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31 janvier 2012 2 31 /01 /janvier /2012 09:00

Chalcogenides and pnictides

The known chalcogenides of americium include the sulfide AmS2,[69] selenides AmSe2 and Am3Se4,[69][70] and tellurides Am2Te3and AmTe2.[71] The pnictides of americium (243Am) of the AmX type are known for the elements phosphorus, arsenic,[72]antimony and bismuth. They crystallize in the rock-salt lattice.[70] Sony VAIO VGN-BZ12XN Battery

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 siliconin vacuum at 1050 °C (AmSi) and 1150?1200 °C (AmSix). AmSi is a black solid isomorphic with LaSi, it has an orthorhombic crystal symmetry. Sony VAIO VGN-BZ31VT Battery

AmSix has a bright silvery lustre and a tetragonal crystal lattice (space group I41/amd), it is isomorphic with PuSi2 and ThSi2.[73]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.[74][75] Sony VAIO VGN-BZ31XT Battery

Organometallic compounds

Analogous to uranocene, americium forms an organometallic compound with two cyclooctatetraene ligands, that is (?8-C8H8)2Am.[76] It also makes trigonal (?5-C5H5)3Am complexes with three cyclopentadienyl rings.[77] Sony VAIO VGN-BZ560 Battery

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.[78] Sony VAIO VGN-BZ560N24 Battery

Biological aspects

Americium is an artificial element, and thus a biological function involving the element, like all elements heavier than tungsten, and thus all artificial elements, would be impossible.[79][80] It has been proposed to use bacteria for removal of americium and other heavy metals from rivers and streams. Sony VAIO VGN-BZ560N30 Battery

Thus, Enterobacteriaceae of the genus Citrobacter precipitate americium ions from aqueous solutions, binding them into a metal-phosphate complex at their cell walls.[81] Several studies have been reported on the biosorption and bioaccumulation of americium by bacteria[82][83] and fungi.[84] Sony VAIO VGN-BZ560P Battery

Fission

The isotope 242m1Am (half-life 141 years) has the largest cross sections for absorption of thermal neutrons (5,700 barns),[85] that results in a small critical massfor 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). Sony VAIO VGN-BZ560P20 Battery

It can be lowered to 3–5 kg with a metal reflector and should become even smaller with a water reflector.[86] 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. Sony VAIO VGN-BZ560P22 Battery

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.[87] Scarcity and high price yet hinder application of americium as a nuclear fuel in nuclear reactors.[88]

There are proposals of very compact 10-kW high-flux reactors using as little as 20 grams of 242m1Am. Sony VAIO VGN-BZ560P28 Battery

Such low-power reactors would be relatively safe to use asneutron sources for radiation therapy in hospitals.[89]

About 19 isotopes and 8 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. Sony VAIO VGN-BZ560P30 Battery

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.[12]

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%).Sony VAIO VGN-BZ560P34 Battery

Because many of the resulting states are metastable, they also emit gamma-rays with the discrete energies between 26.3 and 158.5 keV.[90]

Americium-242 is a short-lived isotope with a half-life of 16.02 h.[12] It mostly (82.7%) converts by ?-decay to 242Cm, but also by electron capture to 242Pu (17.3%). Sony VAIO VGN-BZ561 Battery

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 ?-decay to 238Np. The latter breaks down to 238Pu and then to234U.[12]

Americium-243 transforms by ?-emission into 239Np, which converts by ?-decay to 239Pu, and the 239Pu changes into 235U by emitting an ?-particle. Sony VAIO VGN-BZ561N20 Battery

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.[91] This isotope is preferred against 226Ra because it emits 5 times more alpha particles and relatively little of harmful ?-radiation. Sony VAIO VGN-BZ561P20 Battery

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 differenttransuranic 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. Sony VAIO VGN-BZ562P Battery

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. Sony VAIO VGN-BZ563P Battery

Sony VAIO VGN-BZAAFS Battery

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.[92][93][94][95] Sony VAIO VGN-BZAAHS Battery

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 ofradioisotope thermoelectric generators, for example in spacecraft.[96] Sony VAIO VGN-BZAANS Battery

