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Isotopes of neptunium
Neptunium (Np) is an artificial element, and thus a standard atomic mass 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 with a half-life of 2.14 million years, with a half-life of 154,000 years, and 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 (t1/2 22.5 hours).
The isotopes of neptunium range in atomic weight from 225.0339 u () to 244.068 u (). The primary decay mode before the most stable isotope, , is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium.
==Some notable isotopes==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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