Credit: Andrei Marincas | Shutterstock
Atomic Number: 93
Atomic Symbol: Np
Atomic Weight: 237
Melting Point: 1,191 F (644 C)
Boiling Point: 7,052 F (3,900 C)
Word origin: Neptunium is named after the planet Neptune, which is named for the Roman god of the sea. Neptune is the next planet out from the sun from Uranus, and Neptunium is next to Uranium on the periodic table.
Discovery: Neptunium was discovered in 1940 at the University of California, Berkeley, by professor Edwin McMillan and graduate student Philip Abelson. They bombarded uranium with cyclotron-produced slow neutrons, which resulted in the reactacts fusing to form the new element neptunium.
237Np was the first transuranium element to be produced synthetically and the first actinide series transuranium element to be discovered. Its discovery came after several false findings of the element, including Enrico Fermi’s attempt to bombard uranium with neutrons. That experiment resulted in the discovery of fission, or splitting atoms.
Properties of neptunium
As a metal, neptunium has a silver appearance. It is chemically reactive and is found it at least three allotropes. It is the most dense of all actinides and the fifth most dense of all naturally occurring elements. With 5,594 degrees F (3,090 C) between its melting point and boiling point, neptunium has the largest liquid range of any element. [See Periodic Table of the Elements]
A recent discovery revealed that neptunium has superconductor capabilities as an alloy. This was surprising because its compounds are usually magnetic, which eliminates superconductivity.
In solutions, neptunium exhibits five oxidization states, state V being the most stable.
When introduced to the human body, neptunium does not accumulate in the digestive tract. If injected, however, it will accumulate in bones until it is eventually released. Neptunium’s decay product 233Pa is highly radioactive and can penetrate skin, paper and gloves. To be safe, care should be taken when handling all neptunium materials.
Sources of neptunium
Since the longest living and most stable isotope of neptunium, 237Np, has a half-life of 2.14 million years, all of the natural neptunium that was present at Earth’s formation has decayed away. As a result, neptunium is primarily produced synthetically. As a metal, it can be made from the reaction of NpF3 with liquid or gaseous barium or lithium at around 1,200 C (2,192 F).
Neptunium is often extracted from spent nuclear fuel rods by the kilogram. Reactor plutonium production discharges 95 percent more plutonium than neptunium, but that 5 percent still amounts to more than 50 tons of neptunium per year.
Traces of neptunium isotopes can, however, sometimes be found naturally as decay products of transmutation reactions in concentrated uranium ores. Neptunium can also be milked from alpha decay of 241Am.
Uses of neptunium
Neptunium is fissionable. Therefore, it could theoretically be used as fuel in a fast neutron reactor or in a nuclear weapon. When 237Np is irradiated with neutrons, it results in 238Pu, an alpha emitter for radioisotope thermal generators used in spacecraft and military applications.
Many of neptunium’s uses are related to the study of chemistry. 237Np is a component in devices that detect high-energy (MeV) neutrons. Because 237Np (half-life 2.14 million years) decays to a strongly radioactive and short-lived protactinium daughter (half-life 26.9 days), its study can help determine the amount of time since the neptunium was last separated and purified in questionable samples.
Isotopes of neptunium
Neptunium has 25 known isotopes. They range in atomic weight from 225 to 244. Twenty of the isotopes have half-lives of less than 4.5 days, and most are less than 50 minutes. The five remaining isotopes, however, are known as metastable isotopes and have extremely long half-lives. The most stable is 237Np, which has a half-life of 2.14 million years. 236Np has a half-life of 154,000 years, and 235Np has a half-life of 396 days.
Most common heavy nuclei decay to make isotopes of lead. Neptunium, however, has a decay chain known as the neptunium series. The series includes 237Np, which decays from protactinium to uranium, and eventually to bismuth-209 and thallium-205. Most isotopes heavier than 237Np beta-decay to form plutonium. Most isotopes lighter than 237Np decay by electron capture with alpha emission. Those products are mostly isotopes of uranium.
(Source: Los Alamos National Laboratory)