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Americium,95Am
Americium
Pronunciation/ˌæməˈrɪsiəm/​(AM-ə-RISS-ee-əm)
Appearancesilvery white
Mass number[243]
Americium in theperiodic 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
Groupf-block groups(no number)
Periodperiod 7
Blockf-block
Electron configuration[Rn] 5f77s2
Electrons per shell2, 8, 18, 32, 25, 8, 2
Physical properties
PhaseatSTPsolid
Melting point1449K​(1176 °C, ​2149 °F)
Boiling point2880 K ​(2607 °C, ​4725 °F)(calculated)
Density(nearr.t.)12 g/cm3
Heat of fusion14.39kJ/mol
Molar heat capacity28[1]J/(mol·K)
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
atT(K) 1239 1356
Atomic properties
Oxidation states+2,+3,+4, +5, +6, +7 (anamphotericoxide)
ElectronegativityPauling scale: 1.3
Ionization energies
  • 1st: 578 kJ/mol
Atomic radiusempirical: 173pm
Covalent radius180±6 pm
Color lines in a spectral range
Spectral linesof americium
Other properties
Natural occurrencesynthetic
Crystal structure​double hexagonal close-packed(dhcp)
Double hexagonal close packed crystal structure for americium
Thermal conductivity10 W/(m⋅K)
Electrical resistivity0.69 µΩ⋅m[1]
Magnetic orderingparamagnetic
Molar magnetic susceptibility+1000.0×10−6cm3/mol[2]
CAS Number7440-35-9
History
Namingafter theAmericas
DiscoveryGlenn T. Seaborg,Ralph A. James,Leon O. Morgan,Albert Ghiorso(1944)
Isotopes of americium
Main isotopes[3] 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 –
Category: Americium
|references

Americiumis asynthetic chemical element;it hassymbolAmandatomic number95. It isradioactiveand atransuranicmember of theactinideseries in theperiodic table,located under thelanthanideelementeuropiumand was thus named after theAmericasby analogy.[4][5][6]

Americium was first produced in 1944 by the group ofGlenn T. SeaborgfromBerkeley, California,at theMetallurgical Laboratoryof theUniversity of Chicago,as part of theManhattan Project.Although it is the third element in the transuranic series, it was discovered fourth, after the heaviercurium.The discovery was kept secret and only released to the public in November 1945. Most americium is produced byuraniumorplutoniumbeing bombarded withneutronsinnuclear reactors– onetonneofspent nuclear fuelcontains about 100 grams of americium. It is widely used in commercialionization chambersmoke detectors,as well as inneutron sourcesand industrial gauges. Several unusual applications, such as nuclear batteries or fuel for space ships with nuclearpropulsion,have been proposed for theisotope242mAm, but they are as yet hindered by the scarcity and high price of thisnuclear isomer.

Americium is a relatively softradioactivemetal with silvery appearance. Its most commonisotopesare241Amand243Am. In chemical compounds, americium usually assumes theoxidation state+3, especially in solutions. Several other oxidation states are known, ranging from +2 to +7, and can be identified by their characteristicoptical absorptionspectra. The crystal lattices of solid americium and its compounds contain small intrinsic radiogenic defects, due tometamictizationinduced 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

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

Although americium was likely produced in previous nuclear experiments, it wasfirst intentionally synthesized,isolated and identified in late autumn 1944, at theUniversity of California, Berkeley,byGlenn T. Seaborg,Leon O. Morgan,Ralph A. James,andAlbert Ghiorso.They used a 60-inchcyclotronat the University of California, Berkeley.[7]The element was chemically identified at the Metallurgical Laboratory (nowArgonne National Laboratory) of theUniversity of Chicago.Following the lighterneptunium,plutonium,and heaviercurium,americium was the fourthtransuranium elementto be discovered. At the time, theperiodic tablehad been restructured by Seaborg to its present layout, containing the actinide row below thelanthanideone. This led to americium being located right below its twin lanthanide element europium; it was thus by analogy named after theAmericas:"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."[8][9][10]

