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Curium ( ) is a synthetic chemical element with the symbol Cm and atomic number 96. A radioactive metallic transuranic element of the actinide series, curium is produced by bombarding plutonium with alpha particles (helium ions) and was named after Marie Curie and her husband Pierre.

Characteristics

The isotope curium-248 has been synthesized only in milligram quantities, but curium-242 and curium-244 are made in gram amounts, which allows for the determination of some of the element's properties. Curium-244 can be made in quantity by subjecting plutonium to neutron bombardment. Curium does not occur in nature. There are few commercial applications for curium but it may one day be useful in radioisotope thermoelectric generators. Curium bio-accumulates in bone tissue where its radiation destroys bone marrow and thus stops red blood cell creation.

A rare earth homolog, curium is somewhat chemically similar to gadolinium but with a more complex crystal structure. Chemically reactive, its metal is silvery-white in color and the element is more electropositive than aluminium (most trivalent curium compound are slightly yellow).

Curium has been studied greatly as a potential fuel for radioisotope thermoelectric generators (RTG). Curium-242 can generate up to 120 watts of thermal energy per gram (W/g); however, its very short half-life makes it undesirable as a power source for long-term use. Curium-242 can decay by alpha emission to plutonium-238 which is the most common fuel for RTGs. Curium-244 has also been studied as an energy source for RTGs having a maximum energy density ~3 W/g, but produces a large amount of neutron radiation from spontaneous fission. Curium-243 with a ~30 year half-life and good energy density of ~1.6 W/g would seem to make an ideal fuel, but it produces significant amounts of gamma and beta radiation from radioactive decay products.

Compounds

Some compounds are:

History

Curium was first synthesized at the University of California, Berkeleymarker by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso in 1944. The team named the new element after Marie Curie and her husband Pierre who are famous for discovering radium and for their work in radioactivity. It was chemically identified at the Metallurgical Laboratory (now Argonne National Laboratorymarker) at the University of Chicagomarker. It was actually the third transuranium element to be discovered even though it is the fourth in the series. Curium-242 (half-life 163 days) and one free neutron were made by bombarding alpha particles onto a plutonium-239 target in the 60-inch cyclotron at Berkeley.

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Due to the fact that the discovery of the new elements, curium and americium, was closely related to the Manhattan Project the results were confidential and publication was impossible. Seaborg announced the discovery of the elements on the radio show for kids, the Quiz Kids, five days before the official presentation at an American Chemical Society meeting on November 11, 1945. Seaborg also patented the synthesis of the new elements.

Louis Werner and Isadore Perlman created a visible sample of curium-242 hydroxide at the University of California in 1947 by bombarding americium-241 with neutrons. Curium was made in its elemental form in 1951 for the first time.

Isotopes

19 radioisotopes of curium have been characterized, with the most stable being Cm-247 with a half-life of 1.56 × 107 years, Cm-248 with a half-life of 3.40 × 105 years, Cm-250 with a half-life of 9000 years, and Cm-245 with a half-life of 8500 years. All of the remaining radioactive isotopes have half-lives that are less than 30 years, and the majority of these have half-lives that are less than 33 days. This element also has 4 meta states, with the most stable being Cm-244m (t½ 34 ms). The isotopes of curium range in atomic weight from 233.051 u (Cm-233) to 252.085 u (Cm-252).

Nuclear fuel cycle

Transmutation flow between 238Pu and 244Cm in LWR.
Fission percentage is 100 minus shown percentages.
Total rate of transmutation varies greatly by nuclide.
245Cm–248Cm are long-lived with negligible decay.

Thermal neutron cross section (barns)
242Cm 243Cm 244Cm 245Cm 246Cm 247Cm
Fission 5 617 1.04 2145 0.14 81.90
Capture 16 130 15.20 369 1.22 57
C/F ratio 3.20 0.21 14.62 0.17 8.71 0.70
LEU spent fuel 20 years after 53 MWd/kg burnup
3 common isotopes 51 3700 390
Fast reactor MOX fuel (avg 5 samples, burnup 66-120GWd/t)
Total curium 3.09 % 27.64% 70.16% 2.166% 0.0376% 0.000928%


The odd-mass number isotopes are fissile, the even-mass number isotopes are not and can only capture neutrons, but very slowly. Therefore in a thermal reactor the even-mass isotopes accumulate as burnup increases.

The MOX which is to be used in power reactors should contain little or no curium as the neutron activation of 248Cm will create californium which is a strong neutron emitter. The californium would pollute the back end of the fuel cycle and increase the dose to workers. 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.

Applications

The Curium isotopes 244Cm and 242Cm are strong alpha emitters with a halflife in the months to years range and produce considerable heat during this process. These properties make them useful for applications as alpha particle source and as heat generator in radioisotope thermoelectric generators (RTG).

A 244Curium source is used for the Alpha particle X-ray spectrometer on board several American and European space missions, for example the Mars Exploration Rover and the Rosetta/Philae. The use in RTG is proposed for several future missions.

References

  1. Gmelins Handbuch der anorganischen Chemie, System Nr. 71, Band 7 a, Transurane, Teil A 2, p. 289.
  2. G. T. Seaborg, R. A. James, A. Ghiorso: "The New Element Curium (Atomic Number 96)", NNES PPR (National Nuclear Energy Series, Plutonium Project Record), Vol. 14B, The Transuranium Elements: Research Papers, Paper No. 22.2, McGraw-Hill Book Co., Inc., New York, 1949; Abstract; Typoskript (January 1948).
  3. Glen T. Seaborg "Element" Issue date: December 1964
  4. L. B. Werner, I. Perlman: "Isolation of Curium", NNES PPR (National Nuclear Energy Series, Plutonium Project Record), Vol. 14 B, The Transuranium Elements: Research Papers, Paper No. 22.5, McGraw-Hill Book Co., Inc., New York, 1949.


Literature

  • Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1.


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