Synthesis
Chemically, neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200 °C. Most Np is produced in nuclear reactions:
- 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 through beta decay. During beta decay, the excited 237U emits an electron, while the atomic weak interaction converts a neutron to a proton, thus creating 237Np.
- 237U is also produced via an (n,2n) reaction with 238U. This only happens with very energetic neutrons.
- 237Np is the product of alpha decay of 241Am.
Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure 237Np.
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