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= Americium =
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Introduction
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Americium is a synthetic chemical element; it has symbol Am and atomic
number 95. It is radioactive and a transuranic member of the actinide
series in the periodic table, located under the lanthanide element
europium and was thus named after the Americas by analogy.
Americium was first produced in 1944 by the group of Glenn T. Seaborg
from Berkeley, California, at the Metallurgical Laboratory of the
University of Chicago, as part of the Manhattan Project. Although it
is the third element in the transuranic series, it was discovered
fourth, after the heavier curium. The discovery was kept secret and
only released to the public in November 1945. Most americium is
produced by uranium or plutonium being bombarded with neutrons in
nuclear reactors - one tonne of spent nuclear fuel contains about 100
grams of americium. It is widely used in commercial ionization chamber
smoke detectors, as well as in neutron sources and industrial gauges.
Several unusual applications, such as nuclear batteries or fuel for
space ships with nuclear propulsion, have been proposed for the
isotope 242mAm, but they are as yet hindered by the scarcity and high
price of this nuclear isomer.
Americium is a relatively soft radioactive metal with a silvery
appearance. Its most common isotopes are 241Am and 243Am. In chemical
compounds, americium usually assumes the oxidation state +3,
especially in solutions. Several other oxidation states are known,
ranging from +2 to +7, and can be identified by their characteristic
optical absorption spectra. The crystal lattices of solid americium
and its compounds contain small intrinsic radiogenic defects, due to
metamictization induced by self-irradiation with alpha particles,
which accumulates with time; this can cause a drift of some material
properties over time, more noticeable in older samples.
History
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Although americium was likely produced in previous nuclear
experiments, it was first intentionally synthesized, isolated and
identified in late autumn 1944, at the University of California,
Berkeley, by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and
Albert Ghiorso. They used a 60-inch cyclotron at the University of
California, Berkeley. The element was chemically identified at the
Metallurgical Laboratory (now Argonne National Laboratory) of the
University of Chicago. Following the lighter neptunium, plutonium, and
heavier curium, americium was the fourth transuranium element to be
discovered. At the time, the periodic table had been restructured by
Seaborg to its present layout, containing the actinide row below the
lanthanide one. This led to americium being located right below its
twin lanthanide element europium; it was thus by analogy named after
the Americas: "The name americium (after the Americas) and the symbol
Am are suggested for the element on the basis of its position as the
sixth member of the actinide rare-earth series, analogous to europium,
Eu, of the lanthanide series."
The new element was isolated from its oxides in a complex, multi-step
process. First plutonium-239 nitrate (239PuNO3) solution was coated on
a platinum foil of about 0.5 cm2 area, the solution was evaporated and
the residue was converted into plutonium dioxide (PuO2) by calcining.
After cyclotron irradiation, the coating was dissolved with nitric
acid, and then precipitated as the hydroxide using concentrated
aqueous ammonia solution. The residue was dissolved in perchloric
acid. Further separation was carried out by ion exchange, yielding a
certain isotope of curium. The separation of curium and americium was
so painstaking that those elements were initially called by the
Berkeley group as 'pandemonium' (from Greek for 'all demons' or
'hell') and 'delirium' (from Latin for 'madness').
Initial experiments yielded four americium isotopes: 241Am, 242Am,
239Am and 238Am. Americium-241 was directly obtained from plutonium
upon absorption of two neutrons. It decays by emission of a α-particle
to 237Np; the half-life of this decay was first determined as years
but then corrected to 432.2 years.
:
: The times are half-lives
The second isotope 242Am was produced upon neutron bombardment of the
already-created 241Am. Upon rapid β-decay, 242Am converts into the
isotope of curium 242Cm (which had been discovered previously). The
half-life of this decay was initially determined at 17 hours, which
was close to the presently accepted value of 16.02 h.
:
The discovery of americium and curium in 1944 was closely related to
the Manhattan Project; the results were confidential and declassified
only in 1945. Seaborg leaked the synthesis of the elements 95 and 96
on the U.S. radio show for children 'Quiz Kids' five days before the
official presentation at an American Chemical Society meeting on 11
November 1945, when one of the listeners asked whether any new
transuranium element besides plutonium and neptunium had been
discovered during the war. After the discovery of americium isotopes
241Am and 242Am, their production and compounds were patented listing
only Seaborg as the inventor. The initial americium samples weighed a
few micrograms; they were barely visible and were identified by their
radioactivity. The first substantial amounts of metallic americium
weighing 40-200 micrograms were not prepared until 1951 by reduction
of americium(III) fluoride with barium metal in high vacuum at 1100
°C.