Sony VAIO VGN-BZAAPS Battery

Although americium produces less heat and electricity – the power yield is 114.7 mW/g for 241Am and 6.31 mW/g for 243Am[1] (cf. 390 mW/g for 238Pu)[96] – 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.[97]

Another proposed space-related application of americium is a fuel for space ships with nuclear propulsion. Sony VAIO VGN-CR11H/B Battery

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.[98][99] Sony VAIO VGN-CR11S/L Battery

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.[100]

One more proposal which utilizes the high nuclear fission rate of 242mAm is a nuclear battery. Sony VAIO VGN-CR11S/P 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.[101] With all the potential benefits, the current applications of 242mAm are as yet hindered by the scarcity and high price of thisnuclear isomer.[100] Sony VAIO VGN-CR11S/W Battery

]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 (?,n) nuclear reaction: Sony VAIO VGN-CR11Z/R Battery

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.[102] Sony VAIO VGN-CR120E/L Battery

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% to242Pu. In the nuclear reactor, 242Am is also up-converted by neutron capture to 243Am and 244Am, which transforms by ?-decay to 244Cm: Sony VAIO VGN-CR120E/P Battery

Irradiation of 241Am by 12C or 22Ne ions yields the isotopes 247Es (einsteinium) or 260Db (dubnium), respectively.[102]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. Sony VAIO VGN-CR120E/R Battery

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 of243Am with 18O.[7] Sony VAIO VGN-CR120E/W Battery

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 fixednuclear density gauges and nuclear densometers. Sony VAIO VGN-CR125E/B Battery

For example, the element has been employed to gauge glass thickness to help create flat glass.[22] 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).[103] Sony VAIO VGN-CR13/B Battery

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. Sony VAIO VGN-CR13/L Battery

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.[104]

If consumed, americium is excreted within a few days and only 0.05% is absorbed in the blood. Sony VAIO VGN-CR13/P Battery

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 corticaland trabecular surfaces and slowly redistributes over the bone with time. Sony VAIO VGN-CR13/R Battery

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.[17][105][106]

Americium often enters landfills from discarded smoke detectors. Sony VAIO VGN-CR13/W Battery

The rules associated with the disposal of smoke detectors are relaxed in most jurisdictions. In the U.S., the "Radioactive Boy Scout" David Hahn was able to concentrate americium from smoke detectors after managing to buy a hundred of them at remainder prices and also stealing a few. Sony VAIO VGN-CR131E/L Battery

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. Sony VAIO VGN-CR13G Battery

Curium is a transuranic radioactive chemical element with the symbol Cm and atomic number96. This radioactive transuranic element of the actinide series was named after Marie Sk?odowska-Curie and her husbandPierre Curie. Curium was first intentionally produced and identified in summer 1944 by the group of Glenn T. Seaborgat the University of California, Berkeley. Sony VAIO VGN-CR13G/B Battery

The discovery was kept secret and only released to the public in November 1945. Most curium is produced by bombarding uranium or plutoniumwith neutrons in nuclear reactors – one tonne of spent nuclear fuel contains about 20 grams of curium.

Curium is a hard, dense silvery metal with a relatively high melting point and boiling point for an actinide. Sony VAIO VGN-CR13G/L Battery

Whereas it is paramagneticat ambient conditions, it becomes antiferromagnetic upon cooling, and other magnetic transitions are also observed for many curium compounds. In compounds, curium usually exhibits valence +3 and sometimes +4, and the +3 valence is predominant in solutions. Curium readily oxidizes, and its oxides are a dominant form of this element. Sony VAIO VGN-CR13G/P Battery

It forms strongly fluorescent complexes with various organic compounds, but there is no evidence of its incorporation into bacteria and archaea. When introduced into the human body, curium accumulates in the bones, lungs and liver where it promotes cancer.

