Americium-241

Americium-241 (²⁴¹
Am
, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of 432.2 years. ²⁴¹
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 ²⁴¹
Pu
, EC from ²⁴¹
Cm
, and α from ²⁴⁵
Bk
. ²⁴¹
Am
is fissile^[1] and the critical mass of a bare sphere is 57.6–75.6 kilograms (127.0–166.7 lb) and a sphere diameter of 19–21 centimetres (7.5–8.3 in).^[2] Americium-241 has a specific activity of 3.43 Ci/g (126.91 GBq/g).^[3] It is commonly found in the form of americium-241 dioxide (²⁴¹
Am
O
₂). This isotope also has one meta state, ^241m
Am
, with an excitation energy of 2.2 MeV (0.35 pJ) and a half-life of 1.23 μs. The presence of americium-241 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 ²⁴¹
Pu
contain a buildup of ²⁴¹
Am
. A chemical removal of americium-241 from reworked plutonium (e.g. during reworking of plutonium pits) may be required in some cases.

Americium-241, ²⁴¹Am

Small button containing ^241 Am O 2 from a smoke alarm
General
Symbol	^241Am
Names	americium-241, Am-241
Protons (Z)	95
Neutrons (N)	146
Nuclide data
Natural abundance	0 (synthetic)
Half-life (t1/2)	432.2 years
Isotope mass	241.056829144 Da
Spin	5/2−
Excess energy	52936.008 keV
Binding energy	7543.272 keV
Parent isotopes	^241Pu (β^−) ^241Cm (EC) ^245Bk (α)
Decay products	^237Np
Decay modes
Decay mode	Decay energy (MeV)
α-decay (alpha)	5.486
γ-emission (gamma)	0.0595409
CD (cluster decay)	93.923
Isotopes of americium Complete table of nuclides

Nucleosynthesis

Americium-241 has been produced in small quantities in nuclear reactors for decades, and many kilograms of ²⁴¹
Am
have been accumulated by now.^[4]: 1262 Nevertheless, since it was first offered for sale in 1962, its price, about US$1,500 per gram of ²⁴¹
Am
, remains almost unchanged owing to the very complex separation procedure.^[5]

Americium-241 is not synthesized directly from uranium – the most common reactor material – but from the plutonium isotope ²³⁹
Pu
. The latter needs to be produced first, according to the following nuclear process:

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

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

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

The plutonium present in spent nuclear fuel contains about 12% of ²⁴¹
Pu
. Because it converts to ²⁴¹
Am
, ²⁴¹
Pu
can be extracted and may be used to generate further ²⁴¹
Am
.^[5] However, this process is rather slow: half of the original amount of ²⁴¹
Pu
decays to ²⁴¹
Am
after about 14 years, and the ²⁴¹
Am
amount reaches a maximum after 70 years.^[6]

The obtained ²⁴¹
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 ²⁴¹
Am
convert to ²⁴²
Am
and 10% to its nuclear isomer ^242m
Am
:^[7]

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

Decay

Main article: Radioactive decay

Americium-241 decays mainly via alpha decay, with a weak gamma ray byproduct. The α-decay is shown as follows:

${\displaystyle \mathrm {^{241\!\,}_{\ 95}Am\ {\overset {432.2y}{\longrightarrow }}\ _{\ 93}^{237}Np~+~_{2}^{4}\alpha ^{2+}+\gamma ~59.5409~keV} }$ 

The α-decay energies are 5.486 megaelectronvolts (0.8790 picojoules) for 85% of the time (the one which is widely accepted for standard α-decay energy), 5.433 MeV (0.8705 pJ) for 13% of the time, and 5.388 MeV (0.8633 pJ) for the remaining 2%.^[8] The γ-ray energy is 59.5409 keV (9.53950 fJ) for the most part, with little amounts of other energies such as 13.9 keV (2.23 fJ), 17.8 keV (2.85 fJ) and 26.4 keV (4.23 fJ).^[9]

The second most common type of decay that americium-241 undergoes is spontaneous fission, with a branching ratio of 3.6×10^−12[10] and happening 1.2 times a second per gram of ²⁴¹
Am
. It is written as such (the asterisk denotes an excited nucleus):

${\displaystyle \mathrm {^{241}_{\ 95}Am\longrightarrow ~_{\ 95}^{241}Am^{*}\longrightarrow 3_{0}^{1}n~+~fission~products~+energy~(\gamma )} }$ 

The least common (rarest) type of decay for americium-241 is ³⁴
Si
cluster decay, with a branching ratio of less than 7.4×10⁻¹⁶.^[10] It is written as follows:

