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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.
Contents
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).
The isotopes of neptunium range in atomic weight from 225.0339 u (225
Np
) to 244.068 u (244
Np
). The primary decay mode before the most stable isotope, 237
Np
, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before 237
Np
are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium. Uranium-237 and Neptunium-239 are regarded as the leading hazardous radioisotopes in the first hour-to-week period following nuclear fallout from a nuclear detonation, with Np-239 dominating "the spectrum for several days".
Neptunium-235
Neptunium-235 has 142 neutrons and a half-life of 400 days. This isotope of Neptunium either decays by:
This particular isotope of neptunium has a weight of 235.0440633 u.
Neptunium-236
Neptunium-236 has 143 neutrons and a half-life of 154,000 years. It can decay by the following methods:
This particular isotope of neptunium has a mass of 236.04657 u. It is a fissile material with a critical mass of 6.79 kg.
236
Np
is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237
Np
, however it is nearly impossible to separate in any significant quantities from its parent 237
Np
. It is for this reason that, despite its low critical mass and high neutron cross section, it has not been researched as a nuclear fuel in weapons or reactors.
Neptunium-237
237
Np
decays via the neptunium series, which terminates with thallium-205, which is stable, unlike most other actinides, which decay to stable isotopes of lead.
In 2002, 237
Np
was shown to be capable of sustaining a chain reaction with fast neutrons, as in a nuclear weapon, with a critical mass of around 60 kg. However, it has a low probability of fission on bombardment with thermal neutrons, which makes it unsuitable as a fuel for conventional nuclear power plants (as opposed to accelerator-driven systems, etc.).
237
Np
is the only neptunium isotope produced in significant quantity in the nuclear fuel cycle, both by successive neutron capture by uranium-235 (which fissions most but not all of the time) and uranium-236, or (n,2n) reactions where a fast neutron occasionally knocks a neutron loose from uranium-238 or isotopes of plutonium. Over the long term, 237
Np
also forms in spent nuclear fuel as the decay product of americium-241.
237
Np
was projected to be one of the most mobile nuclides at the Yucca Mountain nuclear waste repository.
Use in plutonium-238 production
When exposed to neutron bombardment 237
Np
can capture a neutron and become 238
Pu
, this product being useful as an thermal energy source for the production of electricity in deep space probes and, of recent note, the Mars Science Laboratory (Curiosity rover). These applications are economically practical where photovoltaic power sources are weak or inconsistent.