All known isotopes of curium are radioactive and have a small critical mass for a sustained nuclear chain reaction. Sony VAIO VGN-CR13G/R Battery

They predominantly emit ?-particles, and the heat released in this process can potentially produce electricity inradioisotope thermoelectric generators. This application is hindered by the scarcity, high cost and radioactivity of curium isotopes. Curium is used in production of heavier actinides and of the 238Pu radionuclide for power sources inartificial pacemakers. Sony VAIO VGN-CR13G/W Battery

It served as the ?-source in the alpha particle X-ray spectrometers installed on theSojourner, Mars, Mars 96, Athena, Spirit and Opportunity rovers to analyze the composition and structure of the rocks on the surface of Mars and the Moon. Such a spectrometer will also be used by the Philae lander of the Rosettaspacecraft to probe the surface of the 67P/Churyumov-Gerasimenko comet. Sony VAIO VGN-CR13T/L Battery

Although curium had likely been produced in previous nuclear experiments, it was first intentionally synthesized, isolated and identified in 1944, at the University of California, Berkeley by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso. In their experiments, they used a 60-inch (150 cm) cyclotron.[2] Sony VAIO VGN-CR13T/P Battery

Curium was chemically identified at the Metallurgical Laboratory (now Argonne National Laboratory) at the University of Chicago. It was the third transuranium element to be discovered even though it is the fourth in the series – the lighter element americium was unknown at the time.[3][4] Sony VAIO VGN-CR13T/R Battery

The sample was prepared as follows: first plutonium nitrate solution was coated on aplatinum foil of about 0.5 cm2 area, the solution was evaporated and the residue was converted into plutonium dioxide (PuO2) by annealing. Following cyclotron irradiation of the oxide, the coating was dissolved with nitric acid and then precipitated as the hydroxide using concentrated aqueous ammonia solution. Sony VAIO VGN-CR13T/W Battery

The residue was dissolved inperchloric acid, and further separation was carried out by ion exchange to yield a certain isotope of curium. The separation of curium and americium was so painstaking that the Berkeley group initially called those elements pandemonium (from Greek for all demons orhell) and delirium (from Latin for madness). Sony VAIO VGN-CR15/B Battery

The curium-242 isotope was produced in July–August 1944 by bombarding 239Pu with ?-particles to produce curium with the release of a neutron:

Curium-242 was unambiguously identified by the characteristic energy of the ?-particles emitted during the decay: Sony VAIO VGN-CR150E/B Battery

The half-life of this alpha decay was first measured as 150 days and then corrected to 162.8 days.[9]

Another isotope 240Cm was produced in a similar reaction in March 1945:

The half-life of the 240Cm ?-decay was correctly determined as 26.7 days.[9] Sony VAIO VGN-CR190 Battery

The discovery of curium, as well as americium, 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, Sony VAIO VGN-CR190E/L Battery

when one of the listeners asked whether any new transuranium element beside plutonium andneptunium had been discovered during the war.[5] The discovery of curium (242Cm and 240Cm), their production and compounds were later patented listing only Seaborg as the inventor.[10] Sony VAIO VGN-CR190E/P Battery

"As the name for the element of atomic number 96 we should like to propose "curium" , with symbol Cm. The evidence indicates that element 96 contains seven 5f electrons and is thus analogous to the element gadolinium with its seven 4f electrons in the regular rare earth series. On this base element 96 is named after the Curies in a manner analogous to the naming of gadolinium, in which the chemist Gadolin was honored."[3] Sony VAIO VGN-CR190E/R Battery

The first curium samples were barely visible, and were identified by their radioactivity. Louis Werner and Isadore Perlman created the first substantial sample of 30 µg curium-242 hydroxide at the University of California in 1947 by bombarding americium-241 with neutrons. Sony VAIO VGN-CR190E/W Battery

Macroscopic amounts of curium fluoride were obtained in 1950 by W. W. T. Crane, J. C. Wallmann and B. B. Cunningham. Its magnetic susceptibility was very close to that of GdF3 providing the first experimental evidence for the +3 valence of curium in its compounds.[12] Curium metal was produced only in 1951 by reduction of curium fluoride with barium.[15][16] Sony VAIO VGN-CR19VN/B Battery

Physical

A synthetic, radioactive element, curium is a hard dense metal with silvery-white appearance and physical and chemical properties resembling those of gadolinium. Its melting point of 1340 °C is significantly higher than that of the previous transuranic elements neptunium (637 °C), plutonium (639 °C) and americium (1173 °C). Sony VAIO VGN-CR19XN/B Battery

