Fermium - Production

Production

Fermium is produced by the bombardment of lighter actinides with neutrons in a nuclear reactor. Fermium-257 is the heaviest isotope that is obtained via neutron capture, and can only be produced in nanogram quantities. The major source is the 85 MW High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory in Tennessee, USA, which is dedicated to the production of transcurium (Z > 96) elements. In a "typical processing campaign" at Oak Ridge, tens of grams of curium are irradiated to produce decigram quantities of californium, milligram quantities of berkelium and einsteinium and picogram quantities of fermium. However, nanogram and microgram quantities of fermium can be prepared for specific experiments. The quantities of fermium produced in 20–200 kiloton thermonuclear explosions is believed to be of the order of milligrams, although it is mixed in with a huge quantity of debris; 40 picograms of 257Fm was recovered from 10 kilograms of debris from the 'Hutch' test (16 July 1969).

After production, the fermium must be separated from other actinides and from lanthanoid fission products. This is usually achieved by ion exchange chromatography, with the standard process using a cation exchanger such as Dowex 50 or T eluted with a solution of ammonium α-hydroxyisobutyrate. Smaller cations form more stable complexes with the α-hydroxyisobutyrate anion, and so are preferentially eluted from the column. A rapid fractional crystallization method has also been described.

Although the most stable isotope of fermium is 257Fm, with a half-life of 100.5 days, most studies are conducted on 255Fm (t_½ = 20.07(7) hours) as this isotope can be easily isolated as required as the decay product of 255Es (t_½ = 39.8(12) days).