Of all the elements currently known about 90 are naturally occurring. How are the others made? Particle accelerators allow atoms to be smashed together at high energies and new elements can be formed as a result of this.
There are three main research laboratories involved in new element discovery – the University of California at Berkeley, the Joint Institute for Nuclear Research in Dubna, Russia, and the Society for Heavy Ion Research in Darmstadt, Germany.
Elements with an atomic number greater than 92 (uranium) are all man-made either in nuclear power stations or by bombarding heavy elements with a beam of high-energy particles in particle accelerators. The bombarding particle fuses with the nucleus of the target atom, forming a new nucleus. This only lasts for a short time before decomposing. By analysing the decomposition products, new elements can be detected. These elements are known as transuranium elements.
Even although only minute quantities of the man-made transuranium elements are produced, they do have uses. One of the most important practical applications of uranium and the transuranium elements is in the generation of electricity in nuclear power plants. In a thermal reactor, some of the 235U used as the ‘fuel’ is converted to 239Pu. This isotope of plutonium is fissionable. Like 235U, it too can be used as fuel in a nuclear reactor. It has been estimated that all of the thermal nuclear reactors worldwide collectively produce about 110 tonnes of 239Pu annually. Nuclear reactors can make their own fuel
Unfortunately, 235U and 239Pu also serve as the main fuel in nuclear weapons. Nuclear reactors also produce highly radioactive isotopes of the transuranium elements, some having very long half-lives. The management of these wastes needs careful consideration.
Another special application of the transuranium elements is their use in radionuclide power sources. Suitable radionuclides need to:
- be easily shielded
- emit weakly penetrating radiation
- have reasonably long half-lives
- be corrosion-resistant
- be insoluble in water
- be cheap and readily available.
Plutonium-238 and curium-245 both have these properties. The decay energy of the radionuclide is absorbed in a suitable material, giving rise to heat. Using thermo-electrical devices, this heat can be converted into electricity. A few grams to kilograms of such nuclides, in specially designed containers, provide sources with power levels up to hundreds of watts. These can be used in space applications such as remote sensing, communication satellites and deep space missions.
The research continues
One of the most well known researchers in this field was Glen Seaborg. Working at the Lawrence Berkley Laboratory in California, USA, he discovered 9 additional transuranium elements (besides plutonium) during the years 1944–1974 – americium (95), curium (96), berkelium (97), californium (98), einsteinium (99), fermium (100), mendelevium (101), nobelium (102) and seaborgium (106).
Typical syntheses reactions are:
seaborgium (Sg) 249Cf + 18O
263Sg + 4n
mendelevium (Md) 253Es + 4He
256Md + n
Will more new element discoveries be made? It would be hard to say no, but now that the limits of nuclear stability are being reached, it does seem unlikely given our present understanding.
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