The new element was isolated from itsoxidesin a complex, multi-step process. Firstplutonium-239 nitrate (239PuNO3) solution was coated on aplatinumfoil of about 0.5 cm2area, the solution was evaporated and the residue was converted into plutonium dioxide (PuO2) bycalcining.After cyclotron irradiation, the coating was dissolved withnitric acid,and then precipitated as the hydroxide using concentrated aqueousammonia solution.The residue was dissolved inperchloric acid.Further separation was carried out byion 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 aspandemonium[11](from Greek forall demonsorhell) anddelirium(from Latin formadness).[12][13]

Initial experiments yielded four americium isotopes:241Am,242Am,239Am and238Am.Americium-241was directly obtained from plutonium upon absorption of two neutrons. It decays by emission of aα-particleto237Np; thehalf-lifeof this decay was first determined as510±20years but then corrected to 432.2 years.[14]

The times arehalf-lives

The second isotope242Am was produced upon neutron bombardment of the already-created241Am. Upon rapidβ-decay,242Am converts into the isotope of curium242Cm (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.[14]

The discovery of americium and curium in 1944 was closely related to theManhattan 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 childrenQuiz Kidsfive days before the official presentation at anAmerican Chemical Societymeeting 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.[12]After the discovery of americium isotopes241Am and242Am, their production and compounds were patented listing only Seaborg as the inventor.[15]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 ofamericium(III) fluoridewithbariummetal in high vacuum at 1100 °C.[16]

Occurrence

[edit]
See also:Nuclear reprocessing
Americium was detected in the fallout from theIvy Mikenuclear test.

The longest-lived and most common isotopes of americium,241Am and243Am, have half-lives of 432.2 and 7,370 years, respectively. Therefore, anyprimordialamericium (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.[17][18][19]Extraterrestrial long-lived247Cm is probably also deposited on Earth and has243Am as one of its intermediate decay products, but again this has not been confirmed.[19]

Existing americium is concentrated in the areas used for the atmosphericnuclear weapons testsconducted between 1945 and 1980, as well as at the sites of nuclear incidents, such as theChernobyl 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.[20]Trinitite,the glassy residue left on the desert floor nearAlamogordo, New Mexico,after theplutonium-basedTrinitynuclear bomb teston 16 July 1945, contains traces of americium-241. Elevated levels of americium were also detected at thecrash siteof a USBoeing B-52bomber aircraft, which carried four hydrogen bombs, in 1968 inGreenland.[21]

In other regions, the average radioactivity of surface soil due to residual americium is only about 0.01picocuriesper gram (0.37mBq/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 inloamsoils.[22]

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, mostly241Am and243Am.[23]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 asnuclear transmutation,but it is still being developed for americium.[24][25]Thetransuranic elementsfrom americium tofermiumoccurred naturally in thenatural nuclear fission reactoratOklo,but no longer do so.[26]

Americium is also one of the elements that have theoretically been detected inPrzybylski's Star.[27]

Synthesis and extraction

[edit]

Isotope nucleosynthesis

[edit]
Chromatographicelutioncurves revealing the similarity between the lanthanides Tb, Gd, and Eu and the corresponding actinides Bk, Cm, and Am

Americium has been produced in small quantities innuclear reactorsfor decades, and kilograms of its241Am and243Am isotopes have been accumulated by now.[28]Nevertheless, since it was first offered for sale in 1962, its price, about US$1,500 per gram (US$43,000/oz) of241Am, remains almost unchanged owing to the very complex separation procedure.[29]The heavier isotope243Am is produced in much smaller amounts; it is thus more difficult to separate, resulting in a higher cost of the order US$100,000–US$160,000 per gram (US$2,800,000–US$4,500,000/oz).[30][31]

Americium is not synthesized directly from uranium – the most common reactor material – but from the plutonium isotope239Pu. The latter needs to be produced first, according to the following nuclear process:

The capture of two neutrons by239Pu (a so-called (n,γ) reaction), followed by a β-decay, results in241Am:

The plutonium present in spent nuclear fuel contains about 12% of241Pu. Because itbeta-decaysto241Am,241Pu can be extracted and may be used to generate further241Am.[29]However, this process is rather slow: half of the original amount of241Pu decays to241Am after about 15 years, and the241Am amount reaches a maximum after 70 years.[32]