In comparison, gadolinium melts at 1312 °C. The boiling point of curium is 3110 °C. With a density of 13.52 g/cm3, curium is significantly lighter than neptunium (20.45 g/cm3) and plutonium (19.8 g/cm3), but is heavier than most other metals. Between two crystalline forms of curium, the ?-Cm is more stable at ambient conditions. Sony VAIO VGN-CR20 Battery

It has a hexagonal symmetry, space group P63/mmc, lattice parameters a = 365 pm and c = 1182 pm, and four formula units per unit cell.[17] The crystal consists of a double-hexagonal close packing with the layer sequence ABAC and so is isotypic with ?-lanthanum. At pressures above 23 GPa, at room temperature, ?-Cm transforms into ?-Cm, which has a face-centered cubicsymmetry, space group Fm3m and the lattice constant a = 493 pm.[17] Sony VAIO VGN-CR21/B Battery

Upon further compression to 43 GPa, curium transforms to an orthorhombic ?-Cm structure similar to that of ?-uranium, with no further transitions observed up to 52 GPa. These three curium phases are also referred to as Cm I, II and III.[18][19]

Curium has peculiar magnetic properties. Sony VAIO VGN-CR21E/L Battery

Whereas its neighbor element actinium shows no deviation from Curie-Weissparamagnetismin the entire temperature range, ?-Cm transforms to an antiferromagnetic state upon cooling to 65–52 K,[20][21] and ?-Cm exhibits a ferrimagnetic transition at about 205 K. Meanwhile, curium pnictides show ferromagnetic transitions upon cooling: Sony VAIO VGN-CR21E/P Battery

244CmN and 244CmAs at 109 K,248CmP at 73 K and 248CmSb at 162 K. Similarly, the lanthanide analogue of curium, gadolinium, as well as its pnictides also show magnetic transitions upon cooling, but the transition character is somewhat different: Gd and GdN become ferromagnetic, and GdP, GdAs and GdSb show antiferromagnetic ordering.[22] Sony VAIO VGN-CR21E/W Battery

In accordance with magnetic data, electrical resistivity of curium increases with temperature – about twice between 4 and 60 K – and then remains nearly constant up to room temperature. There is a significant increase in resistvity over time (about 10 µOhm·cm/h) due to self-damage of the crystal lattice by alpha radiation. Sony VAIO VGN-CR21S/L Battery

This makes uncertain the absolute resistivity value for curium (about 125 µ?·cm). The resistivity of curium is similar to that of gadolinium and of the actinides plutonium and neptunium, but is significantly higher than that of americium, uranium, polonium and thorium.[1][23] Sony VAIO VGN-CR21S/P Battery

Under ultraviolet illumination, curium(III) ions exhibit strong and stable yellow-orange fluorescence with a maximum in the range about 590–640 nm depending on their environment.[24] The fluorescence originates from the transitions from the first excited state 6D7/2 and the ground state 8S7/2. Analysis of this fluorescence allows monitoring interactions between Cm(III) ions in organic and inorganic complexes.[25] Sony VAIO VGN-CR21S/W Battery

Chemical

Curium ions in solution assume the oxidation state of +3, which is the most stable oxidation state for curium.[26] The +4 oxidation state is observed only in a few solid phases, such as CmO2 and CmF4.[27][28] Chemical behavior of curium is different from actinides thorium and uranium, and is similar to that of americium and many lanthanides. Sony VAIO VGN-CR21Z/N Battery

In aqueous solution, the Cm3+ ion is colorless to pale green,[29] and Cm4+ ion is pale yellow.[30] The optical absorption of Cm3+ions contains three sharp peaks at 375.4, 381.2 and 396.5 nanometers and their strength can be directly converted into the concentration of the ions.[31] Curium ions are hard Lewis acids and thus form most stable complexes with hard bases.[32] Sony VAIO VGN-CR21Z/R Battery

The bonding is mostly ionic, with a small covalent component.[33]Curium in its complexes commonly exhibits a 9-fold coordination environment, within a tricapped trigonal prismaticgeometry.[34]

About 20 radioisotopes and 7 nuclear isomers between 233Cm and 252Cm are known for curium, and no stable isotopes. Sony VAIO VGN-CR220E/R Battery

