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List of isotopes

Actinides vs fission products

Notable isotopes

Neptunium-235

Neptunium-235 has 142 neutrons and a half-life of 396.1 days. This isotope decays by:

• Alpha emission: the decay energy is 5.2 MeV and the decay product is protactinium-231.
• Electron capture: the decay energy is 0.125 MeV and the decay product is uranium-235

This isotope of neptunium has a weight of 235.044 063 3 u.

Neptunium-236

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

• Electron capture: the decay energy is 0.93 MeV and the decay product is uranium-236. This usually decays (with a half-life of 23 million years) to thorium-232.
• Beta emission: the decay energy is 0.48 MeV and the decay product is plutonium-236. This usually decays (half-life 2.8 years) to uranium-232, which usually decays (half-life 69 years) to thorium-228, which decays in a few years to lead-208.
• Alpha emission: the decay energy is 5.007 MeV and the decay product is protactinium-232. This decays with a half-life of 1.3 days to uranium-232.

This particular isotope of neptunium has a mass of 236.04657 u. It is a fissile material; it has an estimated critical mass of 6.79 kg (15.0 lb),³² though precise experimental data is not available.³³

236Np is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237Np,³⁴ however, it is nearly impossible to separate in any significant quantities from its parent 237Np.³⁵ It is for this reason that despite its low critical mass and high neutron cross section, it has not been researched extensively as a nuclear fuel in weapons or reactors.³⁶ Nevertheless, 236Np has been considered for use in mass spectrometry and as a radioactive tracer, because it decays predominantly by beta emission with a long half-life.³⁷ Several alternative production routes for this isotope have been investigated, namely those that reduce isotopic separation from 237Np or the isomer 236mNp. The most favorable reactions to accumulate 236Np were shown to be proton and deuteron irradiation of uranium-238.³⁸

Neptunium-237

237Np 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, 237Np 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.³⁹ However, it has a low probability of fission on bombardment with thermal neutrons, which makes it unsuitable as a fuel for light water nuclear power plants (as opposed to fast reactor or accelerator-driven systems, for example).

Inventory in spent nuclear fuel

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 considered to be one of the most mobile radionuclides at the site of the Yucca Mountain nuclear waste repository (Nevada) where oxidizing conditions prevail in the unsaturated zone of the volcanic tuff above the water table.

Raw material for 238Pu production

Main articles: Plutonium-238 and Radioisotope thermoelectric generator

When exposed to neutron bombardment 237Np can capture a neutron, undergo beta decay, and become 238Pu, this product being useful as a thermal energy source in a radioisotope thermoelectric generator (RTG or RITEG) for the production of electricity and heat. The first type of thermoelectric generator SNAP (Systems for Nuclear Auxiliary Power) was developed and used by NASA in the 1960's and during the Apollo missions to power the instruments left on the Moon surface by the astronauts. Thermoelectric generators were also embarked on board of deep space probes such as for the Pioneer 10 and 11 missions, the Voyager program, the Cassini–Huygens mission, and New Horizons. They also deliver electrical and thermal power to the Mars Science Laboratory (Curiosity rover) and Mars 2020 mission (Perseverance rover) both exploring the cold surface of Mars. Curiosity and Perseverance rovers are both equipped with the last version of multi-mission RTG, a more efficient and standardized system dubbed MMRTG.

These applications are economically practical where photovoltaic power sources are weak or inconsistent due to probes being too far from the sun or rovers facing climate events that may obstruct sunlight for long periods (like Martian dust storms). Space probes and rovers also make use of the heat output of the generator to keep their instruments and internals warm.⁴⁰

Shortage of 237Np stockpiles

The long half-life (T1/2 ~ 88 years) of 238Pu and the absence of γ-radiation that could interfere with the operation of on-board electronic components, or irradiate people, makes it the radionuclide of choice for electric thermogenerators.

237Np is therefore a key radionuclide for the production of 238Pu, which is essential for deep space probes requiring a reliable and long-lasting source of energy without maintenance.

Stockpiles of 238Pu built up in the United States since the Manhattan Project, thanks to the Hanford nuclear complex (operating in Washington State from 1943 to 1977) and the development of atomic weapons, are now almost exhausted. The extraction and purification of sufficient new quantities of 237Np from irradiated nuclear fuels is therefore necessary for the resumption of 238Pu production in order to replenish the stocks needed for space exploration by robotic probes.