The obtained241Am can be used for generating heavier americium isotopes by further neutron capture inside a nuclear reactor. In alight water reactor(LWR), 79% of241Am converts to242Am and 10% to itsnuclear isomer242mAm:[note 1][33]

Americium-242has a half-life of only 16 hours, which makes its further conversion to243Am extremely inefficient. The latter isotope is produced instead in a process where239Pu captures four neutrons under highneutron flux:

Metal generation

[edit]

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 innitric acid,and the bulk of uranium and plutonium is removed using aPUREX-type extraction (Plutonium–URaniumEXtraction) withtributyl phosphatein ahydrocarbon.The lanthanides and remaining actinides are then separated from the aqueous residue (raffinate) by adiamide-based extraction, to give, after stripping, a mixture of trivalent actinides and lanthanides. Americium compounds are then selectively extracted using multi-stepchromatographicand centrifugation techniques[34]with an appropriate reagent. A large amount of work has been done on thesolvent extractionof americium. For example, a 2003EU-funded project codenamed "EUROPART" studiedtriazinesand other compounds as potential extraction agents.[35][36][37][38][39]Abis-triazinyl bipyridinecomplex was proposed in 2009 as such a reagent is highly selective to americium (and curium).[40]Separation of americium from the highly similar curium can be achieved by treating a slurry of their hydroxides in aqueoussodium bicarbonatewithozone,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.[41]

Metallic americium is obtained byreductionfrom its compounds.Americium(III) fluoridewas first used for this purpose. The reaction was conducted using elementalbariumas reducing agent in a water- and oxygen-free environment inside an apparatus made oftantalumandtungsten.[16][42][43]

An alternative is the reduction ofamericium dioxideby metalliclanthanumorthorium:[43][44]

Physical properties

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

In theperiodic table,americium is located to the right of plutonium, to the left of curium, and below the lanthanideeuropium,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 lowerbulk modulusthan the actinides before it: Th, Pa, U, Np and Pu.[45]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).[44][46]

At ambient conditions, americium is present in its most stable α form which has ahexagonal crystal symmetry,and aspace groupP63/mmc with cell parametersa= 346.8pmandc= 1124 pm, and four atoms perunit cell.The crystal consists of a double-hexagonal close packingwith the layer sequence ABAC and so is isotypic with α-lanthanum and several actinides such as α-curium.[42][46]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 aface-centered cubic(fcc) symmetry, space group Fm3m and lattice constanta= 489 pm. Thisfccstructure is equivalent to the closest packing with the sequence ABC.[42][46]Upon further compression to 23 GPa, americium transforms to anorthorhombicγ-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.[45]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 anfccphase which is different from β-Am, and at 1075 °C it converts to abody-centered cubicstructure. The pressure-temperature phase diagram of americium is thus rather similar to those of lanthanum,praseodymiumandneodymium.[47]

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 producedstructure defectsis relatively low, by broadening ofX-ray diffractionpeaks. This effect makes somewhat uncertain the temperature of americium and some of its properties, such as electricalresistivity.[48]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 atliquid heliumto 69 μOhm·cm at room temperature; this behavior is similar to that of neptunium, uranium, thorium andprotactinium,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.[1]

Americium isparamagneticin a wide temperature range, from that ofliquid helium,to room temperature and above. This behavior is markedly different from that of its neighbor curium which exhibits antiferromagnetic transition at 52 K.[49]Thethermal expansioncoefficient of americium is slightly anisotropic and amounts to(7.5±0.2)×10−6/°Calong the shorteraaxis and(6.2±0.4)×10−6/°Cfor the longerchexagonal axis.[46]Theenthalpy of dissolutionof americium metal inhydrochloric acidat standard conditions is−620.6±1.3 kJ/mol,from which thestandard enthalpy change of formation(ΔfH°) of aqueous Am3+ion is−621.2±2.0 kJ/mol.Thestandard potentialAm3+/Am0is−2.08±0.01 V.[50]

Chemical properties

[edit]