The longest half-lives have been reported for 247Cm (15.6 million years) and 248Cm (348,000 years). Other long-lived isotopes are 245Cm (half-life 8500 years), 250Cm (8,300 years) and246Cm (4,760 years). Curium-250 is unusual by that it predominantly (about 86%) decays via spontaneous fission. Sony VAIO VGN-CR23/B Battery

The most commonly used curium isotopes are 242Cm and 244Cm with the half-lives of 162.8 days and 18.1 years, respectively.[9]

All isotopes between 242Cm and 248Cm, as well as 250Cm, undergo a self-sustaining nuclear chain reaction and thus in principle can act as a nuclear fuel in a reactor. Sony VAIO VGN-CR23/L Battery

As in most transuranic elements, the nuclear fission cross section is especially high for the odd-mass curium isotopes243Cm,245Cm and 247Cm. These can be used in thermal-neutron reactors, whereas a mixture of curium isotopes is only suitable for fast breeder reactors since the even-mass isotopes are not fissile in a thermal reactor and accumulate as burn-up increases.[39] Sony VAIO VGN-CR23/N Battery

The mixed-oxide (MOX) fuel, which is to be used in power reactors, should contain little or no curium because the neutron activation of 248Cm will createcalifornium. This is strong neutron emitter, and would pollute the back end of the fuel cycle and increase the dose to reactor personnel. Sony VAIO VGN-CR23/P Battery

Hence, if the minor actinides are to be used as fuel in a thermal neutron reactor, the curium should be excluded from the fuel or placed in special fuel rods where it is the only actinide present.[40]

The table to the right lists the critical masses for curium isotopes for a sphere, without a moderator and reflector. Sony VAIO VGN-CR23/R Battery

With a metal reflector (30 cm of steel), the critical masses of the odd isotopes are about 3–4 kg. When using water (thickness ~20–30 cm) as the reflector, the critical mass can be as small as 59 gram for 245Cm, 155 gram for 243Cm and 1550 gram for 247Cm. There is a significant uncertainty in these critical mass values. Sony VAIO VGN-CR23/W Battery

Whereas it is usually of the order 20%, the values for 242Cm and 246Cm were listed as large as 371 kg and 70.1 kg, respectively, by some research groups.[39][41]

Currently, curium is not used as a nuclear fuel owing to its low availability and high price.[42] 245Sony VAIO VGN-CR240E/B Battery

Cm and 247Cm have a very small critical mass and therefore could be used in portable nuclear weapons, but none have been reported thus far. Curium-243 is not suitable for this purpose because of its short half-life and strong ? emission which would result in excessive heat.[43] Curium-247 would be highly suitable, having a half-life 647 times that of Plutonium 239. Sony VAIO VGN-CR240N/B Battery

Occurrence

The longest-lived isotope of curium, 247Cm, has a half-life of 15.6 million years. Therefore, all primordialcurium, that is curium present on the Earth during its formation, should have decayed by now. Curium is produced artificially, in small quantities for research purposes. Sony VAIO VGN-CR25G/N Battery

Furthermore, it occurs in spent nuclear fuel. Curium is present in nature in certain areas used for the atmospheric nuclear weapons tests, which were conducted between 1945 and 1980.[44] So the analysis of the debris at the testing site of the first U.S. hydrogen bomb, Ivy Mike, (1 November 1952, Enewetak Atoll), Sony VAIO VGN-CR290EAL Battery

beside einsteinium, fermium, plutoniumand americium also revealed isotopes of berkelium, californium and curium, in particular 245Cm, 246Cm and smaller quantities of 247Cm, 248Cm and 249Cm. For reasons of military secrecy, this result was published only in 1956.[45]

Atmospheric curium compounds are poorly soluble in common solvents and mostly adhere to soil particles. Sony VAIO VGN-CR290EAN Battery

Soil analysis revealed about 4,000 times higher concentration of curium at the sandy soil particles than in water present in the soil pores. An even higher ratio of about 18,000 was measured in loam soils.[46]

A few atoms of curium can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits.[47] Sony VAIO VGN-CR290EAP Battery

