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Neptunium (₉₃Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was Np in 1940, produced by bombarding
U
with neutrons to produce
U
, which then underwent beta decay to
Np
.

Trace quantities are found in nature from neutron capture reactions by uranium atoms, a fact not discovered until 1951.

Twenty-five neptunium radioisotopes have been characterized, with the most stable being
Np
with a half-life of 2.14 million years,
Np
with a half-life of 154,000 years, and
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 five meta states, with the most stable being
Np
(t_1/2 22.5 hours).

The isotopes of neptunium range from
Np
to
Np
, though the intermediate isotope
Np
has not yet been observed. The primary decay mode before the most stable isotope,
Np
, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before
Np
are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium. Neptunium is the heaviest element for which the location of the proton drip line is known; the lightest bound isotope is Np.

List of isotopes

1. Np – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. Modes of decay:

   CD:	Cluster decay
   EC:	Electron capture
   IT:	Isomeric transition
   SF:	Spontaneous fission

5. Bold italics symbol as daughter – Daughter product is nearly stable.
6. ( ) spin value – Indicates spin with weak assignment arguments.
7. ^ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
8. Heaviest known nucleus, as of 2019, that is beyond the proton drip line.
9. Fissile nuclide
10. Most common nuclide
11. ^ Produced by neutron capture in uranium ore
12. Intermediate decay product of Pu

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), though precise experimental data is not available.

Np
is produced in small quantities via the (n,2n) and (γ,n) capture reactions of
Np
, however, it is nearly impossible to separate in any significant quantities from its parent
Np
. 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. Nevertheless,
Np
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. Several alternative production routes for this isotope have been investigated, namely those that reduce isotopic separation from
Np
or the isomer
Np
. The most favorable reactions to accumulate
Np
were shown to be proton and deuteron irradiation of uranium-238.

Neptunium-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,
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 light water nuclear power plants (as opposed to fast reactor or accelerator-driven systems, for example).

Inventory in spent nuclear fuel

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

Np
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
Pu production

When exposed to neutron bombardment
Np
can capture a neutron, undergo beta decay, and become
Pu
, 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.

Shortage of
Np stockpiles

The long half-life (T_½ ~ 88 years) of
Pu
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.

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

Stockpiles of
Pu
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
Np
from irradiated nuclear fuels is therefore necessary for the resumption of
Pu
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 U can be made by neutron capture in uranium-238.

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 dominating "the spectrum for several days."

References

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2. 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.
3. ^ Zhang, Z. Y.; Gan, Z. G.; Yang, H. B.; et al. (2019). "New isotope Np: 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.
4. Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 030003-1–030003-442. doi:10.1088/1674-1137/41/3/030003.
5. Yang, H; Ma, L; Zhang, Z; Yang, C; Gan, Z; Zhang, M; et al. (2018). "Alpha decay properties of the semi-magic nucleus Np". Physics Letters B. 777: 212–216. Bibcode:2018PhLB..777..212Y. doi:10.1016/j.physletb.2017.12.017.
6. Ma, L.; Zhang, Z. Y.; Gan, Z. G.; et al. (2020). "Short-Lived α-emitting isotope Np and the Stability of the N=126 Magic Shell". Physical Review Letters. 125 (3): 032502. Bibcode:2020PhRvL.125c2502M. doi:10.1103/PhysRevLett.125.032502. PMID 32745401. S2CID 220965400.
7. Sun, M. D.; et al. (2017). "New short-lived isotope Np and the absence of the Z = 92 subshell closure near N = 126". Physics Letters B. 771: 303–308. Bibcode:2017PhLB..771..303S. doi:10.1016/j.physletb.2017.03.074.
8. Huang, T. H.; et al. (2018). "Identification of the new isotope Np" (pdf). Physical Review C. 98 (4): 044302. Bibcode:2018PhRvC..98d4302H. doi:10.1103/PhysRevC.98.044302. S2CID 125251822.
9. Asai, M.; Suekawa, Y.; Higashi, M.; et al. Discovery of 234 Np isomer and its decay properties (PDF) (Report) (in Japanese).
10. ^ Niwase, T.; Watanabe, Y. X.; Hirayama, Y.; et al. (2023). "Discovery of New Isotope U 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.
11. 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.
12. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
13. 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 Bk with a half-life greater than 9 . No growth of Cf was detected, and a lower limit for the β half-life can be set at about 10 . No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 ."
14. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
15. Excluding those "classically stable" nuclides with half-lives significantly in excess of Th; e.g., while Cd has a half-life of only fourteen years, that of Cd is eight quadrillion years.
16. 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.
17. ^ Reed, B. C. (2017). "An examination of the potential fission-bomb weaponizability of nuclides other than U and Pu". American Journal of Physics. 85: 38–44. doi:10.1119/1.4966630.
18. Analysis of the Reuse of Uranium Recovered from the Reprocessing of Commercial LWR Spent Fuel, United States Department of Energy, Oak Ridge National Laboratory.
19. **Jukka Lehto; Xiaolin Hou (2011). "15.15: Neptunium". Chemistry and Analysis of Radionuclides (1st ed.). John Wiley & Sons. 231. ISBN 978-3527633029.
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25. Bounding Analysis of Effects of Fractionation of Radionuclides in Fallout on Estimation of Doses to Atomic Veterans DTRA-TR-07-5. 2007