${\displaystyle \mathrm {^{241\!\,}_{\ 95}Am\longrightarrow _{\ 81}^{207}Tl+_{14}^{34}Si} }$ 

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 (the ionization-type) uses ²⁴¹
Am
O
₂ (americium-241 dioxide) as its source of ionizing radiation.^[11] This isotope is preferred over ²²⁶
Ra
because it emits 5 times more alpha particles and relatively little harmful gamma radiation. With its half-life of 432.2 years, the americium in a smoke detector decreases and includes about 3% neptunium after 19 years, and about 5% after 32 years. The amount of americium in a typical new smoke detector is 0.29 micrograms (4.5×10⁻⁶ grains) (about 1/3000 the weight of a small grain of sand) with an activity of 1 microcurie (37 kBq). Some old industrial smoke detectors (notably from the Pyrotronics Corporation) can contain up to 80 microcuries (3,000 kBq). The amount of ²⁴¹
Am
declines slowly as it decays into neptunium-237 (²³⁷
Np
), a different transuranic element with a much longer half-life (about 2.14 million years). 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.^{[12][13][14][15]}

Manufacturing process

The process for making the americium used in the buttons on ionization-type smoke detectors begins with americium dioxide. The ²⁴¹
Am
O
₂ is thoroughly mixed with gold, shaped into a briquette, and fused by pressure and heat at over 1,470 °F (800 °C). 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 0.008 inches (0.20 mm), with the gold cover representing about one percent of the thickness. The resulting foil strip, which is about 0.8 inches (20 mm) wide, is cut into sections 39 inches (1 m) long. The sources are punched out of the foil strip. Each disc, about 0.2 inches (5.1 mm) 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.^[16]

RTG (radioisotope thermoelectric generator) power generation

As ²⁴¹
Am
has a roughly similar half-life to ²³⁸
Pu
(432.2 years vs. 87 years), it has been proposed as an active isotope of radioisotope thermoelectric generators, for use in spacecraft.^[17] Even though americium-241 produces less heat and electricity than plutonium-238 (the power yield is 114.7 milliwatts per gram [3.25 watts per ounce] for ²⁴¹
Am
vs. 570 mW/g [16 W/oz] for ²³⁸
Pu
)^[17] 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^[18] as a result of the global shortage of plutonium-238 and easy access to americium-241 in Europe from nuclear waste reprocessing.^[19][20]

Its shielding requirements in an RTG are the second lowest of all possible isotopes: only ²³⁸
Pu
requires less. An advantage over ²³⁸
Pu
is that it is produced as nuclear waste and is nearly isotopically pure. Prototype designs of ²⁴¹
Am
RTGs expect 2–2.2 W_e/kg for 5–50 W_e RTGs design, putting ²⁴¹
Am
RTGs at parity with ²³⁸
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.^[21]

Neutron source

Oxides of ²⁴¹
Am
pressed with beryllium can be very efficient neutron sources, since they emit alpha particles during radioactive decay:

${\displaystyle \mathrm {^{241\!\,}_{\ 95}Am\ {\overset {432.2y}{\longrightarrow }}\ _{\ 93}^{237}Np\ +\ _{2}^{4}\alpha ^{2+}+\ \gamma ~59.5~keV} }$ 

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

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

The most widespread use of ²⁴¹
Am
Be 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. ²⁴¹
Am
neutron sources are also used in well logging applications, as well as in neutron radiography, tomography, and other radiochemical investigations.^[22]

Production of other elements

 

Chart displaying actinides and their decays and transmutations.

Americium-241 is sometimes used as a starting material for the production of other transuranic elements and transactinides – for example, neutron bombardment of ²⁴¹
Am
yields ²⁴²
Am
:

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

From there, 82.7% of ²⁴²
Am
decays to ²⁴²
Cm
and 17.3% to ²⁴²
Pu
:

82.7% → ${\displaystyle \mathrm {^{241}_{\ 95}Am\ {\xrightarrow {(n,\gamma )}}\ _{\ 95}^{242}Am\ {\xrightarrow[{16.02\ h}]{\beta ^{-}}}\ _{\ 96}^{242}Cm} }$ 

17.3%→ ${\displaystyle \mathrm {^{241}_{\ 95}Am\ {\xrightarrow {(n,\gamma )}}\ _{\ 95}^{242}Am\ {\xrightarrow[{16.02\ h}]{\beta ^{+}}}\ _{\ 94}^{242}Pu} }$ 