Occurrence
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The longest-lived and most common isotopes of americium, 241Am and
243Am, have half-lives of 432.6 and 7,350 years, respectively.
Therefore, any primordial americium (americium that was present on
Earth during its formation) should have decayed by now. Trace amounts
of americium probably occur naturally in uranium minerals as a result
of neutron capture and beta decay (238U → 239Pu → 240Pu → 241Am),
though the quantities would be tiny and this has not been confirmed.
Extraterrestrial long-lived 247Cm is probably also deposited on Earth
and has 243Am as one of its intermediate decay products, but again
this has not been confirmed.
Existing americium is concentrated in the areas used for the
atmospheric nuclear weapons tests conducted between 1945 and 1980, as
well as at the sites of nuclear incidents, such as the Chernobyl
disaster. For example, the analysis of the debris at the testing site
of the first U.S. hydrogen bomb, Ivy Mike, (1 November 1952, Enewetak
Atoll), revealed high concentrations of various actinides including
americium; but due to military secrecy, this result was not published
until later, in 1956. Trinitite, the glassy residue left on the desert
floor near Alamogordo, New Mexico, after the plutonium-based Trinity
nuclear bomb test on 16 July 1945, contains traces of americium-241.
Elevated levels of americium were also detected at the crash site of a
US Boeing B-52 bomber aircraft, which carried four hydrogen bombs, in
1968 in Greenland.
In other regions, the average radioactivity of surface soil due to
residual americium is only about 0.01 picocuries per gram (0.37
mBq/g). Atmospheric americium compounds are poorly soluble in common
solvents and mostly adhere to soil particles. Soil analysis revealed
about 1,900 times higher concentration of americium inside sandy soil
particles than in the water present in the soil pores; an even higher
ratio was measured in loam soils.
Americium is produced mostly artificially in small quantities, for
research purposes. A tonne of spent nuclear fuel contains about 100
grams of various americium isotopes, mostly 241Am and 243Am. Their
prolonged radioactivity is undesirable for the disposal, and therefore
americium, together with other long-lived actinides, must be
neutralized. The associated procedure may involve several steps, where
americium is first separated and then converted by neutron bombardment
in special reactors to short-lived nuclides. This procedure is well
known as nuclear transmutation, but it is still being developed for
americium.
The transuranic elements up to fermium, including americium, should
have been present in the natural nuclear fission reactor at Oklo, but
any quantities produced then would have long since decayed away.
Isotope nucleosynthesis
=========================
Americium has been produced in small quantities in nuclear reactors
for decades, and kilograms of its 241Am and 243Am isotopes have been
accumulated by now. Nevertheless, since it was first offered for sale
in 1962, its price, about of 241Am, remains almost unchanged owing to
the very complex separation procedure. The heavier isotope 243Am is
produced in much smaller amounts; it is thus more difficult to
separate, resulting in a higher cost of the order 100,000-160,000 $/g.
Americium 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:
: ^{238}_{92}U ->[\ce{(n,\gamma)}] ^{239}_{92}U ->[\beta^-][23.5
\ \ce{min}] ^{239}_{93}Np ->[\beta^-][2.3565 \ \ce{d}]
^{239}_{94}Pu
The capture of two neutrons by 239Pu (a so-called (n,γ) reaction),
followed by a β-decay, results in 241Am:
: ^{239}_{94}Pu ->[\ce{2(n,\gamma)}] ^{241}_{94}Pu
->[\beta^-][14.35 \ \ce{yr}] ^{241}_{95}Am
The plutonium present in spent nuclear fuel contains about 12% of
241Pu. Because it beta-decays 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
15 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:
:
Americium-242 has a half-life of only 16 hours, which makes its
further conversion to 243Am extremely inefficient. The latter isotope
is produced instead in a process where 239Pu captures four neutrons
under high neutron flux:
: ^{239}_{94}Pu ->[\ce{4(n,\gamma)}] \ ^{243}_{94}Pu
->[\beta^-][4.956 \ \ce{h}] ^{243}_{95}Am
Metal generation
==================
Most synthesis routines yield a mixture of different actinide isotopes
in oxide forms, from which isotopes of americium can be separated. In
a typical procedure, the spent reactor fuel (e.g. MOX fuel) is
dissolved in nitric acid, and the bulk of uranium and plutonium is
removed using a PUREX-type extraction (Plutonium-URanium EXtraction)
with tributyl phosphate in a hydrocarbon. The lanthanides and
remaining actinides are then separated from the aqueous residue
(raffinate) by a diamide-based extraction, to give, after stripping, a
mixture of trivalent actinides and lanthanides. Americium compounds
are then selectively extracted using multi-step chromatographic and
centrifugation techniques with an appropriate reagent. A large amount
of work has been done on the solvent extraction of americium. For
example, a 2003 EU-funded project codenamed "EUROPART" studied
triazines and other compounds as potential extraction agents. A
'bis'-triazinyl bipyridine complex was proposed in 2009 as such a
reagent is highly selective to americium (and curium). Separation of
americium from the highly similar curium can be achieved by treating a
slurry of their hydroxides in aqueous sodium bicarbonate with ozone,
at elevated temperatures. Both Am and Cm are mostly present in
solutions in the +3 valence state; whereas curium remains unchanged,
americium oxidizes to soluble Am(IV) complexes which can be washed
away.
