Americium

Americium, ₉₅Am

Americium
Pronunciation	/ˌæməˈrɪsiəm/ ​(AM-ə-RISS-ee-əm)
Appearance	silvery white
Mass number	[243]
Americium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Eu ↑ Am ↓ — plutonium ← americium → curium
Atomic number (Z)	95
Group	f-block groups (no number)
Period	period 7
Block	f-block
Electron configuration	[Rn] 5f7 7s2
Electrons per shell	2, 8, 18, 32, 25, 8, 2
Physical properties
Phase at STP	solid
Melting point	1449 K ​(1176 °C, ​2149 °F)
Boiling point	2880 K ​(2607 °C, ​4725 °F) (calculated)
Density (near r.t.)	12 g/cm3
Heat of fusion	14.39 kJ/mol
Molar heat capacity	28 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1239 1356
Atomic properties
Oxidation states	common: +3 +2, +4, +5, +6, +7
Electronegativity	Pauling scale: 1.3
Ionization energies	1st: 578 kJ/mol
Atomic radius	empirical: 173 pm
Covalent radius	180±6 pm
Spectral lines of americium
Other properties
Natural occurrence	synthetic
Crystal structure	​double hexagonal close-packed (dhcp)
Thermal conductivity	10 W/(m⋅K)
Electrical resistivity	0.69 µΩ⋅m
Magnetic ordering	paramagnetic
Molar magnetic susceptibility	+1000.0×10−6 cm3/mol
CAS Number	7440-35-9
History
Naming	after the Americas
Discovery	Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, Albert Ghiorso (1944)
Isotopes of americium
Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 241Am synth 432.2 y α 237Np SF – 242m1Am synth 141 y IT 242Am α 238Np SF – 243Am synth 7350 y α 239Np SF –

Americium (/ˌæməˈrɪsiəm/ AM-ə-RIS-ee-əm) is a synthetic element that has the symbol Am and atomic number 95. This transuranic element of the actinide series is located in the periodic table below the lanthanide element europium, and thus by analogy was named after another continent, America.

Americium was first produced in late autumn 1944 by the group of Glenn T. Seaborg at the University of California, Berkeley. 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 bombarding uranium or plutonium with alpha particles (helium ions) 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 a nuclear battery 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 silvery-white appearance. Its most common isotopes are ²⁴¹Am and ²⁴³Am. In chemical compounds, they usually assume the oxidation state +3, especially in solutions. Several other oxidation states are known, which range from +2 to +7 and can be identified via their characteristic optical absorption spectra. The crystal lattice of solid americium and its compounds contains intrinsic defects, which are induced by self-irradiation with alpha particles and accumulate with time; this results in a drift of some material properties.

History

The 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley, in August 1939.

The triangle in the glass tube contains the first sample of americium (as the hydroxide), produced in 1944.

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 lead to americium being located right below its twin lanthanide element europium; it was thus by analogy named after another continent, America: "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 annealing. 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 one neutron. It decays by emission of a α-particle to ²³⁷Np; the half-life of this decay was first determined as 510 ± 20 years but then corrected to 432.2 years.

$\mathrm {^{239}_{\ 94}Pu\ {\xrightarrow {(n,\gamma )}}\ _{\ 94}^{240}Pu\ {\xrightarrow {(n,\gamma )}}\ _{\ 94}^{241}Pu\ {\xrightarrow[{14.35\ yr}]{\beta ^{-}}}\ _{\ 95}^{241}Am\ \left(\ {\xrightarrow[{432.2\ yr}]{\alpha }}\ _{\ 93}^{237}Np\right)}$

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.

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

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, the Quiz Kids, five days before the official presentation at an American Chemical Society meeting on November 11, 1945, when one of the listeners asked whether any new transuranium element beside 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

See also: Nuclear reprocessing

Americium was detected in the fallout from the Ivy Mike nuclear test.