Isotope preparation

Curium is produced in small quantities in nuclear reactors, and by now only kilograms of it have been accumulated for the 242Cm and 244Cm and grams or even milligrams for heavier isotopes. This explains the high price of curium, which has been be quoted at 160–185 USD per milligram,[12] with a more recent estimate at 2,000 USD/g for 242Cm and 170 USD/g for 244Cm.[48] Sony VAIO VGN-CR290EAR Battery

In nuclear reactors, curium is formed from 238U in a series of nuclear reactions. In the first chain, 238U captures a neutron and converts into 239U, which via ?– decay transforms into 239Np and 239Pu. Such nuclear fusion process is used in the breeder reactors for producing fissile material. Sony VAIO VGN-CR290EAW Battery

(the times are half-lives).

Further neutron capture followed by ?–-decay produces the 241Am isotope of americium which further converts into 242Cm:For research purposes, curium is obtained by irradiating not uranium but plutonium, which is available in large amounts from spent nuclear fuel. Much higher neutron flux is used for the irradiation that results in a different reaction chain and formation of 244Cm:[4]Sony VAIO VGN-CR29XN/B Battery

Curium-244 decays into 240Pu by emission of alpha particle, but it also absorbs neutrons resulting in a small amount of heavier curium isotopes. Among those, 247Cm and 248Cm are popular in scientific research because of their long half-lives. However, the production rate of 247Cm in thermal neutron reactors is relatively low because of it is prone to undergo fission induced by thermal neutrons.[49] Sony VAIO VGN-CR305E/RC Battery

Synthesis of 250Cm via neutron absorption is also rather unlikely because of the short half-life of the intermediate product 249Cm (64 min), which converts by ?– decay to the berkelium isotope 249Bk.[49]

(for A = 244–248)

The above cascade of (n,?) reactions produces a mixture of different curium isotopes. Sony VAIO VGN-CR31S/D Battery

Their post-synthesis separation is cumbersome, and therefore a selective synthesis is desired. Curium-248 is favored for research purposes because of its long half-life. The most efficient preparation method of this isotope is via ?-decay of the californium isotope 252Cf, which is available in relatively large quantities due to its long half-life (2.65 years). Sony VAIO VGN-CR323/W Battery

About 35–50 mg of 248Cm is being produced by this method every year. The associated reaction produces 248Cm with isotopic purity of 97%.[49]

Another interesting for research isotope 245Cm can be obtained from the ?-decay of 249Cf, and the latter isotope is produced in minute quantities from the ?–-decay of the berkelium isotope 249Bk. Sony VAIO VGN-CR33 Battery

Metal preparation

Most synthesis routines yield a mixture of different actinide isotopes as oxides, from which a certain isotope of curium needs to be separated. An example procedure could be to dissolve spent reactor fuel (e.g. MOX fuel) in nitric acid, and remove the bulk of the uranium and plutonium using a PUREX (Plutonium – URanium EXtraction) type extraction with tributyl phosphate in a hydrocarbon. Sony VAIO VGN-CR382 Battery

The lanthanides and the 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. A curium compound is then selectively extracted using multi-step chromatographic and centrifugation techniques with an appropriate reagent.[50] Sony VAIO VGN-CR490EBL Battery

Bis-triazinyl bipyridinecomplex has been recently proposed as such reagent which is highly selective to curium.[51] Separation of curium from a very similar americium can also be achieved by treating a slurry of their hydroxides in aqueous sodium bicarbonate with ozone at elevated temperature. Both americium and curium are present in solutions mostly in the +3 valence state;Sony VAIO VGN-CR490EBN Battery

Sony VAIO VGN-CR490EBP Battery

whereas americium oxidizes to soluble Am(IV) complexes, curium remains unchanged and can thus be isolated by repeated centrifugation.[52]

Metallic curium is obtained by reduction of its compounds. Initially, curium(III) fluoride was used for this purpose. Sony VAIO VGN-CR490EBR Battery

The reaction was conducted in the environment free from water and oxygen, in the apparatus made of tantalum and tungsten, using elementalbariumor lithium as reducing agents.