Americium metal readily reacts with oxygen and dissolves in aqueousacids.The most stableoxidation statefor americium is +3.[51]The chemistry of americium(III) has many similarities to the chemistry oflanthanide(III) compounds. For example, trivalent americium forms insolublefluoride,oxalate,iodate,hydroxide,phosphateand other salts.[51]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),AmVO+2;(yellow),AmVIO2+2(brown) andAmVIIO5−6(dark green).[52][53]The absorption spectra have sharp peaks, due tof-ftransitions' 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.[54][55][56][57]

Americium compounds with oxidation state +4 and higher are strong oxidizing agents, comparable in strength to thepermanganateion (MnO−4) in acidic solutions.[58]Whereas the Am4+ions are unstable in solutions and readily convert to Am3+,compounds such asamericium dioxide(AmO2) andamericium(IV) fluoride(AmF4) are stable in the solid state.

The pentavalent oxidation state of americium was first observed in 1951.[59]In acidic aqueous solution theAmO+2ion is unstable with respect todisproportionation.[60][61][62]The reaction

3[AmO2]++ 4H+→ 2[AmO2]2++ Am3++ 2H2O

is typical. The chemistry of Am(V) and Am(VI) is comparable to the chemistry ofuraniumin those oxidation states. In particular, compounds likeLi3AmO4andLi6AmO6are comparable touranatesand the ionAmO2+2is comparable to theuranylion,UO2+2.Such compounds can be prepared by oxidation of Am(III) in dilute nitric acid withammonium persulfate.[63]Other oxidising agents that have been used includesilver(I) oxide,[57]ozoneandsodium persulfate.[56]

Chemical compounds

[edit]
Main article:Americium compounds

Oxygen compounds

[edit]

Three americium oxides are known, with the oxidation states +2 (AmO), +3 (Am2O3) and +4 (AmO2).Americium(II) oxidewas prepared in minute amounts and has not been characterized in detail.[64]Americium(III) oxideis a red-brown solid with a melting point of 2205 °C.[65]Americium(IV) oxideis 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.[66]

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 AmO2at 300 °C, the decomposition completes at about 470 °C.[51]The initial oxalate dissolves in nitric acid with the maximum solubility of 0.25 g/L.[67]

Halides

[edit]

Halidesof americium are known for the oxidation states +2, +3 and +4,[68]where the +3 is most stable, especially in solutions.[69]

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 sodiumamalgamyields Am(II) salts – the black halides AmCl2,AmBr2and 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:

  • OrthorhombicAmCl2:a=896.3±0.8 pm,b=757.3±0.8 pmandc=453.2±0.6 pm
  • TetragonalAmBr2:a=1159.2±0.4 pmandc=712.1±0.3 pm.[70]They can also be prepared by reacting metallic americium with an appropriate mercury halide HgX2,where X = Cl, Br or I:[71]

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

The tetravalent americium(IV) fluoride (AmF4) is obtained by reacting solid americium(III) fluoride with molecularfluorine:[72][73]

Another known form of solid tetravalent americium fluoride is KAmF5.[72][74]Tetravalent americium has also been observed in the aqueous phase. For this purpose, black Am(OH)4was dissolved in 15-MNH4F with the americium concentration of 0.01 M. The resulting reddish solution had a characteristic optical absorption spectrum which is similar to that of AmF4but 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.[55]

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 touranium(III) chloride(space group P63/m) and the melting point of 715 °C.[68]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.[69]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 amonocliniccrystal structure.[75]

Oxyhalides of americium in the form AmVIO2X2,AmVO2X, AmIVOX2and AmIIIOX can be obtained by reacting the corresponding americium halide with oxygen or Sb2O3,and AmOCl can also be produced by vapor phasehydrolysis:[71]

AmCl3+ H2O -> AmOCl + 2HCl

Chalcogenides and pnictides

[edit]

The knownchalcogenidesof americium include thesulfideAmS2,[76]selenidesAmSe2and Am3Se4,[76][77]andtelluridesAm2Te3and AmTe2.[78]Thepnictidesof americium (243Am) of the AmX type are known for the elementsphosphorus,arsenic,[79]antimonyandbismuth.They crystallize in therock-saltlattice.[77]

