Fermium

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Lore & Background

Fermium was first discovered in the fallout from the 'Ivy Mike' nuclear test (1 November 1952), the first successful test of a hydrogen bomb. Initial examination of the debris from the explosion had shown the production of a new isotope of plutonium, 244Pu: this could only have formed by the absorption of six neutrons by a uranium-238 nucleus followed by two β− decays. Element 99 (einsteinium) was quickly discovered on filter papers which had been flown through clouds from the explosion. The discovery of fermium (Z = 100) required more material, as the yield was expected to be at least an order of magnitude lower than that of element 99, and so contaminated coral from the Enewetak atoll (where the test had taken place) was shipped to the University of California Radiation Laboratory in Berkeley, California, for processing and analysis. About two months after the test, a new component was isolated emitting high-energy α-particles (7.1 MeV) with a half-life of about a day. With such a short half-life, it could only arise from the β− decay of an isotope of einsteinium, and so had to be an isotope of the new element 100: it was quickly identified as 255Fm (t = 20.07(7) h). The discovery of the new elements, and the new data on neutron capture, was initially kept secret on the orders of the U.S. military until 1955 due to Cold War tensions. Nevertheless, the Berkeley team was able to prepare elements 99 and 100 by civilian means, through the neutron bombardment of plutonium-239, and published this work in 1954 with the disclaimer that it was not the first studies that had been carried out on the elements. The 'Ivy Mike' studies were declassified and published in 1955. A group at the Nobel Institute for Physics in Stockholm independently discovered the element, producing an isotope later confirmed to be 250Fm (t1/2 = 30 min) by bombarding a 238U target with 16O ions, and published their work in May 1954. Nevertheless, the priority of the Berkeley team was generally recognized, and with it the prerogative to name the new element in honour of Enrico Fermi, the developer of the first artificial self-sustained nuclear reactor. Fermi was still alive when the name was proposed, but had died by the time it became official.

Reader's Guide

Fermium holds significance as the heaviest element that can be formed by neutron bombardment of lighter elements and the last element that can be prepared in macroscopic quantities, though pure fermium metal has not been prepared. Its discovery in the debris of the first hydrogen bomb test demonstrated the power of neutron capture in nuclear explosions to create new elements, advancing understanding of nuclear physics. The element's chemistry is typical for the late actinides, with a preponderance of the +3 oxidation state but also an accessible +2 oxidation state. Owing to the small amounts produced and all its isotopes having relatively short half-lives, there are currently no uses for fermium outside basic scientific research. The discovery was initially kept secret on U.S. military orders until 1955 due to Cold War tensions. Fermium is produced by the bombardment of lighter actinides with neutrons in a nuclear reactor. 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 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; 4.0 picograms of 257Fm was recovered from 10 kilograms of debris from the 'Hutch' test (16 July 1969). The Hutch experiment produced an estimated total of 250 micrograms of 257Fm. After production, the fermium must be separated from other actinides and from lanthanide fission products, usually achieved by ion-exchange chromatography. Although the most stable isotope of fermium is 257Fm, with a half-life of 100.5 days, most studies are conducted on 255Fm (t1/2 = 20.07(7) hours), since this isotope can be easily isolated as required as the decay product of 255Es (t1/2 = 39.8(12) days).

Did You Know?

• Fermium was discovered in the fallout from the 'Ivy Mike' nuclear test on 1 November 1952, the first successful test of a hydrogen bomb.
• The discovery was initially kept secret on the orders of the U.S. military until 1955 due to Cold War tensions.
• A group at the Nobel Institute for Physics in Stockholm independently discovered fermium in May 1954, producing 250Fm by bombarding a 238U target with 16O ions.
• The element was named after Enrico Fermi, the developer of the first artificial self-sustained nuclear reactor; Fermi was still alive when the name was proposed, but had died by the time it became official.
• The Hutch test (16 July 1969) produced an estimated total of 250 micrograms of 257Fm, but only 4.0 picograms were recovered from 10 kilograms of debris.