Americium-241

• Comment: You have some problems with some of your references. Can you resolve those? Thanks. LaMona (talk) 16:18, 14 December 2015 (UTC)

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Americium-241, ²⁴¹Am

Small button containing 241AmO2 from a smoke alarm
General
Symbol	241Am
Names	americium-241, 241Am, Am-241
Protons (Z)	95
Neutrons (N)	146
Nuclide data
Natural abundance	synthetic
Half-life (t1/2)	432.2 years
Spin	5/2-
Excess energy	52,930 keV
Parent isotopes	241Pu (β−) 241Cm (EC) 245Bk (α)
Decay products	237Np
Decay modes
Decay mode	Decay energy (MeV)
α-decay (alpha)	5.5486
Isotopes of americium Complete table of nuclides

Americium-241 (²⁴¹Am) is an isotope of americium. Like all isotopes of americium, it is radioactive. ²⁴¹Am is the most common isotope of americium. It is the most prevalent isotope of americium in nuclear waste. Americium-241 has a half-life of 432.2 years. It is commonly found in ionization type smoke detectors. It is a potential fuel for long-lifetime radioisotope thermoelectric generators (RTGs). Its common parent nuclides are β^- from ²⁴¹Pu, EC from ²⁴¹Cm and α from ²⁴⁵Bk. ²⁴¹Am is fissile and the critical mass of a bare sphere is 57.6-75.6 kilograms and a sphere diameter of 19–21 centimeters.. Americium-241 has a specific activity of 3.43 Ci/g (Curies per gram or 117.29 Gigabequerels (GBq) per gram). It is commonly found in the form of americium-241 dioxide (²⁴¹AmO₂). This isotope also has one meta state; ^241mAm, with an exitation energy of 2.2 MeV, and a half-life of 1.23 μs. Its presence 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 plutonium-241 contain a buildup of ²⁴¹Am. A chemical removal of americium-241 from reworked plutonium (e.g. during reworking of plutonium pits) may be required.

Chart displaying actinides and their decays and transmutations.

Nucleosynthesis

Americium-241 has been produced in small quantities in nuclear reactors for decades, and kilograms of ²⁴¹Am have been accumulated by now. Nevertheless, since it was first offered for sale in 1962, its price, about 1,500 USD per gram of ²⁴¹Am, 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 ²³⁹Pu. The latter needs to be produced first, according to the following nuclear process:

$\mathrm {^{238}_{\ 92}U\ {\xrightarrow {(n,\gamma )}}\ _{\ 92}^{239}U\ {\xrightarrow[{23.5\ min}]{\beta ^{-}}}\ _{\ 93}^{239}Np\ {\xrightarrow[{2.3565\ d}]{\beta ^{-}}}\ _{\ 94}^{239}Pu}$

The capture of two neutrons by ²³⁹Pu (a so-called (n,γ) reaction), followed by a β-decay, results in ²⁴¹Am:

$\mathrm {^{239}_{\ 94}Pu\ {\xrightarrow {2(n,\gamma )}}\ _{\ 94}^{241}Pu\ {\xrightarrow[{14.35\ yr}]{\beta ^{-}}}\ _{\ 95}^{241}Am}$

The plutonium present in spent nuclear fuel contains about 12% of ²⁴¹Pu. Because it spontaneously converts to ²⁴¹Am, ²⁴¹Pu can be extracted and may be used to generate further ²⁴¹Am. However, this process is rather slow: half of the original amount of ²⁴¹Pu decays to ²⁴¹Am after about 14 years, and the ²⁴¹Am amount reaches a maximum after 70 years.

The obtained ²⁴¹Am can be used for generating heavier americium isotopes by further neutron capture inside a nuclear reactor. In a light water reactor (LWR), 79% of ²⁴¹Am converts to ²⁴²Am and 10% to its nuclear isomer ^242mAm:

79%:   $\mathrm {^{241}_{\ 95}Am\ {\xrightarrow {(n,\gamma )}}\ _{\ 95}^{242}Am}$

Applications

Ionization-type smoke detector

Main article: 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 (ionization-type) uses ²⁴¹AmO₂ (americium-241 dioxide) as its source of ionizing radiation. This isotope is preferred over ²²⁶Ra because it emits 5 times more alpha particles and relatively little harmful gamma radiation. The amount of americium in a typical new smoke detector is 1 microcurie (37 kBq) or 0.28 microgram. This amount declines slowly as the americium-241 decays into neptunium-237, a different transuranic element with a much longer half-life (about 2.14 million years). With its half-life of 432.2 years, the americium in a smoke detector includes about 3% neptunium after 19 years, and about 5% after 32 years. The radiation passes through an ionization chamber, an air-filled space between two electrodes, and permits a small, constant current between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and therfore affects this current, triggering the alarm. 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.

Radionuclide

As ²⁴¹Am has a roughly similar half-life to ²³⁸Pu (432.2 years vs. 87 years), it has been proposed as an active isotope of radioisotope thermoelectric generators, for example in spacecraft., G.L. Kulcinski, NEEP 602 Course Notes (Spring 2000), Nuclear Power in Space, University of Wisconsin Fusion Technology Institute (see last page)</ref> Although americium produces less heat and electricity – the power yield is 114.7 mW/g for ²⁴¹Am (cf. 390 mW/g for ²³⁸Pu) – and its radiation poses more threat to humans owing to neutron emission, the European Space Agency is considering to use americium-241 for its space probes.

Neutron source

Oxides of ²⁴¹Am pressed with beryllium can be efficient neutron sources. Here americium acts as the alpha source, and beryllium produces neutrons owing to its large cross-section for the (α,n) nuclear reaction:

$\mathrm {^{241\!\,}_{\ 95}Am\ \longrightarrow \ _{\ 93}^{237}Np\ +\ _{2}^{4}He\ +\ \gamma }$

$\mathrm {^{9}_{4}Be\ +\ _{2}^{4}He\ \longrightarrow \ _{\ 6}^{12}C\ +\ _{0}^{1}n\ +\ \gamma }$

The most widespread use of ²⁴¹AmBe 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. ²⁴¹Am neutron sources are also used in well logging applications, as well as in neutron radiography, tomography and other radiochemical investigations.

Hazards

Americium-241 is a form of americium therfore having the same general hazards. Americium and its isotopes are 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-emmiters. Americium and its isotopes are also very chemically toxic as well, in the form of heavy-metal toxicity.

Americium-241 is an α-emmiter with a weak γ-ray byproduct. It is crucial to handle americium-241 with knowing proper safety precautions, as without them it would be extemely dangerous. Its specific gamma dose constant is 3.14 x 10^-1 mR/hr/mCi or 8.48 x10^-5 mSv/hr/MBq at 1 meter

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 ²⁴¹Am is 50 years in the bones and 20 years 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. 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 which he had before the accident.