Another possibility is the reduction of curium(IV) oxide using a magnesium-zinc alloy in a melt of magnesium chlorideandmagnesium fluoride.[56] Sony VAIO VGN-CR490EBT Battery

Oxides

Curium readily reacts with oxygen forming mostly Cm2O3 and CmO2 oxides,[44] but the divalent oxide CmO is also known.[57]Black CmO2 can be obtained by burning curiumoxalate (Cm2(C2O4)3), nitrate (Cm(NO3)3) or hydroxide in pure oxygen.[28][58]Upon heating to 600–650 °C in vacuum (about 0.01 Pa), it transforms into the whitish Cm2O3:[ Sony VAIO VGN-CR490EBW Battery

Alternatively, Cm2O3 can be obtained by reducing CmO2 with molecular hydrogen:[60]

Furthermore, a number of ternary oxides of the type M(II)CmO3 are known, where M stays for a divalent metal, such as barium.[61]Sony VAIO VGN-CR50B/W Battery

Halides

The colorless curium(III) fluoride (CmF3) can be produced by introducing fluoride ions into curium(III)-containing solutions. The brown tetravalent curium(IV) fluoride (CmF4) on the other hand is only obtained by reacting curium(III) fluoride with molecular fluorine:[4] Sony VAIO VGN-CR51B/W Battery

A series of ternary fluorides are known of the form A7Cm6F31, where A stands for alkali metal.[62]

The colorless curium(III) chloride (CmCl3) is produced in the reaction of curium(III) hydroxide (Cm(OH)3) with anhydrous hydrogen chloride gas. Sony VAIO VGN-CR520E/J Battery

It can further be converted into other halides, such as curium(III) bromide (colorless to light green) and curium(III) iodide (colorless), by reacting it with the ammonia salt of the corresponding halide at elevated temperature of about 400–450 °C:[63]

An alternative procedure is heating curium oxide to about 600 °C with the corresponding acid (such as hydrobromic for curium bromide).[64][65] Sony VAIO VGN-CR52B/W Battery

Chalcogenides and pnictides

Sulfides, selenides and tellurides of curium have been obtained by treating curium with gaseous sulfur, selenium or tellurium in vacuum at elevated temperature. The pnictides of curium of the type CmX are known for the elements nitrogen, phosphorus, arsenic and antimony.[4] Sony VAIO VGN-CR590EBL Battery

They can be prepared by reacting either curium(III) hydride (CmH3) or metallic curium with these elements at elevated temperatures.

Organometallic compounds and biological aspects

Organometallic complexes analogous to uranocene are known also for other actinides, such as thorium, protactinium, neptunium, plutonium and americium. Sony VAIO VGN-CR590EBN Battery

The molecular orbital theory predicts a stable "curocene" complex (?8-C8H8)2Cm, but it has not been reported experimentally yet.[69][70]

Formation of the complexes of the type Cm(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 Cm3+ ions has been confirmed by EXAFS. Sony VAIO VGN-CR590EBP Battery

Some of these BTP-type complexes selectively interact with curium and therefore are useful in its selective separation from lanthanides and another actinides.[24][71] Dissolved Cm3+ ions bind with many organic compounds, such as hydroxamic acid,[72]urea,[73] fluorescein[74] and adenosine triphosphate.[75] Sony VAIO VGN-CR590EBR Battery

Many of these compounds are related to biological activity of variousmicroorganisms. The resulting complexes exhibit strong yellow-orange emission under UV light excitation, which is convenient not only for their detection, but also for studying the interactions between the Cm3+ ion and the ligands via changes in the half-life (of the order ~0.1 ms) and spectrum of the fluorescence. Sony VAIO VGN-CR590EBT Battery

Curium has no biological significance.[76] There are a few reports on biosorption of Cm3+ by bacteria and archaea, however no evidence for incorporation of curium into them.

