Neptunium Np

Discovery

In the mid-1930s Enrico Fermi's group bombarded uranium with neutrons and believed they had created element 93. Most of what they actually made, however, turned out to be fission fragments, a misunderstanding cleared up only when fission itself was recognised in 1938.

The genuine element 93 was identified in 1940 by Edwin McMillan and Philip Abelson at the Radiation Laboratory in Berkeley, California. They irradiated uranium with cyclotron neutrons and followed a new activity with a half-life of about 2.4 days that grew from the beta decay of uranium-239. Its chemistry matched neither uranium nor any fission product. It was neptunium-239, and with it the periodic table was pushed beyond uranium for the first time.

McMillan began hunting for element 94 as well but was called away to wartime radar research, and Glenn Seaborg's team took over. McMillan and Seaborg shared the 1951 Nobel Prize in Chemistry for their work on the transuranium elements. Because reactors produce it steadily, neptunium later became available in kilogram quantities.

Origin of the name

Uranium had been named after Uranus, so its successor took the name of Neptune, the next planet out from the Sun. Neptune in turn is the Roman god of the sea. The symbol Np uses the first two letters. The planetary pattern continued with plutonium.

Main uses

• Making plutonium-238: Irradiating neptunium-237 in a reactor turns it, via neptunium-238, into plutonium-238, whose steady decay heat powers radioisotope thermoelectric generators on spacecraft travelling far from the Sun.
• Neutron detection: Neptunium-237 fissions when struck by fast neutrons, so it is used in detectors that measure high-energy neutron flux.
• Fuel-cycle research: Neptunium is one of the long-lived components of nuclear waste, and researchers study ways to separate it or 'burn' it in advanced reactors.

Isotopes

Neptunium has no stable isotopes. The longest-lived, neptunium-237, has a half-life of about 2.14 million years, short by geological standards, so any primordial neptunium vanished long ago and the decay chain that starts from it, the neptunium series, is essentially extinct in nature. Neptunium-239, the isotope first discovered, has a half-life of about 2.4 days and beta-decays into plutonium-239, which is exactly how reactors breed plutonium. Tiny amounts of neptunium are continuously formed in uranium ores by natural neutrons.

In everyday life

Neptunium plays no part in everyday life. Almost all of it on Earth comes from reactors and from atmospheric weapons tests of the twentieth century. It is an alpha emitter that can accumulate in bone and liver if taken into the body, so it is handled only in specialised facilities. The metal is remarkably dense, around 20 grams per cubic centimetre.