Neptunium-237

Main isotopes of neptunium

iso NA half-life DM DE (MeV) DP 235Np syn 396.1 d α 5.192 231Pa ε 0.124 235U 236Np syn 1.54×105 y ε 0.940 236U β− 0.940 236Pu α 5.020 232Pa 237Np trace 2.144×106 y α 4.959 233Pa 239Np trace 2.356 d β− 0.218 239Pu

Neptunium (Np) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was ²³⁹Np in 1940, produced by bombarding ²³⁸U with neutrons to produce ²³⁹U, which then underwent beta decay to ²³⁹Np.

Trace quantities are found in nature from neutron capture by uranium atoms.

Twenty neptunium radioisotopes have been characterized, with the most stable being 237
Np
with a half-life of 2.14 million years, 236
Np
with a half-life of 154,000 years, and 235
Np
with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has 4 meta states, with the most stable being 236m
Np
(t_1/2 22.5 hours).

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