Metallic americium is obtained by reduction from its compounds.
Americium(III) fluoride was first used for this purpose. The reaction
was conducted using elemental barium as reducing agent in a water- and
oxygen-free environment inside an apparatus made of tantalum and
tungsten.
:
An alternative is the reduction of americium dioxide by metallic
lanthanum or thorium:
:
Physical properties
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In the periodic table, americium is located to the right of plutonium,
to the left of curium, and below the lanthanide europium, with which
it shares many physical and chemical properties. Americium is a highly
radioactive element. When freshly prepared, it has a silvery-white
metallic lustre, but then slowly tarnishes in air. With a density of
12 g/cm3, americium is less dense than both curium (13.52 g/cm3) and
plutonium (19.8 g/cm3); but has a higher density than europium (5.264
g/cm3)--mostly because of its higher atomic mass. Americium is
relatively soft and easily deformable and has a significantly lower
bulk modulus than the actinides before it: Th, Pa, U, Np and Pu. Its
melting point of 1173 °C is significantly higher than that of
plutonium (639 °C) and europium (826 °C), but lower than for curium
(1340 °C).
At ambient conditions, americium is present in its most stable α form
which has a hexagonal crystal symmetry, and a space group P63/mmc with
cell parameters 'a' = 346.8 pm and 'c' = 1124 pm, and four atoms per
unit cell. The crystal consists of a double-hexagonal close packing
with the layer sequence ABAC and so is isotypic with α-lanthanum and
several actinides such as α-curium. The crystal structure of americium
changes with pressure and temperature. When compressed at room
temperature to 5 GPa, α-Am transforms to the β modification, which has
a face-centered cubic ('fcc') symmetry, space group Fmm and lattice
constant 'a' = 489 pm. This 'fcc' structure is equivalent to the
closest packing with the sequence ABC. Upon further compression to 23
GPa, americium transforms to an orthorhombic γ-Am structure similar to
that of α-uranium. There are no further transitions observed up to 52
GPa, except for an appearance of a monoclinic phase at pressures
between 10 and 15 GPa. There is no consistency on the status of this
phase in the literature, which also sometimes lists the α, β and γ
phases as I, II and III. The β-γ transition is accompanied by a 6%
decrease in the crystal volume; although theory also predicts a
significant volume change for the α-β transition, it is not observed
experimentally. The pressure of the α-β transition decreases with
increasing temperature, and when α-americium is heated at ambient
pressure, at 770 °C it changes into an 'fcc' phase which is different
from β-Am, and at 1075 °C it converts to a body-centered cubic
structure. The pressure-temperature phase diagram of americium is thus
rather similar to those of lanthanum, praseodymium and neodymium.