The longest-lived and most common isotopes of americium, ²⁴¹Am and ²⁴³Am, have half-lives of 432.2 and 7,370 years, respectively. Therefore, all primordial americium, that is americium that may have been present on Earth during its formation, would have decayed by now. 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 Chernobyl disaster and Three Mile Island accident. For example, the analysis of the debris at the testing site of the first U.S. hydrogen bomb, Ivy Mike, (November 1, 1952, Enewetak Atoll), revealed high concentrations of various actinides including americium. However, due to the military secrecy, this result was published only in 1956. Elevated levels of americium were also detected at the crash site of the US B-52 bomber, which carried four hydrogen bombs, in 1968 in Greenland.

In other regions, the average radioactivity due to residual americium is only about 0.01 picocuries. Atmospheric americium compounds are poorly soluble in common solvents and mostly adhere to soil particles. Soil analysis revealed about 1,900 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, have to 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.

Synthesis and extraction

Isotope nucleosyntheses

Chromatographic elution curves revealing the similarity between Tb, Gd, Eu lanthanides and corresponding Bk, Cm, Am actinides.

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 1,500 USD per gram 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 that results in a higher cost of the order 100–160 USD/mg.

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:

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

The capture of one neutron 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 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:

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

10%:   $\mathrm {^{241}_{\ 95}Am\ {\xrightarrow {(n,\gamma )}}\ _{\ \ \ 95}^{242m}Am}$

Americium-242 has a half-life of only 16 hours, which makes its further up-conversion to ²⁴³Am, extremely inefficient. The latter isotope is produced instead in a process where ²³⁹Pu captures four neutrons under high neutron flux:

$\mathrm {^{239}_{\ 94}Pu\ {\xrightarrow {4(n,\gamma )}}\ _{\ 94}^{243}Pu\ {\xrightarrow[{4.956\ h}]{\beta ^{-}}}\ _{\ 95}^{243}Am}$

Metal generation

Most synthesis routines yield a mixture of different actinide isotopes in oxide forms, from which isotopes of americium need to 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 recent EU funded project codenamed "EUROPART" studied triazines and other compounds as potential extraction agents. Bis-triazinyl bipyridine complex has been recently proposed as such reagent as 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.

$\mathrm {2\ AmF_{3}\ +\ 3\ Ba\ \longrightarrow \ 2\ Am\ +\ 3\ BaF_{2}}$

An alternative is the reduction of americium dioxide by metallic lanthanum or thorium:

$\mathrm {3\ AmO_{2}\ +\ 4\ La\ \longrightarrow \ 3\ Am\ +\ 2\ La_{2}O_{3}}$

Physical properties

Double-hexagonal close packing with the layer sequence ABAC in the crystal structure of α-americium (A: green, B: blue, C: red).

In the periodic table, americium is located right to plutonium, left to curium, and below the lanthanide europium, with which it shares many similarities in 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 lighter than both curium (13.52 g/cm³) and plutonium (19.8 g/cm³); but is heavier 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 lattice 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 Fm3m 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 lattice due to alpha-particle irradiation is intrinsic to americium. It is especially noticeable at low temperatures, where the mobility of the produced lattice 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 exhibit antiferromagnetic transition at 52 K. The thermal expansion coefficient of americium is slightly anisotropic and amounts to (7.5 ± 0.2)×10⁻⁶/°C along the shorter a axis and (6.2 ± 0.4)×10⁻⁶/°C for the longer c hexagonal axis. The enthalpy of dissolution of americium metal in hydrochloric acid at standard conditions is −620.6 ± 1.3 kJ/mol, from which the standard enthalpy change of formation (Δ_fH°) of aqueous Am³⁺ ion is −621.2 ± 2.0 kJ/mol⁻¹. The standard potential Am³⁺/Am⁰ is 2.08 ± 0.01 V.

Chemical properties

File:Americium34.jpg

Americium ions in solution: Am³⁺ (left) and Am⁴⁺ (right). Am³⁺ is colorless at low and reddish at higher concentrations.