• 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.

v t e Isotopes of the chemical elements
Group 1 2   3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Period Hydrogen and alkali metals Alkaline earth metals Pnicto­gens Chal­co­gens Halo­gens Noble gases ① Isotopes § ListH1 Isotopes § ListHe2 ② Isotopes § ListLi3 Isotopes § ListBe4 Isotopes § ListB5 Isotopes § ListC6 Isotopes § ListN7 Isotopes § ListO8 Isotopes § ListF9 Isotopes § ListNe10 ③ Isotopes § ListNa11 Isotopes § ListMg12 Isotopes § ListAl13 Isotopes § ListSi14 Isotopes § ListP15 Isotopes § ListS16 Isotopes § ListCl17 Isotopes § ListAr18 ④ Isotopes § ListK19 Isotopes § ListCa20 Isotopes § ListSc21 Isotopes § ListTi22 Isotopes § ListV23 Isotopes § ListCr24 Isotopes § ListMn25 Isotopes § ListFe26 Isotopes § ListCo27 Isotopes § ListNi28 Isotopes § ListCu29 Isotopes § ListZn30 Isotopes § ListGa31 Isotopes § ListGe32 Isotopes § ListAs33 Isotopes § ListSe34 Isotopes § ListBr35 Isotopes § ListKr36 ⑤ Isotopes § ListRb37 Isotopes § ListSr38 Isotopes § ListY39 Isotopes § ListZr40 Isotopes § ListNb41 Isotopes § ListMo42 Isotopes § ListTc43 Isotopes § ListRu44 Isotopes § ListRh45 Isotopes § ListPd46 Isotopes § ListAg47 Isotopes § ListCd48 Isotopes § ListIn49 Isotopes § ListSn50 Isotopes § ListSb51 Isotopes § ListTe52 Isotopes § ListI53 Isotopes § ListXe54 ⑥ Isotopes § ListCs55 Isotopes § ListBa56 Isotopes § ListLu71 Isotopes § ListHf72 Isotopes § ListTa73 Isotopes § ListW74 Isotopes § ListRe75 Isotopes § ListOs76 Isotopes § ListIr77 Isotopes § ListPt78 Isotopes § ListAu79 Isotopes § ListHg80 Isotopes § ListTl81 Isotopes § ListPb82 Isotopes § ListBi83 Isotopes § ListPo84 Isotopes § ListAt85 Isotopes § ListRn86 ⑦ Isotopes § ListFr87 Isotopes § ListRa88 Isotopes § ListLr103 Isotopes § ListRf104 Isotopes § ListDb105 Isotopes § ListSg106 Isotopes § ListBh107 Isotopes § ListHs108 Isotopes § ListMt109 Isotopes § ListDs110 Isotopes § ListRg111 Isotopes § ListCn112 Isotopes § ListNh113 Isotopes § ListFl114 Isotopes § ListMc115 Isotopes § ListLv116 Isotopes § ListTs117 Isotopes § ListOg118 ⑧ Isotopes § ListUue119 Isotopes § ListUbn120 Isotopes § ListLa57 Isotopes § ListCe58 Isotopes § ListPr59 Isotopes § ListNd60 Isotopes § ListPm61 Isotopes § ListSm62 Isotopes § ListEu63 Isotopes § ListGd64 Isotopes § ListTb65 Isotopes § ListDy66 Isotopes § ListHo67 Isotopes § ListEr68 Isotopes § ListTm69 Isotopes § ListYb70   Isotopes § ListAc89 Isotopes § ListTh90 Isotopes § ListPa91 Isotopes § ListU92 Isotopes § ListNp93 Isotopes § ListPu94 Isotopes § ListAm95 Isotopes § ListCm96 Isotopes § ListBk97 Isotopes § ListCf98 Isotopes § ListEs99 Isotopes § ListFm100 Isotopes § ListMd101 Isotopes § ListNo102
Table of nuclides Categories: Isotopes Tables of nuclides Metastable isotopes Isotopes by element

• Isotopes of neptunium
• Neptunium
• Lists of isotopes by element