As with many other actinides, self-damage of the crystal structure due
to alpha-particle irradiation is intrinsic to americium. It is
especially noticeable at low temperatures, where the mobility of the
produced structure defects is relatively low, by broadening of X-ray
diffraction peaks. This effect makes somewhat uncertain the
temperature of americium and some of its properties, such as
electrical resistivity. So for americium-241, the resistivity at 4.2 K
increases with time from about 2 μOhm·cm to 10 μOhm·cm after 40 hours,
and saturates at about 16 μOhm·cm after 140 hours. This effect is less
pronounced at room temperature, due to annihilation of radiation
defects; also heating to room temperature the sample which was kept
for hours at low temperatures restores its resistivity. In fresh
samples, the resistivity gradually increases with temperature from
about 2 μOhm·cm at liquid helium to 69 μOhm·cm at room temperature;
this behavior is similar to that of neptunium, uranium, thorium and
protactinium, but is different from plutonium and curium which show a
rapid rise up to 60 K followed by saturation. The room temperature
value for americium is lower than that of neptunium, plutonium and
curium, but higher than for uranium, thorium and protactinium.
Americium is paramagnetic in a wide temperature range, from that of
liquid helium, to room temperature and above. This behavior is
markedly different from that of its neighbor curium which exhibits
antiferromagnetic transition at 52 K. The thermal expansion
coefficient of americium is slightly anisotropic and amounts to along
the shorter 'a' axis and for the longer 'c' hexagonal axis. The
enthalpy of dissolution of americium metal in hydrochloric acid at
standard conditions is , from which the standard enthalpy change of
formation (Δf'H'°) of aqueous Am3+ ion is . The standard potential
Am3+/Am0 is .
Chemical properties
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Americium metal readily reacts with oxygen and dissolves in aqueous
acids. The most stable oxidation state for americium is +3. The
chemistry of americium(III) has many similarities to the chemistry of
lanthanide(III) compounds. For example, trivalent americium forms
insoluble fluoride, oxalate, iodate, hydroxide, phosphate and other
salts. Compounds of americium in oxidation states +2, +4, +5, +6 and
+7 have also been studied. This is the widest range that has been
observed with actinide elements. The color of americium compounds in
aqueous solution is as follows: Am3+ (yellow-reddish), Am4+
(yellow-reddish), {{chem2|Am^{V}O2+}}; (yellow),
{{chem2|Am^{VI}O2(2+)}} (brown) and {{chem2|Am^{VII}O6(5-)}} (dark
green). The absorption spectra have sharp peaks, due to 'f'-'f'
transitions' in the visible and near-infrared regions. Typically,
Am(III) has absorption maxima at ca. 504 and 811 nm, Am(V) at ca. 514
and 715 nm, and Am(VI) at ca. 666 and 992 nm.
Americium compounds with oxidation state +4 and higher are strong
oxidizing agents, comparable in strength to the permanganate ion () in
acidic solutions. Whereas the Am4+ ions are unstable in solutions and
readily convert to Am3+, compounds such as americium dioxide (AmO2)
and americium(IV) fluoride (AmF4) are stable in the solid state.
The pentavalent oxidation state of americium was first observed in
1951. In acidic aqueous solution the ion is unstable with respect to
disproportionation. The reaction
:
is typical. The chemistry of Am(V) and Am(VI) is comparable to the
chemistry of uranium in those oxidation states. In particular,
compounds like and are comparable to uranates and the ion is
comparable to the uranyl ion, . Such compounds can be prepared by
oxidation of Am(III) in dilute nitric acid with ammonium persulfate.
Other oxidising agents that have been used include silver(I) oxide,
ozone and sodium persulfate.
Oxygen compounds
==================
Three americium oxides are known, with the oxidation states +2 (AmO),
+3 (Am2O3) and +4 (AmO2). Americium(II) oxide was prepared in minute
amounts and has not been characterized in detail. Americium(III) oxide
is a red-brown solid with a melting point of 2205 °C. Americium(IV)
oxide is the main form of solid americium which is used in nearly all
its applications. As most other actinide dioxides, it is a black solid
with a cubic (fluorite) crystal structure.
The oxalate of americium(III), vacuum dried at room temperature, has
the chemical formula Am2(C2O4)3·7H2O. Upon heating in vacuum, it loses
water at 240 °C and starts decomposing into AmO2 at 300 °C, the
decomposition completes at about 470 °C. The initial oxalate dissolves
in nitric acid with the maximum solubility of 0.25 g/L.
Halides
=========
Halides of americium are known for the oxidation states +2, +3 and +4,
where the +3 is most stable, especially in solutions.