In the nuclear reactor, ²⁴²
Am
is also up-converted by neutron capture to ²⁴³
Am
and ²⁴⁴
Am
, which transforms by β-decay to ²⁴²
Cm
:

${\displaystyle \mathrm {^{242}_{\ 95}Am{\xrightarrow {(n,\gamma )}}~_{\ 95}^{243}Am\ {\xrightarrow {(n,\gamma )}}\ _{\ 95}^{244}Am\ {\xrightarrow[{10.1\ h}]{\beta ^{-}}}\ _{\ 96}^{244}Cm} }$ 

Irradiation of ²⁴¹
Am
by ¹²
C
or ²²
Ne
ions yields the isotopes ²⁵³
Es
(einsteinium) or ²⁶³
Db
(dubnium), respectively.^[23] Furthermore, the element berkelium (²⁴³
Bk
isotope) had been first intentionally produced and identified by bombarding ²⁴¹
Am
with alpha particles, in 1949, by the same Berkeley group, using the same 60-inch (1,500 mm) cyclotron that had been used for many previous experiments. Similarly, nobelium was produced at the Joint Institute for Nuclear Research, Dubna, Russia, in 1965 in several reactions, one of which included irradiation of ²⁴³
Am
with ¹⁵
N
ions. Besides, one of the synthesis reactions for lawrencium, discovered by scientists at Berkeley and Dubna, included bombardment of ²⁴³
Am
with ¹⁸
O
.^[4]: 1262

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 59.5409 keV (9.53950 fJ) gamma ray emissions from ²⁴¹
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.^[4]: 1262 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).^[24]

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. ²⁴¹
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.^[25]

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 0.03 microcuries (1.1 kBq) is the maximum permissible body burden for ²⁴¹
Am
.^[26]

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 1 metre (3 ft 3 in).^[27]

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 ²⁴¹
Am
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.^[28]

 

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.^{[29][30][31][32]} 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.^[33][34]

