Fermium Fm

Discovery

After the Ivy Mike thermonuclear test of November 1952, Albert Ghiorso and his colleagues first identified element 99 in the debris. About two months later they isolated another new component that emitted high-energy alpha particles and had a half-life of about a day. A substance that short-lived could only persist if it was being continually fed by the beta decay of a longer-lived einsteinium isotope, so it had to belong to element 100. They identified it as fermium-255, with a half-life of about 20 hours.

As with einsteinium, the result was classified and only published in 1955. In the meantime a team at the Nobel Institute in Stockholm independently reported producing an isotope of element 100 by bombarding uranium with oxygen ions.

The discovery prompted a natural question: could reactors keep adding neutrons to reach still heavier elements? The answer was no. Fermium-258 splits by spontaneous fission in well under a millisecond, so everything beyond fermium has had to be made by smashing nuclei together in accelerators.

Origin of the name

The element is named after Enrico Fermi, who led the construction of the first nuclear reactor. The symbol is Fm. The name was chosen during Fermi's lifetime but announced only after declassification, following his death in November 1954. Elements 99 and 100 thus carry the names of two giants of twentieth-century physics side by side.

Main uses

• Basic research: As the heaviest element reachable by neutron capture, fermium is used to study where the neutron-capture route ends and why very heavy nuclei collapse through spontaneous fission.
• Nuclear structure: The way fermium isotopes' fission behaviour changes sharply with mass number offers a demanding test for theories of nuclear fission.
• Production runs yield amounts measured in picograms, so there are no practical applications.

Isotopes

Fermium has no stable isotopes. The longest-lived is fermium-257, with a half-life of about 100.5 days. Fermium-255, the first to be identified, lasts about 20 hours. When fermium-257 captures one more neutron it becomes fermium-258, which fissions spontaneously in roughly 0.4 milliseconds, so the neutron-capture path cannot go any further. This barrier is often called the fermium gap.

In everyday life

Fermium does not occur in nature and has no link to everyday life. All the fermium ever made would be invisible to the naked eye, and the pure metal has never been prepared. It is radioactive and would be harmful inside the body in principle, but in practice nobody is exposed to it.