Isotopes of neptunium

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Main isotopes of neptunium
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
235Np syn 396.1 d α 231Pa
ε 235U
236Np syn 1.54×105 y ε 236U
β 236Pu
α 232Pa
237Np trace 2.144×106 y α 233Pa
239Np trace 2.356 d β 239Pu

Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so thus 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 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 reactions by uranium atoms, a fact not discovered until 1951.

Twenty neptunium radioisotopes have been characterized, with the most stable being 237
with a half-life of 2.14 million years, 236
with a half-life of 154,000 years, and 235
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
(t1/2 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (225
) to 244.068 u (244
). The primary decay mode before the most stable isotope, 237
, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before 237
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".[1][2]

Actinides vs fission products

Actinides and fission products by half-life
Actinides[3] by decay chain Half-life
range (y)
Fission products of 235U by yield[4]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ№ 243Cmƒ 29–97 137Cs 151Smþ 121mSn
248Bk[5] 249Cfƒ 242mAmƒ 141–351

No fission products
have a half-life
in the range of
100–210 k years ...

241Amƒ 251Cfƒ[6] 430–900
226Ra 247Bk 1.3 k – 1.6 k
240Puƒ№ 229Th 246Cmƒ 243Amƒ 4.7 k – 7.4 k
245Cmƒ 250Cm 8.3 k – 8.5 k
239Puƒ№ 24.1 k
230Th 231Pa 32 k – 76 k
236Npƒ 233Uƒ№ 234U 150 k – 250 k 99Tc 126Sn
248Cm 242Puƒ 327 k – 375 k 79Se
1.53 M 93Zr
237Npƒ№ 2.1 M – 6.5 M 135Cs 107Pd
236U 247Cmƒ 15 M – 24 M 129I
244Pu 80 M

... nor beyond 15.7 M years[7]

232Th 238U 235Uƒ№ 0.7 G – 14.1 G

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 y: Medium-lived fission product
‡  over 200,000 y: Long-lived fission product

Notable isotopes


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 235.044 063 3 u.


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.
  • Beta emission: the decay energy is 0.48 MeV and the decay product is plutonium-236.
  • Alpha emission: the decay energy is 5.007 MeV and the decay product is protactinium-232

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.[8]

is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237
,[9] however it is nearly impossible to separate in any significant quantities from its parent 237
.[10] 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 decay scheme (simplified)

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
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.[11] 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.).

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

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
can capture a neutron and become 238
, this product being useful as an thermal energy source in a radio-isotope thermoelectric generator for the production of electricity and heat in deep space probes (such as the New Horizons and Voyager probes) and, of recent note, the Mars Science Laboratory (Curiosity rover). 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. Space probes and rovers also make use of the heat output of the generator to keep their instruments and internals warm.[12]

List of isotopes

Z(p) N(n)  
isotopic mass (u)
half-life decay
mode(s)[13][n 1]
spin and
excitation energy
93 132 225.03391(8) 3# ms [>2 µs] α 221Pa 9/2−#
93 133 226.03515(10)# 35(10) ms α 222Pa
93 134 227.03496(8) 510(60) ms α (99.95%) 223Pa 5/2−#
β+ (.05%) 227U
93 135 228.03618(21)# 61.4(14) s β+ (59%) 228U
α (41%) 224Pa
β+, SF (.012%) (various)
93 136 229.03626(9) 4.0(2) min α (51%) 225Pa 5/2+#
β+ (49%) 229U
93 137 230.03783(6) 4.6(3) min β+ (97%) 230U
α (3%) 226Pa
93 138 231.03825(5) 48.8(2) min β+ (98%) 231U (5/2)(+#)
α (2%) 227Pa
93 139 232.04011(11)# 14.7(3) min β+ (99.99%) 232U (4+)
α (.003%) 228Pa
93 140 233.04074(5) 36.2(1) min β+ (99.99%) 233U (5/2+)
α (.001%) 229Pa
93 141 234.042895(9) 4.4(1) d β+ 234U (0+)
93 142 235.0440633(21) 396.1(12) d EC 235U 5/2+
α (.0026%) 231Pa
[n 2]
93 143 236.04657(5) 1.54(6)×105 y EC (87.3%) 236U (6−)
β (12.5%) 236Pu
α (.16%) 232Pa
60(50) keV 22.5(4) h EC (52%) 236U 1
β (48%) 236Pu
[n 2][n 3]
93 144 237.0481734(20) 2.144(7)×106 y α 233Pa 5/2+
SF (2×10−10%) (various)
CD (4×10−12%) 207Tl
93 145 238.0509464(20) 2.117(2) d β 238Pu 2+
2300(200)# keV 112(39) ns
93 146 239.0529390(22) 2.356(3) d β 239Pu 5/2+
93 147 240.056162(16) 61.9(2) min β 240Pu (5+)
20(15) keV 7.22(2) min β (99.89%) 240Pu 1(+)
IT (.11%) 240Np
93 148 241.05825(8) 13.9(2) min β 241Pu (5/2+)
93 149 242.06164(21) 2.2(2) min β 242Pu (1+)
0(50)# keV 5.5(1) min 6+#
93 150 243.06428(3)# 1.85(15) min β 243Pu (5/2−)
93 151 244.06785(32)# 2.29(16) min β 244Pu (7−)
  1. ^ Abbreviations:
    CD: Cluster decay
    EC: Electron capture
    IT: Isomeric transition
    SF: Spontaneous fission
  2. ^ a b Fissile nuclide
  3. ^ Most common nuclide


  • 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.


  1. ^ [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]
  2. ^ Bounding Analysis of Effects of Fractionation of Radionuclides in Fallout on Estimation of Doses to Atomic Veterans DTRA-TR-07-5. 2007
  3. ^ 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.
  4. ^ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  5. ^ 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 y. No growth of Cf248 was detected, and a lower limit for the β half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
  6. ^ This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
  7. ^ 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 nearly eight quadrillion years.
  8. ^ Final Report, Evaluation of nuclear criticality safety data and limits for actinides in transport Archived 2011-05-19 at the Wayback Machine., Republic of France, Institut de Radioprotection et de Sûreté Nucléaire, Département de Prévention et d'étude des Accidents.
  9. ^ Analysis of the Reuse of Uranium Recovered from the Reprocessing of Commercial LWR Spent Fuel, United States Department of Energy, Oak Ridge National Laboratory.
  10. ^ **Jukka Lehto; Xiaolin Hou (2011). "15.15: Neptunium". Chemistry and Analysis of Radionuclides (1st ed.). John Wiley & Sons. 231. ISBN 3527633022. 
  11. ^ P. Weiss (26 October 2002). "Neptunium Nukes? Little-studied metal goes critical". Science News. 162 (17): 259. Archived from the original on 15 December 2012. Retrieved 7 November 2013. 
  12. ^ Witze, Alexandra (2014-11-27). "Nuclear power: Desperately seeking plutonium". Nature. 515 (7528): 484–486. doi:10.1038/515484a. 
  13. ^ "Universal Nuclide Chart". nucleonica. (Registration required (help)). 

Isotopes of uranium Isotopes of neptunium Isotopes of plutonium
Table of nuclides
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