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.²⁴² Cm 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 („ 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

Bibliography