Neptunium-239

Neptunium-239 has 146 neutrons and a half-life of 2.356 days. It is produced via β− decay of the short-lived uranium-239, and undergoes another β− decay to plutonium-239. This is the primary route for making plutonium, as 239U can be made by neutron capture in uranium-238.⁴¹

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 239Np dominating "the spectrum for several days".⁴²⁴³

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (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
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (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.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.

References

1. Peppard, D. F.; Mason, G. W.; Gray, P. R.; Mech, J. F. (1952). "Occurrence of the (4n + 1) series in nature" (PDF). Journal of the American Chemical Society. 74 (23): 6081–6084. doi:10.1021/ja01143a074. https://digital.library.unt.edu/ark:/67531/metadc172698/m2/1/high_res_d/metadc172698.pdf ↩

2. Zhang, Z. Y.; Gan, Z. G.; Yang, H. B.; et al. (2019). "New isotope 220Np: Probing the robustness of the N = 126 shell closure in neptunium". Physical Review Letters. 122 (19): 192503. Bibcode:2019PhRvL.122s2503Z. doi:10.1103/PhysRevLett.122.192503. PMID 31144958. S2CID 169038981. /wiki/Physical_Review_Letters ↩

3. mNp – Excited nuclear isomer. /wiki/Nuclear_isomer ↩

4. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits. ↩

5. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS). ↩

6. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae. https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf ↩

7. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae. https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf ↩

8. Modes of decay: EC:Electron captureCD:Cluster decayIT:Isomeric transitionSF:Spontaneous fission /wiki/Electron_capture ↩

9. Bold italics symbol as daughter – Daughter product is nearly stable. ↩

10. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae. https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf ↩

11. ( ) spin value – Indicates spin with weak assignment arguments. ↩

12. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN). ↩

13. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN). ↩

14. Heaviest known nucleus, as of 2025[update], that is beyond the proton drip line.[4] https://en.wikipedia.org/w/index.php?title=Isotopes_of_neptunium&action=edit ↩

15. Zhang, Z. Y.; Gan, Z. G.; Yang, H. B.; et al. (2019). "New isotope 220Np: Probing the robustness of the N = 126 shell closure in neptunium". Physical Review Letters. 122 (19): 192503. Bibcode:2019PhRvL.122s2503Z. doi:10.1103/PhysRevLett.122.192503. PMID 31144958. S2CID 169038981. /wiki/Physical_Review_Letters ↩

16. Huang, T. H.; et al. (2018). "Identification of the new isotope 224Np" (pdf). Physical Review C. 98 (4): 044302. Bibcode:2018PhRvC..98d4302H. doi:10.1103/PhysRevC.98.044302. S2CID 125251822. https://www.researchgate.net/publication/328038265 ↩

17. Asai, M.; Suekawa, Y.; Higashi, M.; et al. (2020). Discovery of 234 Np isomer and its decay properties (PDF) (Report) (in Japanese). http://www.radiochem.org/jnrs-abst/pdf/64-501.pdf ↩

18. Fissile nuclide /wiki/Fissile ↩

19. Order of ground state and isomer is uncertain. ↩

20. Produced by neutron capture in uranium ore /wiki/Neutron_capture ↩

21. Produced by neutron capture in uranium ore /wiki/Neutron_capture ↩

22. Intermediate decay product of 244Pu /wiki/Plutonium-244 ↩

23. Order of ground state and isomer is uncertain. ↩

24. Niwase, T.; Watanabe, Y. X.; Hirayama, Y.; et al. (2023). "Discovery of New Isotope 241U and Systematic High-Precision Atomic Mass Measurements of Neutron-Rich Pa-Pu Nuclei Produced via Multinucleon Transfer Reactions" (PDF). Physical Review Letters. 130 (13): 132502-1 – 132502-6. doi:10.1103/PhysRevLett.130.132502. PMID 37067317. S2CID 257976576. https://eprints.whiterose.ac.uk/197980/1/PhysRevLett.130.132502.pdf ↩

