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.  

  Þ 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 µ mass)

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

  3. Individual elements are not produced cleanly in isolation

  Þ separation from other actinides & from lanthanide fission products

  4. Radioactivity

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

  Þ damage to solutions

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

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

  Þ problems with crystallography

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

  5. Instability of most nuclides ~ e.g. No (T_1/2 = 1 hr) & Lw (T_1/2 = 3 min)

  Þ Heavier elements (³ Bk) are produced only in the minutest amounts

  • e.g. typical yields of ²⁵⁸Md (T_1/2 = 3 m) are 1 to 3 atoms per expt.!
  • only a few atoms of No and Lr have ever been isolated

  Þ 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

    Þ prediction of behaviour from utilization of decay systematics

  • Chemical Tracer methods

    Þ 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

If you have any comments please contact stephen.heyes@chem.ox.ac.uk 

Main Introduction I1 I2 I3 I4 I5 Lanthanides L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 L15

Actinides A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 General Data1 Data2 Problems Help

To the Oxford Chemistry IT Centre Homepage

© S.J. Heyes, Oxford, 1997-8