Silicides and borides

[edit]

Americiummonosilicide(AmSi) and "disilicide" (nominally AmSixwith: 1.87 < x < 2.0) were obtained by reduction of americium(III) fluoride with elementarysiliconin vacuum at 1050 °C (AmSi) and 1150−1200 °C (AmSix). AmSi is a black solid isomorphic with LaSi, it has an orthorhombic crystal symmetry. AmSixhas a bright silvery lustre and a tetragonal crystal lattice (space groupI41/amd), it is isomorphic with PuSi2and ThSi2.[80]Boridesof americium include AmB4and AmB6.The tetraboride can be obtained by heating an oxide or halide of americium withmagnesium diboridein vacuum or inert atmosphere.[81][82]

Organoamericium compounds

[edit]
Predicted structure of amerocene [(η8-C8H8)2Am]

Analogous touranocene,americium forms the organometallic compound amerocene with twocyclooctatetraeneligands, with the chemical formula (η8-C8H8)2Am.[83]Acyclopentadienyl complexis also known that is likely to be stoichiometrically AmCp3.[84][85]

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 byEXAFS.Some of these BTP-type complexes selectively interact with americium and therefore are useful in its selective separation from lanthanides and another actinides.[86]

Biological aspects

[edit]

Americium is an artificial element of recent origin, and thus does not have abiological requirement.[87][88]It is harmful tolife.It has been proposed to use bacteria for removal of americium and otherheavy metalsfrom rivers and streams. Thus,Enterobacteriaceaeof the genusCitrobacterprecipitate americium ions from aqueous solutions, binding them into a metal-phosphate complex at their cell walls.[89]Several studies have been reported on thebiosorptionandbioaccumulationof americium by bacteria[90][91]and fungi.[92]

Fission

[edit]

The isotope242mAm (half-life 141 years) has the largest cross sections for absorption of thermal neutrons (5,700barns),[93]that results in a smallcritical massfor a sustainednuclear chain reaction.The critical mass for a bare242mAm 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.[94]Such small critical mass is favorable for portablenuclear weapons,but those based on242mAm are not known yet, probably because of its scarcity and high price. The critical masses of the two readily available isotopes,241Am and243Am, are relatively high – 57.6 to 75.6 kg for241Am and 209 kg for243Am.[95]Scarcity and high price yet hinder application of americium as anuclear fuelinnuclear reactors.[96]

There are proposals of very compact 10-kW high-flux reactors using as little as 20 grams of242mAm. Such low-power reactors would be relatively safe to use asneutron sourcesforradiation therapyin hospitals.[97]

Isotopes

[edit]
See also:Isotopes of americium

About 18isotopesand 11nuclear isomersare known for americium, having mass numbers 229, 230, and 232 through 247.[3]There are two long-lived Alpha -emitters;243Am has a half-life of 7,370 years and is the most stable isotope, and241Am has a half-life of 432.2 years. The most stable nuclear isomer is242m1Am; it has a long half-life of 141 years. The half-lives of other isotopes and isomers range from 0.64 microseconds for245m1Am to 50.8 hours for240Am. 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.[14]

Americium-241decays to237Np 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 emitgamma rayswith the discrete energies between 26.3 and 158.5 keV.[98]

Americium-242is a short-lived isotope with a half-life of 16.02 h.[14]It mostly (82.7%) converts by β-decay to242Cm, but also byelectron captureto242Pu (17.3%). Both242Cm and242Pu transform via nearly the same decay chain through238Pu down to234U.

Nearly all (99.541%) of242m1Am decays byinternal conversionto242Am and the remaining 0.459% by α-decay to238Np. The latter subsequently decays to238Pu and then to234U.[14]

Americium-243transforms by α-emission into239Np, which converts by β-decay to239Pu, and the239Pu changes into235U by emitting an α-particle.

Applications

[edit]
Outside and inside view of an americium-based smoke detector

Ionization-type smoke detector

[edit]
Main article:Smoke detector § Ionization

Americium is used in the most common type of householdsmoke detector,which uses241Am in the form of americium dioxide as its source ofionizing radiation.[99]This isotope is preferred over226Rabecause it emits 5 times more Alpha particles and relatively little harmful gamma radiation.