See also

• Isotopes of americium

References

1. SHINOHARA, Nobuo; HATSUKAWA, Yuichi; HATA, Kentaro; KOHNO, Nobuaki (2012). "Measurement of Fission Product Yields from Neutron-Induced Fission of Americium-241". Journal of Nuclear Science and Technology. 36 (3): 232–241. doi:10.1080/18811248.1999.9726203. ISSN 0022-3131.
2. Dias, Hemanth; Tancock, Nigel; Clayton, Angela (20 October 2003). Critical mass calculations for ²⁴¹Am, ^242mAm and ²⁴³Am. Proceedings of the seventh international conference on nuclear criticality safety. Japan Atomic Energy Research Institute. CiteSeerX 10.1.1.540.1085 – via International Atomic Energy Agency (IAEA).
3. "Americium: Chemical, physical, and radiological information" (PDF). Agency for Toxic Substances and Disease Registry (CDC). pp. 103–111. Retrieved 24 July 2019.
4. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Pergamon Press. ISBN 978-0750633659. LCCN 97036336. OCLC 1005231772. OL 689297M.
5. . World Nuclear Association. January 2009. Archived from the original on 24 December 2008. Retrieved 2 September 2022.
6. . Blue Ridge Environmental Defense League. Archived from the original on 28 July 2022. Retrieved 2 September 2022.
7. Sasahara, Akihiro; Matsumura, Tetsuo; Nicolaou, Giorgos; Papaioannou, Dimitri (7 February 2012) [11 December 2003]. "Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO
   ₂ and MOX Spent Fuels". Journal of Nuclear Science and Technology. Atomic Energy Society of Japan (AESJ). 41 (4): 448–456. doi:10.1080/18811248.2004.9715507. eISSN 1881-1248. ISSN 0022-3131. OCLC 2251715.
8. "AMERICIUM-241".
9. "GAMMA RAY SPECTRUM OF AM-241 IN A BACK SCATTERING GEOMETRY USING A HIGH PURITY GERMANIUM DETECTOR" (PDF).
10. Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
11. . Nuclear Issues Briefing Paper. Uranium Information Centre. 35. May 2002. Archived from the original on 3 March 2008. Retrieved 2 September 2022.{{cite journal}}: CS1 maint: unfit URL (link)
12. Cleary, Thomas G. (8 September 2009). Full-Scale Residential Smoke Alarm Performance. 14th International Conference on Automatic Fire Detection. Duisburg, DE. from the original on 31 July 2021. Retrieved 2 September 2022.  This article incorporates public domain material from the National Institute of Standards and Technology. Residential Smoke Alarm Performance, Thomas Cleary. Building and Fire Research Laboratory, National Institute of Standards and Technology; UL Smoke and Fire Dynamics Seminar. November 2007
13. Bukowski, Richard W.; Peacock, Richard D.; Averill, Jason D.; Cleary, Thomas G.; Bryner, Nelson P.; et al. (1 December 2007). Home Smoke Alarms Analysis of the Response of Several Available Technologies in Residential Fire Settings (Technical report). NIST TN 1455-1. from the original on 7 March 2022. Retrieved 2 September 2022.  This article incorporates public domain material from the National Institute of Standards and Technology.
14. Smoke detectors and americium-241 fact sheet (PDF) (Report). Canadian Nuclear Society. October 2008. (PDF) from the original on 20 May 2022. Retrieved 2 September 2022.
15. Agency for Toxic Substances and Disease Registry (April 2004). Toxicological Profile For Americium (PDF) (Report). Atlanta, GA: United States Department of Health and Human Services. CAS#: 7440-35-9. (PDF) from the original on 27 July 2022. Retrieved 2 September 2022.  This article incorporates public domain material from websites or documents of the United States Department of Health and Human Services.
16. "Smoke Detector". How Products are Made. n.d. Retrieved 1 September 2022.
17. Kulcinski, G.L. (Spring 2000). "Nuclear Power in Space". (PDF). University of Wisconsin Fusion Technology Institute. last page. Archived from the original (PDF) on 4 January 2006.
18. Chahal, Major S. (8 February 2012). "European Space Nuclear Power Programme: UK Activities" (PDF). UK Space Agency. (PDF) from the original on 16 May 2012. Retrieved 1 September 2022 – via United Nations Office for Outer Space Affairs.
19. Clark, Stephen (9 July 2010). "Space agencies tackle waning plutonium stockpiles". Spaceflight Now. from the original on 28 July 2022. Retrieved 2 September 2022. ESA's nuclear program would likely focus on americium, according to Southwood. [...] Americium-241 has a longer half-life than plutonium-238, meaning it could survive longer in space, but the isotope produces less heat and electricity. Americium is also a greater radiation hazard to humans, according to scientists.
20. Greenfieldboyce, Nell (28 September 2009). "Plutonium Shortage Could Stall Space Exploration". NPR. from the original on 12 August 2022. Retrieved 2 September 2022. NASA is running out of the special kind of plutonium needed to power deep space probes, worrying planetary scientists who say the U. S. urgently needs to restart production of plutonium-238.
21. Ambrosi, R.M.; Williams, H.R.; Samara-Ratna, P.; Bannister, N.P.; Vernon, D.; et al. (19 March 2012). Development And Testing Of Americium-241 Radioisotope Thermoelectric Generator: Concept Designs And Breadboard System (PDF). Lunar & Planetary Science Conference with Embedded Nuclear and Emerging Technologies for Space 2012 (NETS 2012). The Woodlands, Texas: Lunar and Planetary Institute. (PDF) from the original on 25 March 2022. Retrieved 2 September 2022.
22. "Industrial Uses of Nuclear Materials". Nuclear Regulatory Commission. 2 December 2020. from the original on 8 August 2022. Retrieved 2 September 2022.
23. Binder, Harry H. (1999). Lexikon der chemischen Elemente: das Periodensystem in Fakten, Zahlen und Daten [Lexicon of the chemical elements: the periodic table in facts, figures and dates] (in German). ISBN 978-3-7776-0736-8. LCCN 99200502. OCLC 40933941. OL 90844M.
24. Nuclear Data Viewer 2.4, NNDC
25. (PDF). Archived from the original (PDF) on 2015-11-29.
26. "Americium Am".
27. "AMERICIUM-241 [241Am]".
28. Frisch, Franz Crystal Clear, 100 x energy, Bibliographisches Institut AG, Mannheim 1977, ISBN 3-411-01704-X, p. 184
29. Ken Silverstein, The Radioactive Boy Scout: When a teenager attempts to build a breeder reactor. Harper's Magazine, November 1998
30. . Fox News. 4 August 2007. Archived from the original on 8 December 2007. Retrieved 28 November 2007.
31. . Detroit Free Press. Associated Press. 27 August 2007. Archived from the original on 29 September 2007. Retrieved 27 August 2007.
32. . Fox News. 4 October 2007. Archived from the original on 2007-11-13. Retrieved 28 November 2007.
33. Cary, Annette (25 April 2008). . Tri-City Herald. Archived from the original on 10 February 2010. Retrieved 17 June 2008.
34. . Billings Gazette. Associated Press. 3 June 2005. Archived from the original on 13 October 2007. Retrieved 17 June 2007.