Americium readily reacts with oxygen and dissolves well in acids. The most common oxidation state for americium is +3, in which americium compounds are rather stable against oxidation and reduction. In this sense, americium is chemically similar to most lanthanides and actinides. The trivalent americium forms insoluble fluoride, oxalate, iodate, hydroxide, phosphate and other salts. Other oxidation states have been observed between +2 and +7, which is the widest range among the actinide elements. Their color in aqueous solutions varies as follows: Am³⁺ (colorless to yellow-reddish), Am⁴⁺ (yellow-reddish), Am^VO₂⁺; (yellow), Am^VIO₂²⁺ (brown) and Am^VIIO₆^5- (dark green). All oxidation states have their characteristic optical absorption spectra, with a few sharp peaks in the visible and mid-infrared regions, and the position and intensity of these peaks can be converted into the concentrations of the corresponding oxidation states. For example, Am(III) has two sharp peaks at 504 and 811 nm, Am(V) at 514 and 715 nm, and Am(VI) at 666 and 992 nm.

Americium compounds with oxidation state +4 and higher are strong oxidizing agents, comparable in strength to the permanganate ion (MnO₄^-) in acidic solutions. Whereas the Am⁴⁺ ions are unstable in solutions and readily convert to Am³⁺, the +4 oxidation state occurs well in solids, such as americium dioxide (AmO₂) and americium(IV) fluoride (AmF₄).

All pentavalent and hexavalent americium compounds are complex salts such as KAmO₂F₂, Li₃AmO₄ and Li₆AmO₆, Ba₃AmO₆, AmO₂F₂. These high oxidation states Am(IV), Am(V) and Am(VI) can be prepared from Am(III) by oxidation with ammonium in dilute nitric acid, with silver(I) oxide in perchloric acid, or with ozone or sodium in sodium solutions. The pentavalent oxidation state of americium was first observed in 1951. It is present in aqueous solution in the form of AmO₂⁺ ions (acidic) or AmO₃^- ions (alkaline) which are however unstable and subject to several rapid disproportionation reactions:

$\mathrm {3\ AmO_{2}^{+}\ +\ 4\ H^{+}\ \longrightarrow \ 2\ AmO_{2}^{2+}\ +\ Am^{3+}\ +\ 2\ H_{2}O}$

$\mathrm {2\ Am(V)\ \longrightarrow \ Am(VI)\ +\ Am(IV)}$

Chemical compounds

Oxygen compounds

Two americium oxides are known, with the oxidation states +3 (Am₂O₃) and +4 (AmO₂). 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 to Am(III) state. The lattice constants for the orthorhombic AmCl₂ are a = 896.3 ± 0.8 pm, b = 757.3 ± 0.8 pm and c = 453.2 ± 0.6 pm; and for the tetragonal AmBr₂ are a = 1159.2 ± 0.4 and c = 712.1 ± 0.3 pm. They can also be prepared by reacting metallic americium with an appropriate mercury halide HgX₂, where X = Cl, Br or I:

$\mathrm {\ Am\ +\ HgX_{2}\ {\xrightarrow {400-500^{\circ }C}}\ AmX_{2}\ +\ Hg\ }$

Americium(III) fluoride (AmF₃) is poorly soluble and precipitates upon reaction of Am³⁺ and fluoride ions in weak acidic solutions:

$\mathrm {Am^{3+}\ _{(aq)}+\ 3\ F^{-}\ _{(aq)}\longrightarrow \ AmF_{3}\ _{(s)}\downarrow }$

The tetravalent americium(IV) fluoride]] (AmF₄) is obtained by reacting solid americium(III) fluoride with molecular fluorine:

