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Occurrence and Production of Neptunium
Natural Occurrence
Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores. 237Np is produced through the reduction of 237NpF3 with barium or lithium vapour at around 1200°C and is most often extracted from spent nuclear fuel rods as a by-product in plutonium production.
Nuclear synthesis
When an 235U atom captures a neutron, it is converted to an excited state of 236U. About 81% of the excited 236U nuclei undergo fission, but the remainder decay to the ground state of 236U by emitting gamma radiation. Further neutron capture creates 237U which has a half-life of 7 days and thus quickly decays to 237Np.
* 237U is also produced via an (n,2n) reaction with 238U.
* 237Np is the decay product of 241Am.
Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure 237Np by chemical separation of neptunium.