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 237Np with a half-life of 2.14 million years, 236Np with a half-life of 154,000 years, and 235Np 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 236mNp (t_½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (225Np) to 244.068 u (244Np). The primary decay mode before the most stable isotope, 237Np, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before 237Np are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium.

Some notable isotopes

Neptunium-235

Neptunium-235 has 142 neutrons and a half-life of 400 days. This isotope of Neptunium either decays by:

• Emitting an alpha particle - Here, the decay energy is 5.2 MeV and the decay product is Protactinium-231.
• Electron capture - Here, the decay energy is 0.125 MeV and the decay product is Uranium-235

This particular isotope of neptunium has a weight of 235.0440633 grams/mole.

Neptunium-236

Neptunium-236 has 143 neutrons and a half-life of 154,000 years. It can decay by the following methods -

• Electron capture - here, the decay energy is 0.95 MeV and the decay product is Uranium-236.
• Beta emission - Here, the decay energy is 0.94 MeV and the decay product is Plutonium-236.
• Alpha emission - Here, the decay energy is 5.024 MeV and the decay product is Protactinium-232

This particular isotope of neptunium has a mass of 236.04657 grams/mole. It is a fissile material with a critical mass of 6.79 kg.^[1]

236Np is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237Np,^[2] however it is nearly impossible to separate in any significant quantities from its parent 237Np.^[3] 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

Neptunium-237 decay scheme (simplified)

237Np decays via the neptunium series to bismuth and thallium, unlike most other actinides which decay to isotopes of lead.

237Np was recently shown to be capable of sustaining a chain reaction with fast neutrons, as in a nuclear weapon.^[7] 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.).

237Np 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, 237Np also forms in spent nuclear fuel as the decay product of americium-241.

237Np is 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 237Np can capture a neutron and become 238Pu, 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.

Table

1. ^ Abbreviations:
   CD: Cluster decay
   EC: Electron capture
   IT: Isomeric transition
   SF: Spontaneous fission
2. ^ Fissile nuclide
3. ^ Most common nuclide

Notes

• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.

References

• Isotope masses from:
  • G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties". Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
• Isotopic compositions and standard atomic masses from:
  • J. R. de Laeter, J. K. Böhlke, P. De Bièvre, H. Hidaka, H. S. Peiser, K. J. R. Rosman and P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry 75 (6): 683–800. doi:10.1351/pac200375060683.
  • M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry 78 (11): 2051–2066. doi:10.1351/pac200678112051. Lay summary.
• Half-life, spin, and isomer data selected from the following sources. See editing notes on .
  • G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties". Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005.
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9 [Amazon-US | Amazon-UK].