Oxidation state F Cl Br I
+4 Americium(IV) fluoride AmF4 pale pink
+3 Americium(III) fluoride AmF3 pink Americium(III) chloride
AmCl3 pink Americium(III) bromide AmBr3 light yellow
Americium(III) iodide AmI3 light yellow
+2 Americium(II) chloride AmCl2 black Americium(II) bromide
AmBr2 black Americium(II) iodide AmI2 black
Reduction of Am(III) compounds with sodium amalgam yields Am(II) salts
- the black halides AmCl2, AmBr2 and AmI2. They are very sensitive to
oxygen and oxidize in water, releasing hydrogen and converting back to
the Am(III) state. Specific lattice constants are:
* Orthorhombic AmCl2: 'a' = , 'b' = and 'c' =
* Tetragonal AmBr2: 'a' = and 'c' = . They can also be prepared by
reacting metallic americium with an appropriate mercury halide HgX2,
where X = Cl, Br or I:
: {Am} + \underset{mercury\ halide}{HgX2} ->[{} \atop 400 - 500
^\circ \ce C] {AmX2} + {Hg}
Americium(III) fluoride (AmF3) is poorly soluble and precipitates upon
reaction of Am3+ and fluoride ions in weak acidic solutions:
: Am^3+ + 3F^- -> AmF3(v)
The tetravalent americium(IV) fluoride (AmF4) is obtained by reacting
solid americium(III) fluoride with molecular fluorine:
: 2AmF3 + F2 -> 2AmF4
Another known form of solid tetravalent americium fluoride is KAmF5.
Tetravalent americium has also been observed in the aqueous phase. For
this purpose, black Am(OH)4 was dissolved in 15-M NH4F with the
americium concentration of 0.01 M. The resulting reddish solution had
a characteristic optical absorption spectrum which is similar to that
of AmF4 but differed from other oxidation states of americium. Heating
the Am(IV) solution to 90 °C did not result in its disproportionation
or reduction, however a slow reduction was observed to Am(III) and
assigned to self-irradiation of americium by alpha particles.
Most americium(III) halides form hexagonal crystals with slight
variation of the color and exact structure between the halogens. So,
chloride (AmCl3) is reddish and has a structure isotypic to
uranium(III) chloride (space group P63/m) and the melting point of 715
°C. The fluoride is isotypic to LaF3 (space group P63/mmc) and the
iodide to BiI3 (space group R). The bromide is an exception with the
orthorhombic PuBr3-type structure and space group Cmcm. Crystals of
americium(III) chloride hexahydrate (AmCl3·6H2O) can be prepared by
dissolving americium dioxide in hydrochloric acid and evaporating the
liquid. Those crystals are hygroscopic and have yellow-reddish color
and a monoclinic crystal structure.
Oxyhalides of americium in the form AmVIO2X2, AmVO2X, AmIVOX2 and
AmIIIOX can be obtained by reacting the corresponding americium halide
with oxygen or Sb2O3, and AmOCl can also be produced by vapor phase
hydrolysis:
: AmCl3 + H2O -> AmOCl + 2HCl
Chalcogenides and pnictides
=============================
The known chalcogenides of americium include the sulfide AmS2,
selenides AmSe2 and Am3Se4, and tellurides Am2Te3 and AmTe2. The
pnictides of americium (243Am) of the AmX type are known for the
elements phosphorus, arsenic, antimony and bismuth. They crystallize
in the rock-salt lattice.
Silicides and borides
=======================
Americium monosilicide (AmSi) and "disilicide" (nominally AmSix with:
1.87 < x < 2.0) were obtained by reduction of americium(III)
fluoride with elementary silicon in vacuum at 1050 °C (AmSi) and
1150−1200 °C (AmSix). AmSi is a black solid isomorphic with LaSi, it
has an orthorhombic crystal symmetry. AmSix has a bright silvery
lustre and a tetragonal crystal lattice (space group 'I'41/amd), it is
isomorphic with PuSi2 and ThSi2. Borides of americium include AmB4 and
AmB6. The tetraboride can be obtained by heating an oxide or halide of
americium with magnesium diboride in vacuum or inert atmosphere.
Organoamericium compounds
===========================
Analogous to uranocene, americium is predicted to form the
organometallic compound amerocene with two cyclooctatetraene ligands,
with the chemical formula (η8-C8H8)2Am. A cyclopentadienyl complex is
also known that is likely to be stoichiometrically AmCp3.
