Chemistry in its element: curium


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Meera Senthilingam

This week's element launches us deep into outer space.

Richard Corfield

Curium is a member of a group of elements, the transuranic elements, that - with the exception of plutonium and neptunium - do not occur naturally on Earth. Curium is a hard, brittle, silvery radioactive metal that tarnishes slowly and which can only be produced in nuclear reactors. The isotope 242Cu was produced in 1944 by Glenn T Seaborg, Ralph A James and Albert Ghioso by bombarding 239Pu with alpha particles in the 60-inch Cyclotron at Berkeley University in the US. Like another synthetic element, americium, the discovery of curium was intimately bound up with the work of the Manhattan Project which Seaborg and his team were working on at the time of their discovery. This meant that neither curium nor americium could be announced to the world until after the end of the war. Seaborg revealed their discovery in November 1945 on the American TV show Quiz Kids just five days before the official unveiling of the new elements at a meeting of the American Chemical Society.

Curium is named in honour of Pierre and Marie Curie, who pioneered the study of radioactivity in the final days of the 19th century. Nineteen radioisotopes of curium are known to exist, the first of which, 242Cu was isolated in the hydroxide form in 1947 and in its elemental form in 1951. The most stable radioisotope is 247Cm which has a half-life of 1.56 × 107 years. 248Cm has a half-life of 3.40 × 105 years, 250Cm a half-life of 9000 years, and 245Cm a half-life of 8500 years. All of the remaining radioactive isotopes have half-lives with a duration that less than 30 years, and the majority of these have half-lives that are less than a month.

Curium has two main uses: as a fuel for Radioisotope Thermal Generators (RTGs) on board satellites, deep space probes, planetary surface rovers and in heart pacemakers, and as a alpha emitter for alpha particle X-Ray spectrometry, again particularly in space applications.

RTGs are electrical generators which produce power from radioactive decay. Usually heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect -where an electrical current is generated at the junctions between two different metals - using an array of thermocouples. However, in some cases such as the Mars Exploration Rovers, the power is used directly to warm the vehicle. For spaceflight use, the fuel must be radioactive enough to produce large quantities of energy per unit of mass and volume. 242Cu produces about 3W of heat energy from radioactive decay per gram which compares favourably with the plutonium and americium sources commonly used in other Radioisotope Thermal Generator applications.

Alpha Particle X-Ray Spectrometers (APXS) are devices that analyse the chemical element composition of a sample from back-scattered alpha particles. Using Rutherford's calculations of the conservation of nuclear energy and linear momentum it is possible to calculate the mass of the nucleus hit by the alpha particle and from this the energy spectrum of the material being analysed. Alpha Particle X-Ray Spectrometers tend to be confined to chemical analyses required during space missions since, although curium is both compact and power efficient, it is also a hazardous radioactive material. APXSs have a long history in space exploration being first used during the later Surveyor (Surveyor 5-7) missions that immediately preceded the Apollo Moon landings. Since the days of Surveyor alpha particle analysers have been included on many other missions including Mars Pathfinder, Mars 96, the Rosetta mission to the comet Comet 67 P/Churyumov- Gerasimenko and the Mars Exploration Rovers.

Back on Earth most curium found in the environment today was generated by the atmospheric testing of nuclear weapons, which ceased worldwide by 1980. More localised pockets of curium contamination have occurred through accidents at weapons production facilities.

As already mentioned, curium is hazardous. It becomes concentrated in bone marrow and because of its significant alpha activity can induce cancers. Despite its rarity and hazards it seems appropriate that an element first synthesised during a global conflict that saw the development of the vehicles that would one day take us to the Moon and beyond is now so pivotal to space exploration, providing our robotic pioneers not only with power but also the ability to analyse extraterrestrial materials as well.

Meera Senthilingam

So, a crucial element in the field of space exploration. That was science writer Richard Corfield bringing us the radio active chemistry of curium. Now next week, the element named after the creator of the periodic table.

Hayley Birch

Brought up in Russia, Mendeleev was the sort of person who, it seems, was incapable of sticking to one discipline and as well as serving as the director of the Russian institute for weights and measures, had a hand in developing the Russian oil industry. Given all this, it's perhaps less surprising than it ought to be that he conceived of the periodic table on the same day that he was supposed to be inspecting a cheese factory.

Meera Senthilingam

So, quite the multi tasker. And to find out the creation, chemistry and history of the Element named after Mendeleev, Mendelevium, join Hayley Birch in next week's Chemistry in its element. Until then, I'm Meera Senthilingam and thank you for listening.


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