25. Niwase, T.; Watanabe, Y. X.; Hirayama, Y.; et al. (2023). "Discovery of New Isotope 241U and Systematic High-Precision Atomic Mass Measurements of Neutron-Rich Pa-Pu Nuclei Produced via Multinucleon Transfer Reactions" (PDF). Physical Review Letters. 130 (13): 132502-1 – 132502-6. doi:10.1103/PhysRevLett.130.132502. PMID 37067317. S2CID 257976576. https://eprints.whiterose.ac.uk/197980/1/PhysRevLett.130.132502.pdf ↩

26. Order of ground state and isomer is uncertain. ↩

27. Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here. /wiki/Polonium ↩

28. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor. /wiki/Thermal_neutron ↩

29. Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4."The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 [years]. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]." /wiki/Bibcode_(identifier) ↩

30. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability". /wiki/Sea_of_instability ↩

31. Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is eight quadrillion years. /wiki/Primordial_nuclide ↩

32. Final Report, Evaluation of nuclear criticality safety data and limits for actinides in transport (PDF) (Report). Republic of France, Institut de Radioprotection et de Sûreté Nucléaire, Département de Prévention et d'étude des Accidents. Archived from the original (PDF) on 2011-05-19. https://web.archive.org/web/20110519171204/http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf ↩

33. Reed, B. C. (2017). "An examination of the potential fission-bomb weaponizability of nuclides other than 235U and 239Pu". American Journal of Physics. 85: 38–44. doi:10.1119/1.4966630. /wiki/Doi_(identifier) ↩

34. Analysis of the Reuse of Uranium Recovered from the Reprocessing of Commercial LWR Spent Fuel, United States Department of Energy, Oak Ridge National Laboratory. http://info.ornl.gov/sites/publications/files/Pub14204.pdf ↩

35. **Jukka Lehto; Xiaolin Hou (2011). "15.15: Neptunium". Chemistry and Analysis of Radionuclides (1st ed.). John Wiley & Sons. 231. ISBN 978-3527633029. 978-3527633029 ↩

36. Reed, B. C. (2017). "An examination of the potential fission-bomb weaponizability of nuclides other than 235U and 239Pu". American Journal of Physics. 85: 38–44. doi:10.1119/1.4966630. /wiki/Doi_(identifier) ↩

37. Jerome, S.M.; Ivanov, P.; Larijani, C.; Parker, D.J.; Regan, P.H. (2014). "The production of Neptunium-236g". Journal of Environmental Radioactivity. 138: 315–322. doi:10.1016/j.jenvrad.2014.02.029. PMID 24731718. /wiki/Journal_of_Environmental_Radioactivity ↩

38. Jerome, S.M.; Ivanov, P.; Larijani, C.; Parker, D.J.; Regan, P.H. (2014). "The production of Neptunium-236g". Journal of Environmental Radioactivity. 138: 315–322. doi:10.1016/j.jenvrad.2014.02.029. PMID 24731718. /wiki/Journal_of_Environmental_Radioactivity ↩

39. P. Weiss (26 October 2002). "Neptunium Nukes? Little-studied metal goes critical". Science News. 162 (17): 259. doi:10.2307/4014034. JSTOR 4014034. Archived from the original on 26 May 2024. Retrieved 7 November 2013. https://archive.today/20240526102438/https://www.webcitation.org/6Cw9Vnt0Q?url=http://www.sciencenews.org/view/generic/id/3246/title/Neptunium_Nukes%3F_Little-studied_metal_goes_critical ↩

40. Witze, Alexandra (2014-11-27). "Nuclear power: Desperately seeking plutonium". Nature. 515 (7528): 484–486. Bibcode:2014Natur.515..484W. doi:10.1038/515484a. PMID 25428482. https://doi.org/10.1038%2F515484a ↩

41. "Periodic Table Of Elements: LANL - Neptunium". Los Alamos National Laboratory. Retrieved 2013-10-13. http://periodic.lanl.gov/93.shtml ↩

42. [Film Badge Dosimetry in Atmospheric Nuclear Tests, By Committee on Film Badge Dosimetry in Atmospheric Nuclear Tests, Commission on Engineering and Technical Systems, Division on Engineering and Physical Sciences, National Research Council. pg24-35] ↩

43. Bounding Analysis of Effects of Fractionation of Radionuclides in Fallout on Estimation of Doses to Atomic Veterans DTRA-TR-07-5. 2007 http://www.dtra.mil/Portals/61/Documents/NTPR/4-Rad_Exp_Rpts/30_DTRA-TR-07-5_Fractionation_Report.pdf ↩