Formation of the complexes of the type Am(n-C3H7-BTP)3, where BTP
stands for 2,6-di(1,2,4-triazin-3-yl)pyridine, in solutions containing
n-C3H7-BTP and Am3+ ions has been confirmed by EXAFS. Some of these
BTP-type complexes selectively interact with americium and therefore
are useful in its selective separation from lanthanides and another
actinides.
Biological aspects
======================================================================
Americium is an artificial element of recent origin, and thus does not
have a biological requirement. It is harmful to life. It has been
proposed to use bacteria for removal of americium and other heavy
metals from rivers and streams. Thus, Enterobacteriaceae of the genus
'Citrobacter' precipitate americium ions from aqueous solutions,
binding them into a metal-phosphate complex at their cell walls.
Several studies have been reported on the biosorption and
bioaccumulation of americium by bacteria and fungi. In the laboratory,
both americium and curium were found to support the growth of
methylotrophs.
Fission
======================================================================
The isotope 242mAm (half-life 141 years) has the largest cross
sections for absorption of thermal neutrons (5,700 barns), that
results in a small critical mass for a sustained nuclear chain
reaction. The critical mass for a bare 242mAm sphere is about 9-14 kg
(the uncertainty results from insufficient knowledge of its material
properties). It can be lowered to 3-5 kg with a metal reflector and
should become even smaller with a water reflector. Such small critical
mass is favorable for portable nuclear weapons, but those based on
242mAm are not known yet, probably because of its scarcity and high
price. The critical masses of the two readily available isotopes,
241Am and 243Am, are relatively high - 57.6 to 75.6 kg for 241Am and
209 kg for 243Am. Scarcity and high price yet hinder application of
americium as a nuclear fuel in nuclear reactors.
There are proposals of very compact 10-kW high-flux reactors using as
little as 20 grams of 242mAm. Such low-power reactors would be
relatively safe to use as neutron sources for radiation therapy in
hospitals.
Isotopes
======================================================================
About 18 isotopes and 11 nuclear isomers are known for americium,
having mass numbers 229, 230, and 232 through 247. There are two
long-lived alpha-emitters; 243Am has a half-life of 7,370 years and is
the most stable isotope, and 241Am has a half-life of 432.2 years. The
most stable nuclear isomer is 242m1Am; it has a long half-life of 141
years. The half-lives of other isotopes and isomers range from 0.64
microseconds for 245m1Am to 50.8 hours for 240Am. As with most other
actinides, the isotopes of americium with odd number of neutrons have
relatively high rate of nuclear fission and low critical mass.
Americium-241 decays to 237Np emitting alpha particles of 5 different
energies, mostly at 5.486 MeV (85.2%) and 5.443 MeV (12.8%). Because
many of the resulting states are metastable, they also emit gamma rays
with the discrete energies between 26.3 and 158.5 keV.
Americium-242 is a short-lived isotope with a half-life of 16.02 h. It
mostly (82.7%) converts by β-decay to 242Cm, but also by electron
capture to 242Pu (17.3%). Both 242Cm and 242Pu transform via nearly
the same decay chain through 238Pu down to 234U.
Nearly all (99.541%) of 242m1Am decays by internal conversion to 242Am
and the remaining 0.459% by α-decay to 238Np. The latter subsequently
decays to 238Pu and then to 234U.
Americium-243 transforms by α-emission into 239Np, which converts by
β-decay to 239Pu, and the 239Pu changes into 235U by emitting an
α-particle.
Ionization-type smoke detector
================================
Americium is used in the most common type of household smoke detector,
which uses 241Am in the form of americium 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 0.29 microgram. This amount declines slowly as
the americium 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 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 element of radioisotope
thermoelectric generators, for example in spacecraft. Although
americium produces less heat and electricity - the power yield is
114.7 mW/g for 241Am and 6.31 mW/g for 243Am (cf. 390 mW/g for 238Pu)
- and its radiation poses more threat to humans owing to neutron
emission, the European Space Agency is considering using americium for
its space probes.
Another proposed space-related application of americium is a fuel for
space ships with nuclear propulsion. It relies on the very high rate
of nuclear fission of 242mAm, which can be maintained even in a
micrometer-thick foil. Small thickness avoids the problem of
self-absorption of emitted radiation. This problem is pertinent to
uranium or plutonium rods, in which only surface layers provide
alpha-particles. The fission products of 242mAm can either directly
propel the spaceship or they can heat a thrusting gas. They can also
transfer their energy to a fluid and generate electricity through a
magnetohydrodynamic generator.
