Curium

History

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.

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.

The sample was prepared as follows: first plutonium nitrate solution was coated on a platinum foil of about 0.5 cm² area, the solution was evaporated and the residue was converted into plutonium(IV) oxide (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. The residue was dissolved in perchloric 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 or hell) and delirium (from Latin for madness).

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

Curium-242 was unambiguously identified by the characteristic energy of the α-particles emitted during the decay:

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

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

The half-life of the ²⁴⁰Cm α-decay was correctly determined as 26.7 days.

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, when one of the listeners asked whether any new transuranium element beside plutonium and neptunium had been discovered during the war. The discovery of curium (²⁴²Cm and ²⁴⁰Cm), their production and compounds were later patented listing only Seaborg as the inventor.

The new element was named after Marie Skłodowska-Curie and her husband Pierre Curie who are noted for discovering radium and for their work in radioactivity. It followed the example of gadolinium, a lanthanide element above curium in the periodic table, which was named after the explorer of the rare earth elements Johan Gadolin:

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. Macroscopic amounts of curium(III) 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. Curium metal was produced only in 1951 by reduction of CmF₃ with barium.

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). 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 α-Cm is more stable at ambient conditions. 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. The crystal consists of a double-hexagonal close packing with the layer sequence ABAC and so is isotypic with α-lanthanum. At pressures above 23 GPa, at room temperature, α-Cm transforms into β-Cm, which has a face-centered cubic symmetry, space group Fm3m and the lattice constant a = 493 pm. Upon further compression to 43 GPa, curium transforms to an orthorhombic γ-Cm structure similar to that of α-uranium, with no further transitions observed up to 52 GPa. These three curium phases are also referred to as Cm I, II and III.

Curium has peculiar magnetic properties. Whereas its neighbor element americium shows no deviation from Curie-Weiss paramagnetism in the entire temperature range, α-Cm transforms to an antiferromagnetic state upon cooling to 65–52 K, and β-Cm exhibits a ferrimagnetic transition at about 205 K. Meanwhile, curium pnictides show ferromagnetic transitions upon cooling: 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.

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 resistivity over time (about 10 µΩ·cm/h) due to self-damage of the crystal lattice by alpha radiation. This makes uncertain the absolute resistivity value for curium (about 125 µΩ·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.

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. The fluorescence originates from the transitions from the first excited state ⁶D_7/2 and the ground state ⁸S_7/2. Analysis of this fluorescence allows monitoring interactions between Cm(III) ions in organic and inorganic complexes.

Chemical

Curium ions in solution almost exclusively assume the oxidation state of +3, which is the most stable oxidation state for curium. The +4 oxidation state is observed mainly in a few solid phases, such as CmO₂ and CmF₄. Aqueous curium(IV) is only known in the presence of strong oxidizers such as potassium persulfate, and is easily reduced to curium(III) by radiolysis and even by water. The chemical behavior of curium is different from the actinides thorium and uranium, and is similar to that of americium and many lanthanides. In aqueous solution, the Cm³⁺ ion is colorless to pale green, and Cm⁴⁺ ion is pale yellow. 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. The +6 oxidation state has only been reported once in solution in 1978, as the curyl ion (CmO2+
2): this was prepared from the beta decay of americium-242 in the americium(V) ion 242
AmO+
2. Failure to obtain Cm(VI) from oxidation of Cm(III) and Cm(IV) may be due to the high Cm⁴⁺/Cm³⁺ ionization potential and the instability of Cm(V).

Curium ions are hard Lewis acids and thus form most stable complexes with hard bases. The bonding is mostly ionic, with a small covalent component. Curium in its complexes commonly exhibits a 9-fold coordination environment, within a tricapped trigonal prismatic geometry.

Isotopes

About 20 radioisotopes and 7 nuclear isomers between ²³³Cm and ²⁵²Cm are known for curium, and no stable isotopes. 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) and ²⁴⁶Cm (4,760 years). Curium-250 is unusual in that it predominantly (about 86%) decays via spontaneous fission. The most commonly used curium isotopes are ²⁴²Cm and ²⁴⁴Cm with the half-lives of 162.8 days and 18.1 years, respectively.

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. As in most transuranic elements, the nuclear fission cross section is especially high for the odd-mass curium isotopes ²⁴³Cm, 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. 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 create californium. This is strong neutron emitter, and would pollute the back end of the fuel cycle and increase the dose to reactor personnel. 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.

The table to the right lists the critical masses for curium isotopes for a sphere, without a moderator and reflector. 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. 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.

Currently, curium is not used as a nuclear fuel owing to its low availability and high price. ²⁴⁵Cm 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 α emission which would result in excessive heat. Curium-247 would be highly suitable, having a half-life 647 times that of plutonium-239.

Occurrence

The longest-lived isotope of curium, ²⁴⁷Cm, has a half-life of 15.6 million years. Therefore, any primordial curium, that is curium present on the Earth during its formation, should have decayed by now, although some of it would be detectable as an extinct radionuclide as an excess of its nearly stable daughter ²³⁵U. Curium is produced artificially, in small quantities for research purposes. 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. So the analysis of the debris at the testing site of the first U.S. hydrogen bomb, Ivy Mike, (1 November 1952, Enewetak Atoll), beside einsteinium, fermium, plutonium and 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.

Atmospheric curium compounds are poorly soluble in common solvents and mostly adhere to soil particles. 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.

The transuranic elements from americium to fermium, including curium, occurred naturally in the natural nuclear fission reactor at Oklo, but no longer do so.

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 quoted at 160–185 USD per milligram, with a more recent estimate at 2,000 USD/g for ²⁴²Cm and 170 USD/g for ²⁴⁴Cm. 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 β⁻ decay transforms into ²³⁹Np and ²³⁹Pu.

