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Lanthanides & Actinides
Synthesis of Trans-Uranium Elements

1932 Chadwick discovers the neutron

1930s Fermi realises that neutron capture by heavy elements is often followed by b-emission (& g-ray production) leading to (Z+1) element.

  • However neutron-bombardment of 238U yields mainly fission products

1940 McMillan & Abelson identify tiny amounts of a short-lived isotope of element 93

1940-60 The Golden Age of Element Synthesis through various "bombardment" techniques

  • Principal investigators: G.T. Seaborg & A. Ghiorso
  • Bombardment particles include:-

    Neutrons, , Deuterons, ,a-particles, , Carbon nuclei,

  • Production of elements beyond Pu requires successive neutron capture

    e.g.  

    reasonable yields need high neutron fluxes

    where are the highest neutron fluxes? - in a thermonuclear explosion!

    hence the discovery of Es & Fm from debris of such an explosion

  • Heaviest elements are (more conveniently!) made by heavy ion bombardment

    e.g.

  • Problems with heavy ion bombardments include:-
  • Principal Difficulties associated with Heavy Element Isolation & Characterization

    1. Powerful accelerators needed for appropriate velocities (inertia mass)

    2. Products are produced only an atom at a time!

    3. Individual elements are not produced cleanly in isolation

    separation from other actinides & from lanthanide fission products

    4. Radioactivity

    remote-handling often necessary (Actinides are also highly toxic)

    damage to solutions

    e.g. generation of radicals, H, OH in H2O leads to reduction of higher Actinide oxidation states

    heating problems (e.g.242 Cm gives out 122 Wg-1)

    problems with crystallography

    • fogging of X-ray film
    • creates defects in crystals

    5. Instability of most nuclides ~ e.g. No (T1/2 = 1 hr) & Lw (T1/2 = 3 min)

    Heavier elements ( Bk) are produced only in the minutest amounts

    • e.g. typical yields of 258Md (T1/2 = 3 m) are 1 to 3 atoms per expt.!
    • only a few atoms of No and Lr have ever been isolated

    Timespan available for experiments can be very limited

    6. Difficulty in identification of a few atoms

    • Separation by ion-exchange techniques (c.f. lanthanides)

      even after purification cumulative daughter product contamination may be a problem

    • Using nuclear decay statistics to detect and count the atoms

      prediction of behaviour from utilization of decay systematics

    • Chemical Tracer methods

      need for accurate prediction of properties

    • Pure chemical properties (where accessible) performed on oxide samples confined in quartz capillaries attached to high-vacuum systems

      computer-controlled apparatus for study of transuranic chemistry

  • For a fascinating account of heavy actinides, including personal recollections of the experiments
    see:  G.T. Seaborg & W.D. Loveland, The Elements beyond Uranium, Wiley, N.Y., 1990


--Info & DownloadsBibliography  [textbook & online resources]

Source: Dr. S.J. Heyes; University of Oxford
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