Isotopes of neptunium

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Isotopes of neptunium (93Np)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
235Np synth 396.1 d α 231Pa
ε 235U
236Np synth 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 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 238
with neutrons to produce 239
, which then underwent beta decay to 239

Trace quantities are found in nature from neutron capture reactions by uranium atoms, a fact not discovered until 1951.[2]

Twenty-five 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 five meta states, with the most stable being 236m
(t1/2 22.5 hours).

The isotopes of neptunium range from 219
to 244
, though the intermediate isotope 221
has not yet been observed. 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. Neptunium is the heaviest element for which the location of the proton drip line is known; the lightest bound isotope is 220Np.[3]

List of isotopes[edit]

[n 1]
Z N Isotopic mass (Da)[4]
[n 2][n 3]

[n 4]

[n 5]
Spin and
[n 6][n 7]
Excitation energy[n 7]
[5][n 8]
93 126 219.03162(9) 0.15+0.72
α 215Pa (9/2−)
93 127 220.03254(21)# 25+14
α 216Pa 1−#
93 129 380+260
α 218Pa 1-#
93 130 223.03285(21)# 2.15+100
α 219Pa 9/2−
93 131 224.03422(21)# 38+26
α (83%) 220m1Pa 1−#
α (17%) 220m2Pa
93 132 225.03391(8) 6(5) ms 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+)
~9 min[9] IT 234Np 5+
EC 234U
93 142 235.0440633(21) 396.1(12) d EC 235U 5/2+
α (.0026%) 231Pa
[n 9]
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 10]
93 144 237.0481734(20) 2.144(7)×106 y α 233Pa 5/2+ Trace[n 11]
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+ Trace[n 11]
93 147 240.056162(16) 61.9(2) min β 240Pu (5+) Trace[n 12]
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−)
This table header & footer:
  1. ^ mNp – 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. ^ a b # – 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. ^ a b Produced by neutron capture in uranium ore
  12. ^ Intermediate decay product of 244Pu

Actinides vs fission products[edit]

Actinides[10] by decay chain Half-life
range (a)
Fission products of 235U by yield[11]
4n 4n + 1 4n + 2 4n + 3 4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 a 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[12] 249Cfƒ 242mAmƒ 141–351 a

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

241Amƒ 251Cfƒ[13] 430–900 a
226Ra 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th 231Pa 32–76 ka
236Npƒ 233Uƒ 234U 150–250 ka 99Tc 126Sn
248Cm 242Pu 327–375 ka 79Se
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs 107Pd
236U 247Cmƒ 15–24 Ma 129I
244Pu 80 Ma

... nor beyond 15.7 Ma[14]

232Th 238U 235Uƒ№ 0.7–14.1 Ga

Notable isotopes[edit]


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

This 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. 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),[15] though precise experimental data is not available.[16]

is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237
,[17] however, it is nearly impossible to separate in any significant quantities from its parent 237
.[18] 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.[16] Nevertheless, 236
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.[19] Several alternative production routes for this isotope have been investigated, namely those that reduce isotopic separation from 237
or the isomer 236m
. The most favorable reactions to accumulate 236
were shown to be proton and deuteron irradiation of uranium-238.[19]


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.[20] 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[edit]

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.

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 238

When exposed to neutron bombardment 237
can capture a neutron, undergo beta decay, and become 238
, 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.[21]

Shortage of 237

The long half-life (T½ ~ 88 years) of 238
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.

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

Stockpiles of 238
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 237
from irradiated nuclear fuels is therefore necessary for the resumption of 238
production in order to replenish the stocks needed for space exploration by robotic probes.


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.[22]

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."[23][24]


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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. ^ a b 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.
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  10. ^ 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.
  11. ^ Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
  12. ^ 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]."
  13. ^ This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
  14. ^ 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.
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