.e. town of science), being home to the Joint Institute for Nuclear Research, an international nuclear physics research center and one o ...

, Russia, in 1965 in several reactions, one of which included irradiation of with ions. Besides, one of the synthesis reactions for lawrencium, discovered by scientists at Berkeley and Dubna, included bombardment of with .

Spectrometer

Americium-241 has been used as a portable source of both gamma rays and alpha particles for a number of medical and industrial uses. The gamma ray emissions from in such sources can be used for indirect analysis of materials in

radiography Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical radiography ("diagnostic" and "therapeu ...

and

X-ray fluorescence X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis ...

spectroscopy, as well as for quality control in fixed nuclear density gauges and nuclear densometers. For example, this isotope has been employed to gauge

glass Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...

thickness to help create flat glass. Americium-241 is also suitable for calibration of gamma-ray spectrometers in the low-energy range, since its spectrum consists of nearly a single peak and negligible Compton continuum (at least three orders of magnitude lower intensity).

Medicine

Gamma rays from americium-241 have been used to provide passive diagnosis of thyroid function. This medical application is now obsolete. Americium-241's gamma rays can provide reasonable quality radiographs, with a 10-minute exposure time. radiographs have only been taken experimentally due to the long exposure time which increases the effective dose to living tissue. Reducing exposure duration reduces the chance of ionization events causing damage to cells and DNA, and is a critical component in the ''"time, distance, shielding"'' maxim used in

radiation protection Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Expos ...

Hazards

Americium-241 has the same general hazards as other americium isotopes: it is both extremely toxic and radioactive. Although α-particles can be stopped by a sheet of paper, there are serious health concerns for ingestion of α-emitters. Americium and its isotopes are also very chemically toxic as well, in the form of heavy-metal toxicity. As little as is the maximum permissible body burden for . Americium-241 is an α-emitter with a weak γ-ray byproduct. Safely handling americium-241 requires knowing and following proper safety precautions, as without them it would be extremely dangerous. Its specific gamma dose constant is 3.14 x 10⁻¹ mR/hr/mCi or 8.48 x10⁻⁵ mSv/hr/MBq at . If consumed, americium-241 is excreted within a few days and only 0.05% is absorbed in the blood. From there, roughly 45% of it goes to the

liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...

and 45% to the bones, and the remaining 10% is excreted. The uptake to the liver depends on the individual and increases with age. In the bones, americium is first deposited over cortical and

trabecula A trabecula (plural trabeculae, from Latin for "small beam") is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ. A trabecula generally ha ...

r surfaces and slowly redistributes over the bone with time. The biological half-life of is in the bones and in the liver, whereas in the

gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sp ...

s (testicles and ovaries) it remains permanently; in all these organs, americium promotes formation of cancer cells as a result of its radioactivity.

Americium-241 often enters landfills from discarded

s. The rules associated with the disposal of smoke detectors are relaxed in most jurisdictions. In the U.S., the "Radioactive Boy Scout" David Hahn was able to concentrate americium-241 from smoke detectors after managing to buy a hundred of them at remainder prices and also stealing a few.

Ken Silverstein Ken Silverstein is an American journalist who worked for the '' Los Angeles Times'' as an investigative reporter, for The Associated Press in Brazil, and has written for ''Mother Jones'', ''Washington Monthly'', '' The Nation'', ''Slate'', ...

The Radioactive Boy Scout: When a teenager attempts to build a breeder reactor
''

Harper's Magazine ''Harper's Magazine'' is a monthly magazine of literature, politics, culture, finance, and the arts. Launched in New York City in June 1850, it is the oldest continuously published monthly magazine in the U.S. (''Scientific American'' is older, b ...

'', November 1998 There have been a few cases of exposure to americium-241, the worst case being that of Harold McCluskey who, at the age of 64, was exposed to 500 times the occupational standard for americium-241 as a result of an explosion in his lab. McCluskey died at the age of 75, not as a result of exposure, but of a

heart disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, h ...

which he had before the accident.

References

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International Atomic Energy Agency (IAEA) The International Atomic Energy Agency (IAEA) is an intergovernmental organization that seeks to promote the peaceful use of nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was established in 195 ...

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Nuclear Regulatory Commission The Nuclear Regulatory Commission (NRC) is an independent agency of the United States government tasked with protecting public health and safety related to nuclear energy. Established by the Energy Reorganization Act of 1974, the NRC began opera ...

, access-date = 2022-09-02 , language = en-us , archive-url = https://web.archive.org/web/20220808144429/https://www.nrc.gov/materials/miau/industrial.html , archive-date = 2022-08-08 , url-status = live , df = dmy-all Isotopes of americium Radioisotope fuels