Plutonium belongs to the class of elements called transuranic elements whose atomic number is higher than 92, the atomic number of uranium. Plutonium is the most economically important of the transuranic elements. Plutonium-239 readily undergoes fission, and it is used for nuclear weapons and for energy. The atomic number of plutonium is 94. Isotopes of plutonium were first prepared and studied by the American chemist Glenn T. Seaborg and his associates at the University of California at Berkeley in 1941. Plutonium has 15 isotopes with mass numbers ranging from 232 to 246. All isotopes of plutonium are radioactive, but they have widely varying half-lives. The half-life is the time it takes for half the atoms of an element to decay. The various isotopes also have different principal decay modes.
The most important isotope of plutonium is Pu-239. It’s virtually nonexistent in nature. It is produced by bombarding uranium-238 with slow neutrons. This forms neptunium-239, which in turn emits a beta particle and forms plutonium-239. Plutonium-239’s principal mode of decay is alpha decay. Various sources give slightly different figures for the half-life. The values found include 24,360, 24,400, 24,110, and 24,000 years. None of theses measurements agree. It is because there are many factors that can affect the accuracy of this measurement. Plutonium-239 is produced artificially, and every time it is produced, it is mixed with varying amounts of other isotopes, notably plutonium-240, plutonium-241 and plutonium-242. Since all the isotopes have nearly the same chemical characteristics, it is very difficult to separate isotopes from each other by chemical techniques. This means, it is virtually impossible to study the properties of pure Plutonium-239. Therefore, the results might be the average half-life of it being mixed with a small amount of the other isotopes. And there will be a slight difference in the density and purity of it every time it is being produced, depending upon the amount of reactants used in the process of its production. In addition, a little spontaneous fission occurs in most plutonium isotopes. So while some of the Plutonium-239 atoms are undergoing decay, a small number of them are splitting into less-massive nuclei. The rate of fission is not a constant.
Plutonium-239 is a fissile material. It can be split by both slow and fast neutrons. Each fission of this isotope resulting from a slow neutron absorption results in the production of a little more than two neutrons. If one of these neutrons, on average, splits another plutonium nucleus, a sustained chain reaction is achieved. So the source of neutrons can have big influence on the outcome of an experiment.