Further neutron capture followed by β⁻-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:

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. 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 β⁻ decay to the berkelium isotope ²⁴⁹Bk.

The above cascade of (n,γ) reactions produces a mixture of different curium isotopes. 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 α-decay of the californium isotope ²⁵²Cf, which is available in relatively large quantities due to its long half-life (2.65 years). About 35–50 mg of ²⁴⁸Cm is being produced by this method every year. The associated reaction produces ²⁴⁸Cm with isotopic purity of 97%.

Another interesting for research isotope ²⁴⁵Cm can be obtained from the α-decay of ²⁴⁹Cf, and the latter isotope is produced in minute quantities from the β⁻-decay of the berkelium isotope ²⁴⁹Bk.

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. 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. Bis-triazinyl bipyridine complex has been recently proposed as such reagent which is highly selective to curium. 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; whereas americium oxidizes to soluble Am(IV) complexes, curium remains unchanged and can thus be isolated by repeated centrifugation.

Metallic curium is obtained by reduction of its compounds. Initially, curium(III) fluoride was used for this purpose. The reaction was conducted in the environment free from water and oxygen, in the apparatus made of tantalum and tungsten, using elemental barium or lithium as reducing agents.

Another possibility is the reduction of curium(IV) oxide using a magnesium-zinc alloy in a melt of magnesium chloride and magnesium fluoride.

Oxides

Curium readily reacts with oxygen forming mostly Cm₂O₃ and CmO₂ oxides, but the divalent oxide CmO is also known. Black CmO₂ can be obtained by burning curium oxalate (Cm₂(C₂O₄)₃), nitrate (Cm(NO₃)₃) or hydroxide in pure oxygen. Upon heating to 600–650 °C in vacuum (about 0.01 Pa), it transforms into the whitish Cm₂O₃:

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

Furthermore, a number of ternary oxides of the type M(II)CmO₃ are known, where M stands for a divalent metal, such as barium.

Thermal oxidation of trace quantities of curium hydride (CmH_2–3) has been reported to produce a volatile form of CmO₂ and the volatile trioxide CmO₃, one of the two known examples of the very rare +6 state for curium. Another observed species was reported to behave similarly to plutonium tetroxide and was tentatively characterized as CmO₄, with curium in the extremely rare +8 state; however, new experiments seem to indicate that CmO₄ does not exist.

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:

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

The colorless curium(III) chloride (CmCl₃) is produced in the reaction of curium(III) hydroxide (Cm(OH)₃) with anhydrous hydrogen chloride gas. 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:

An alternative procedure is heating curium oxide to about 600 °C with the corresponding acid (such as hydrobromic for curium bromide). Vapor phase hydrolysis of curium(III) chloride results in curium oxychloride:

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

Organocurium compounds and biological aspects

Organometallic complexes analogous to uranocene are known also for other actinides, such as thorium, protactinium, neptunium, plutonium and americium. Molecular orbital theory predicts a stable "curocene" complex (η⁸-C₈H₈)₂Cm, but it has not been reported experimentally yet.

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. Some of these BTP-type complexes selectively interact with curium and therefore are useful in its selective separation from lanthanides and another actinides. Dissolved Cm³⁺ ions bind with many organic compounds, such as hydroxamic acid, urea, fluorescein and adenosine triphosphate. Many of these compounds are related to biological activity of various microorganisms. 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.

Curium has no biological significance. 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. 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. 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. Though as an α-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, 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.

A more promising application of ²⁴²Cm is to produce ²³⁸Pu, a more suitable radioisotope for thermoelectric generators such as in cardiac pacemakers. The alternative routes to ²³⁸Pu use the (n,γ) reaction of ²³⁷Np, or the deuteron bombardment of uranium, which both always produce ²³⁶Pu as an undesired by-product — since the latter decays to ²³²U with strong gamma emission. 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). Californium was discovered when a microgram-sized target of curium-242 was irradiated with 35 MeV alpha particles using the 60-inch (150 cm) cyclotron at Berkeley:

Only about 5,000 atoms of californium were produced in this experiment.

X-ray spectrometer

The most practical application of ²⁴⁴Cm — though rather limited in total volume — is as α-particle source in the alpha particle X-ray spectrometers (APXS). These instruments were installed on the Sojourner, Mars, Mars 96, Mars Exploration Rovers and Philae comet lander, as well as the Mars Science Laboratory to analyze the composition and structure of the rocks on the surface of planet Mars. APXS was also used in the Surveyor 5–7 moon probes but with a ²⁴²Cm source.

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). These spectra contain quantitative information on all major elements in the samples except for hydrogen, helium and lithium.

Safety

Owing to its high radioactivity, curium and its compounds must be handled in appropriate laboratories under special arrangements. Whereas curium itself mostly emits α-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. If consumed, curium is excreted within a few days and only 0.05% is absorbed in the blood. 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-life of curium is about 20 years in the liver and 50 years in the bones. Curium is absorbed in the body much more strongly via inhalation, and the allowed total dose of ²⁴⁴Cm in soluble form is 0.3 μC. Intravenous injection of ²⁴²Cm and ²⁴⁴Cm containing solutions to rats increased the incidence of bone tumor, and inhalation promoted pulmonary and liver cancer.

Curium isotopes are inevitably present in spent nuclear fuel with a concentration of about 20 g/tonne. Among them, the ²⁴⁵Cm–²⁴⁸Cm isotopes have decay times of thousands of years and need to be removed to neutralize the fuel for disposal. The associated procedure involves several steps, where curium is first separated and then converted by neutron bombardment in special reactors to short-lived nuclides. This procedure, nuclear transmutation, while well documented for other elements, is still being developed for curium.