One more proposal which utilizes the high nuclear fission rate of
242mAm is a nuclear battery. Its design relies not on the energy of
the emitted by americium alpha particles, but on their charge, that is
the americium acts as the self-sustaining "cathode". A single 3.2 kg
242mAm charge of such battery could provide about 140 kW of power over
a period of 80 days. Even with all the potential benefits, the current
applications of 242mAm are as yet hindered by the scarcity and high
price of this particular nuclear isomer.
In 2019, researchers at the UK National Nuclear Laboratory and the
University of Leicester demonstrated the use of heat generated by
americium to illuminate a small light bulb. This technology could lead
to systems to power missions with durations up to 400 years into
interstellar space, where solar panels do not function.
Neutron source
================
The oxide of 241Am pressed with beryllium is an efficient neutron
source. Here americium acts as the alpha source, and beryllium
produces neutrons owing to its large cross-section for the (α,n)
nuclear reaction:
: ^{241}_{95}Am -> ^{237}_{93}Np + ^{4}_{2}He + \gamma
: ^{9}_{4}Be + ^{4}_{2}He -> ^{12}_{6}C + ^{1}_{0}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.
Production of other elements
==============================
Americium is a starting material for the production of other
transuranic elements and transactinides - for example, 82.7% of 242Am
decays to 242Cm and 17.3% to 242Pu. In the nuclear reactor, 242Am is
also up-converted by neutron capture to 243Am and 244Am, which
transforms by β-decay to 244Cm:
: ^{243}_{95}Am ->[\ce{(n,\gamma)}] ^{244}_{95}Am
->[\beta^-][10.1 \ \ce{h}] ^{244}_{96}Cm
Irradiation of 241Am by 12C or 22Ne ions yields the isotopes 247Es
(einsteinium) or 260Db (dubnium), respectively. Furthermore, the
element berkelium (243Bk isotope) had been first intentionally
produced and identified by bombarding 241Am with alpha particles, in
1949, by the same Berkeley group, using the same 60-inch cyclotron.
Similarly, nobelium was produced at the Joint Institute for Nuclear
Research, Dubna, Russia, in 1965 in several reactions, one of which
included irradiation of 243Am with 15N ions. Besides, one of the
synthesis reactions for lawrencium, discovered by scientists at
Berkeley and Dubna, included bombardment of 243Am with 18O.
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 gamma ray emissions from 241Am 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, the
element 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). Americium-241 gamma rays were
also used to provide passive diagnosis of thyroid function. This
medical application is however obsolete.
Health concerns
======================================================================
As a highly radioactive element, americium and its compounds must be
handled only in an appropriate laboratory under special arrangements.
Although most americium isotopes predominantly emit alpha particles
which can be blocked by thin layers of common materials, many of the
daughter products emit gamma-rays and neutrons which have a long
penetration depth.
If consumed, most of the americium is excreted within a few days, with
only 0.05% absorbed in the blood, of which roughly 45% 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 often enters landfills from discarded smoke detectors. The
rules associated with the disposal of smoke detectors are relaxed in
most jurisdictions. In 1994, 17-year-old David Hahn extracted the
americium from about 100 smoke detectors in an attempt to build a
breeder nuclear reactor. There have been a few cases of exposure to
americium, the worst case being that of chemical operations technician
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 of unrelated pre-existing
disease.
See also
======================================================================
* Actinides in the environment
* :Category:Americium compounds
Bibliography
======================================================================
*
* Penneman, R. A. and Keenan T. K.
[
http://www.osti.gov/bridge/purl.cover.jsp?purl=/4187189-IKQUwY/ The
radiochemistry of americium and curium], University of California, Los
Alamos, California, 1960
*
Further reading
======================================================================
* 'Nuclides and Isotopes - 14th Edition', GE Nuclear Energy, 1989.
*
*
External links
======================================================================
* [
http://www.periodicvideos.com/videos/095.htm Americium] at 'The
Periodic Table of Videos' (University of Nottingham)
*
[
https://web.archive.org/web/20060830050012/http://www.atsdr.cdc.gov/toxprofiles/phs156.html
ATSDR - Public Health Statement: Americium]
*
[
https://web.archive.org/web/20081224123105/http://www.world-nuclear.org/info/inf57.html
World Nuclear Association - Smoke Detectors and Americium ]
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Americium