The amount of americium in a typical new smoke detector is 1microcurie(37kBq) or 0.29microgram.This amount declines slowly as the americium decays intoneptunium-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%neptuniumafter 19 years, and about 5% after 32 years. The radiation passes through anionization chamber,an air-filled space between twoelectrodes,and permits a small, constantcurrentbetween 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 tofalse alarms.[100][101][102][103]

Radionuclide

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As241Am has a roughly similar half-life to238Pu (432.2 years vs. 87 years), it has been proposed as an active element ofradioisotope thermoelectric generators,for example in spacecraft.[104]Although americium produces less heat and electricity – the power yield is 114.7 mW/g for241Am and 6.31 mW/g for243Am[1](cf. 390 mW/g for238Pu)[104]– and its radiation poses more threat to humans owing to neutron emission, theEuropean Space Agencyis considering using americium for its space probes.[105]

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 of242mAm, 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.[106][107]The fission products of242mAm 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 amagnetohydrodynamic generator.[108]

One more proposal which utilizes the high nuclear fission rate of242mAm 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 kg242mAm charge of such battery could provide about 140 kW of power over a period of 80 days.[109]Even with all the potential benefits, the current applications of242mAm are as yet hindered by the scarcity and high price of this particularnuclear isomer.[108]

In 2019, researchers at the UKNational Nuclear Laboratoryand theUniversity of Leicesterdemonstrated 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 intointerstellar space,where solar panels do not function.[110][111]

Neutron source

[edit]

The oxide of241Am pressed withberylliumis an efficientneutron source.Here americium acts as the Alpha source, and beryllium produces neutrons owing to its large cross-section for the (α,n) nuclear reaction:

The most widespread use of241AmBe neutron sources is aneutron 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 inneutron radiography,tomography and other radiochemical investigations.[112]

Production of other elements

[edit]

Americium is a starting material for the production of other transuranic elements andtransactinides– for example, 82.7% of242Am decays to242Cm and 17.3% to242Pu. In the nuclear reactor,242Am is also up-converted by neutron capture to243Am and244Am, which transforms by β-decay to244Cm:

Irradiation of241Am by12C or22Ne ions yields the isotopes247Es (einsteinium) or260Db (dubnium), respectively.[112]Furthermore, the elementberkelium(243Bk isotope) had been first intentionally produced and identified by bombarding241Am with Alpha particles, in 1949, by the same Berkeley group, using the same 60-inch cyclotron. Similarly,nobeliumwas produced at theJoint Institute for Nuclear Research,Dubna,Russia, in 1965 in several reactions, one of which included irradiation of243Am with15N ions. Besides, one of the synthesis reactions forlawrencium,discovered by scientists at Berkeley and Dubna, included bombardment of243Am with18O.[10]

Spectrometer

[edit]

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 from241Am in such sources can be used for indirect analysis of materials inradiographyandX-ray fluorescencespectroscopy, as well as for quality control in fixednuclear density gaugesandnuclear densometers.For example, the element has been employed to gaugeglassthickness to help create flat glass.[28]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).[113]Americium-241 gamma rays were also used to provide passive diagnosis of thyroid function. This medical application is however obsolete.

Health concerns

[edit]

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.[114]

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 theliverand 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 overcorticalandtrabecularsurfaces and slowly redistributes over the bone with time. The biological half-life of241Am is 50 years in the bones and 20 years in the liver, whereas in thegonads(testicles and ovaries) it remains permanently; in all these organs, americium promotes formation of cancer cells as a result of its radioactivity.[22][115][116]

Americium often enters landfills from discardedsmoke detectors.The rules associated with the disposal of smoke detectors are relaxed in most jurisdictions. In 1994, 17-year-oldDavid Hahnextracted the americium from about 100 smoke detectors in an attempt to build a breeder nuclear reactor.[117][118][119][120]There have been a few cases of exposure to americium, the worst case being that ofchemical operations technicianHarold 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.[121][122]

See also

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Notes

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  1. ^The "metastable" state is marked by the letter m.

References

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Bibliography

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Further reading

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