Am-241

Americium-241 (Am, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of . Am is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It is commonly found in ionization type smoke detectors and is a potential fuel for long-lifetime radioisotope thermoelectric generators (RTGs). Its common parent nuclides are β from Pu, EC from Cm, and α from Bk. Am is not fissile, but is fissionable, and the critical mass 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 (AmO). This isotope also has one meta state, Am, with an excitation energy of and a half-life of . The presence of Am in plutonium 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 Pu contain a buildup of Am. Chemical removal of americium-241 from reworked plutonium (e.g., during reworking of plutonium pits) may be required in some cases.

Nucleosynthesis

Americium-241 has been produced in small quantities in nuclear reactors for decades, and many kilograms of Am have been accumulated by now. Nevertheless, since it was first offered for sale in 1962, its price, about per gram of Am, 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 plutonium-239 (Pu). The latter needs to be produced first, according to the following nuclear process:

: $\mathrm$

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

: $\mathrm$

The plutonium present in spent nuclear fuel contains about 12% of Pu. Because it converts to Am, Pu can be extracted and may be used to generate further Am. However, this process is rather slow: half of the original amount of Pu decays to Am after about 14 years, and the Am amount reaches a maximum after 70 years.

The obtained Am can be used for generating heavier americium isotopes by further neutron capture inside a nuclear reactor. In a light water reactor (LWR), 79% of neutron captures on Am convert to Am and 10% to its nuclear isomer Am:

:79%:   $\mathrm$

Decay

Americium-241 decays mainly via alpha decay, with a weak gamma ray 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, with a branching ratio of 3.6×10 and happening 1.2 times a second per gram of Am. 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, 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 smoke detector (the ionization-type) uses (americium-241 dioxide) as its source of ionizing radiation. 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 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) with an activity of . Some old industrial smoke detectors (notably from the Pyrotronics Corporation) can contain up to . The amount of Am declines slowly as it decays into neptunium-237 (Np), a different transuranic element with a much longer half-life (about ). The radiated alpha particles pass through an ionization chamber, an air-filled space between two electrodes, which allows a small, constant electric current 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.

Manufacturing process

The process for making the americium used in the buttons on ionization-type smoke detectors begins with americium dioxide. The AmO 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) 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 Am has a ''roughly'' similar half-life to Pu (432.2 years vs. 87 years), it has been proposed as an active isotope of radioisotope thermoelectric generators, for use in spacecraft. Even though americium-241 produces less heat and electricity than plutonium-238 (the power yield is for Am vs. for Pu) 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 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 Pu requires less. An advantage over Pu is that it is produced as nuclear waste and is nearly isotopically pure. Prototype designs of Am RTGs expect 2–2.2 W/kg for 5–50 W RTGs design, putting Am RTGs at parity with Pu 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 Am pressed with beryllium can be very efficient neutron sources, since they emit alpha particles during radioactive decay:

: $\mathrm$
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 device used to measure the quantity of water present in soil, as well as moisture/density for quality control in highway construction. Am 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 transactinides – for example, neutron bombardment of Am yields Am:
$\mathrm$

From there, 82.7% of Am decays to Cm and 17.3% to Pu:

82.7% → $\mathrm$

17.3%→ $\mathrm$

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

Irradiation of Am by C or Ne ions yields einsteinium-253 (Es) or dubnium-263 (Db), respectively. Furthermore, the element berkelium (Bk isotope) had been first intentionally produced and identified by bombarding Am with alpha particles, in 1949, by the same Berkeley group, using the same cyclotron that had been used for many previous experiments.

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 Am in such sources can be used for indirect analysis of materials in radiography and X-ray fluorescence 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 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. Am 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.

Hazards

Americium-241 has the same general hazards as other americium isotopes: it is both extremely toxic and radioactive. Though 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 Am.

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 or 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 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 trabecular surfaces and slowly redistributes over the bone with time. The biological half-life of Am is in the bones and in the liver, whereas in the gonads (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 smoke detectors. 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
The Radioactive Boy Scout: When a teenager attempts to build a breeder reactor
''Harper's Magazine'', November 1998 There have been a few cases of exposure to americium-241, the worst being Harold McCluskey who, at age 64, was exposed to 500 times the occupational standard for americium-241 as a result of an explosion in his lab. McCluskey died at age 75, not as a result of exposure, but of a heart disease which he had before the accident. Americium-241 has also been detected in the oceans as a result of nuclear testing conducted by various nations.

See also

*Isotopes of americium

References

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{{cite web , url=https://www.atsdr.cdc.gov/toxprofiles/tp156-c4.pdf , title=Americium: Chemical, physical, and radiological information , pages=103–111 , publisher=Agency for Toxic Substances and Disease Registry (CDC) , access-date=24 July 2019

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Isotopes of americium
Radioisotope fuels
Fissile materials