Americium-241

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]

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]

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]

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]

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

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]

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]