Radionuclides

Curium is one of the most radioactive isolable elements. Sony VAIO VGN-CR590EBW Battery

Its two most common isotopes 242Cm and 244Cm are strong alpha emitters (energy 6 MeV); they have relatively short half-lives of 162.8 days and 18.1 years, and produce as much as 120 W/g and 3 W/g of thermal energy, respectively. Therefore, curium can be used in its common oxide form in radioisotope thermoelectric generators like those in spacecraft. Sony VAIO VGN-CR60B/L Battery

This application has been studied for the 244Cm isotope, while 242Cm was abandoned due to its prohibitive price of around 2000 USD/g. Curium-243 with a ~30 year half-life and good energy yield of ~1.6 W/g could make for a suitable fuel, but it produces significant amounts of harmful gamma and beta radiation from radioactive decay products. Sony VAIO VGN-CR60B/P Battery

Though as an ?-emitter, 244Cm requires a much thinner radiation protection shielding, it has a high spontaneous fission rate, and thus the neutron and gamma radiation rate are relatively strong. As compared to a competing thermoelectric generator isotope such as 238Pu,244Cm emits a 500 time greater fluence of neutrons, Sony VAIO VGN-CR60B/R Battery

and its higher gamma emission requires a shield that is 20 times thicker — about 2 inches of lead for a 1 kW source, as compared to 0.1 in for 238Pu. Therefore this application of curium is currently considered impractical.[48]

A more promising application of 242Cm is to produce 238Pu, a more suitable radioisotope for thermoelectric generators such as in cardiac pacemakers. Sony VAIO VGN-CR61B/L Battery

The alternative routes to 238Pu use the (n,?) reaction of 237Np, or the deuteron bombardment of uranium, which both always produce 236Pu as an undesired by-product — since the latter decays to 208Tl with strong gamma emission.[81] Sony VAIO VGN-CR61B/N Battery

Curium is also a common starting material for the production of higher transuranic elements and transactinides. Thus, bombardment of 248Cm with oxygen (18O) or magnesium (26Mg) yielded certain isotopes of seaborgium (265Sg) and hassium (269Hs and 270Hs).[82]

]X-ray spectrometer

The most practical application of 244Cm — though rather limited in total volume — is as ?-particle source in the alpha particle X-ray spectrometers (APXS). Sony VAIO VGN-CR61B/P Battery

These instruments were installed on the Sojourner, Mars, Mars 96, Spirit, Athena and Opportunity rovers to analyze the composition and structure of the rocks on the surface of planet Mars.[84] APXS was also used in the Surveyor 5–7 moon probes but with a 242Cm source. Sony VAIO VGN-CR61B/R Battery

An elaborated APXS setup is equipped with a sensor head containing six curium sources having the total radioactive decay rate of several tens of millicuries (roughly a gigabecquerel). The sources are collimated on the sample, and the energy spectra of the alpha particles and protons scattered from the sample are analyzed (the proton analysis is implemented only in some spectrometers). Sony VAIO VGN-CR62B/L Battery

These spectra contain quantitative information on all major elements in the samples except for hydrogen, helium and lithium.[87]An APXS will also be used by the Philae lander of the Rosetta spacecraft to probe the surface of the67P/Churyumov-Gerasimenkocomet.[88]

Safety

Owing to its high radioactivity, curium and its compounds must be handled in appropriate laboratories under special arrangements.Sony VAIO VGN-CR62B/N Battery

Whereas curium itself mostly emits ?-particles which are absorbed by thin layers of common materials, some of its decay products emit significant fractions of beta and gamma radiation, which require a more elaborate protection.[44] If consumed, curium is excreted within a few days and only 0.05% is absorbed in the blood. Sony VAIO VGN-CR62B/P Battery

From there, about 45% goes to the liver, 45% to the bones, and the remaining 10% is excreted. In the bone, curium accumulates on the inside of the interfaces to the bone marrow and does not significantly redistribute with time; its radiation destroys bone marrow and thus stops red blood cell creation. The biological half-lifeof curium is about 20 years in the liver and 50 years in the bones.[44][46] Sony VAIO VGN-CR62B/R Battery

Curium is absorbed in the body much more strongly via inhalation, and the allowed total dose of 244Cm in soluble form is 0.3 ?C.[12]Intravenous injection of 242Cm and 244Cm containing solutions to rats increased the incidence of bone tumor, and inhalation promoted pulmonary and liver cancer.[44] Sony VAIO VGN-CR70B/W Battery

Curium isotopes are inevitably present in spent nuclear fuel with a concentration of about 20 g/tonne.[89] Among them, the 245Cm–248Cm isotopes have decay times of thousands of years and need to be removed to neutralize the fuel for disposal.[90]Sony VAIO VGN-CR71B/W Battery

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