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Neptunium-239

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
(t1/2 22.5 hours).


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