$\mathrm {2\ AmF_{3}\ +\ F_{2}\ \longrightarrow \ 2\ AmF_{4}}$

Another known form of solid tetravalent americium chloride is KAmF₅. Tetravalent americium has also been observed in the aqueous phase. For this purpose, black Am(OH)₄ was dissolved in 15-M NH₃F 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 R3). The bromide is an exception with the orthorhombic PuBr₃-type structure and space group Cmcm. Crystals of americium 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 Am^VIO₂X₂, Am^VO₂X, Am^IVOX₂ and Am^IIIOX can be obtained by reacting the corresponding americium halide with oxygen or Sb₂O₃, and AmOCl can also be produced by vapor phase hydrolysis:

$\mathrm {AmCl_{3}\ +\ \ H_{2}O\ \longrightarrow \ AmOCl\ +\ 2\ HCl}$

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 AmSi_x 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 (AmSi_x). AmSi is a black solid isomorphic with LaSi, it has an orthorhombic crystal symmetry. AmSi_x has a bright silvery lustre and a tetragonal crystal lattice (space group I4₁/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.

Organometallic compounds

(η⁸-C₈H₈)₂Am structure

Analogous to uranocene, americium forms an organometallic compound with two cyclooctatetraene ligands, that is (η⁸-C₈H₈)₂Am. It also makes trigonal (η⁵-C₅H₅)₃Am complexes with three cyclopentadienyl rings.

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 has no known biological function. 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.

Fission

The isotope ^242m1Am (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 ^242m1Am 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 ^242m1Am are not known yet, probably because of its scarcity and high price. The critical masses of two other 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 ^242m1Am. Such low-power reactors would be relatively safe to use as neutron sources for radiation therapy in hospitals.

Isotopes

See also: Isotopes of americium

About 16 isotopes and 11 nuclear isomers are known for americium. There are two long-lived alpha-emitters, ²⁴¹Am and ²⁴³Am with half-lives of 432.2 and 7,370 years, respectively, and the nuclear isomer ^242m1Am 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 breaks down 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.

Applications

Outside and inside view of an americium-based smoke detector

Ionization detectors

Americium is the only synthetic element to have found its way into the household, where one common type of smoke detector uses ²⁴¹Am in the form of americium dioxide as its source of ionizing radiation. This isotopes is preferred against ²²⁶Ra because it emits 5 times more alpha particles and relatively little of harmful γ-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 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 interrupts this current, setting off the alarm. Compared to the alternative optical sensor, the ionization-type smoke detector is cheaper and can detect particles which are too small to induce significant light scattering; however, it is more prone to false (nuisance) alarms.

Radionuclide

As ²⁴¹Am has a significantly longer half-life than 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 planning to use 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 up 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. With all the potential benefits, the current applications of ^242mAm are as yet hindered by the scarcity and high price of this nuclear isomer.

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:

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

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

The most widespread use of ²⁴¹AmBe neutron sources 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:

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

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 60-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 issues

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, americium 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 often enters landfills from discarded smoke detectors. The rules associated with the disposal of smoke detectors are very relaxed in most municipalities. For instance in the UK it is permissible to dispose of an americium containing smoke detector by placing it in the dustbin with normal household rubbish, but each filled dustbin is limited to contain not more than one smoke detector. In the U.S., the "Radioactive Boy Scout" David Hahn was able to buy and steal thousands of smoke detectors at remainder prices and concentrate the americium from them. There have been cases of humans being contaminated with americium, 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.

See also

• Actinides in the environment
• Category:Americium compounds

Bibliography

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Holleman, Arnold F. and Wiberg, Nils Textbook of Inorganic Chemistry, 102 Edition, de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1.
• Penneman, R. A. and Keenan T. K. The radiochemistry of americium and curium, Univsity of California, Los Alamos, California, 1960

Further reading

• Nuclides and Isotopes – 14th Edition, GE Nuclear Energy, 1989.
• Fioni, Gabriele; Cribier, Michel and Marie, Frédéric. "Can the minor actinide, americium-241, be transmuted by thermal neutrons?". Commissariat à l'énergie atomique.{{cite web}}: CS1 maint: multiple names: authors list (link)
• Guide to the Elements – Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1

External links

• ATSDR – Public Health Statement: Americium
• – World Nuclear Association