Neptunium is a transuranium, radioactive element in the actinide series. It is denoted by the atomic symbol Np, it has the atomic number 93, and it has an atomic weight of 237. It is a silvery metal with a melting point of 1191.2°F (644° C). Its most stable isotope is neptunium-237, and fifteen more have been produced. The pure metal can be obtained by reducing neptunium (III) fluoride with lithium or barium vapor at about 2192°F (1200° C).
The discovery of the neutron in 1932 opened up some exciting new lines of research for scientists. One area that soon attracted a number of researchers involved nuclear changes known as (n,&ggr;) reactions. When substances are bombarded with neutrons (the "n" in n,&ggr;), they may incorporate the neutrons into their nuclei to produce new isotopes with an atomic weight one greater than that of the original isotope. In the process, the new nucleus gives off energy in the form of a gamma ray (the "&ggr;" in n,&ggr;). For example: 6 C 12 + 0 n 1 ----> 6 C 13 + 0 &ggr; 0. The interesting point about n,&ggr; reactions is that the isotope thus formed usually decays by beta emission, resulting in a new isotope whose atomic number is one greater than the original isotope. For example: 6 C 13 --- --> -1 e 0 + 7 N 13.
The application of n,&ggr; reactions to uranium has special significance. When the above pair of reactions was initiated using uranium, the ultimate product was an isotope with atomic number 93. Since no such element was known to exist naturally on the earth, scientists recognized the possibility of using such reactions to create the first synthetic element.
Early efforts to produce element 93 by this method failed in their principal objective but produced a quite different and startling discovery: nuclear fission. During fission, uranium nuclei absorb neutrons and, instead of undergoing n,&ggr; reactions, split apart into two roughly equal parts. Attention shifted immediately to this remarkable discovery and to its potential application as a source of energy.
During his analysis of nuclear fission products in 1940, Edwin McMillan at the University of California at Berkeley isolated a new isotope with a half-life of 2.3 days. Working with Philip Abelson, McMillan was able to demonstrate that the isotope was that of a new element, number 93. The long sought-for new element was found, albeit in a rather roundabout way. McMillan and Abelson decided on the name neptunium for the element since Neptune is the planet next to Uranus in the solar system, and element 93 is the element next to uranium in the periodic table.
We now know that neptunium exists in very small amounts in nature. It is produced when uranium is bombarded by neutrons that are naturally present in the atmosphere or in the earth. The element had never been observed in nature, however, before the work of Abelson and McMillan. Neptunium is now produced as a by-product of the manufacture of plutonium in nuclear reactors. The metal has relatively few uses. Neptunium-237 has been used in devices for the detection of neutrons. It is also used as a source material for the production of uranium- 238.
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