Isotopes of neptunium

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
U
with neutrons to produce ²³⁹
U
, which then underwent beta decay to ²³⁹
Np
.

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

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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
Np
with a half-life of 2.14 million years, ²³⁶
Np
with a half-life of 154,000 years, and ²³⁵
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 ^236m
Np
(t_1/2 22.5 hours).

The isotopes of neptunium range from ²¹⁹
Np
to ²⁴⁴
Np
, though the intermediate isotope ²²¹
Np
has not yet been observed. The primary decay mode before the most stable isotope, ²³⁷
Np
, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before ²³⁷
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 ²²⁰Np.^[3]

List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da)[4] [n 2][n 3]	Half-life	Decay mode [n 4]	Daughter isotope [n 5]	Spin and parity [n 6][n 7]	Isotopic abundance
	Excitation energy[n 7]
219 Np [5][n 8]	93	126	219.03162(9)	0.15+0.72 −0.07 ms	α	215Pa	(9/2−)
220 Np [3]	93	127	220.03254(21)#	25+14 −7 μs	α	216Pa	1−#
222 Np [6]	93	129		380+260 −110 ns	α	218Pa	1-#
223 Np [7]	93	130	223.03285(21)#	2.15+100 −52 μs	α	219Pa	9/2−
224 Np [8]	93	131	224.03422(21)#	38+26 −11 μs	α (83%)	220m1Pa	1−#
					α (17%)	220m2Pa
225 Np	93	132	225.03391(8)	6(5) ms	α	221Pa	9/2−#
226 Np	93	133	226.03515(10)#	35(10) ms	α	222Pa
227 Np	93	134	227.03496(8)	510(60) ms	α (99.95%)	223Pa	5/2−#
					β+ (.05%)	227U
228 Np	93	135	228.03618(21)#	61.4(14) s	β+ (59%)	228U
					α (41%)	224Pa
					β+, SF (.012%)	(various)
229 Np	93	136	229.03626(9)	4.0(2) min	α (51%)	225Pa	5/2+#
					β+ (49%)	229U
230 Np	93	137	230.03783(6)	4.6(3) min	β+ (97%)	230U
					α (3%)	226Pa
231 Np	93	138	231.03825(5)	48.8(2) min	β+ (98%)	231U	(5/2)(+#)
					α (2%)	227Pa
232 Np	93	139	232.04011(11)#	14.7(3) min	β+ (99.99%)	232U	(4+)
					α (.003%)	228Pa
233 Np	93	140	233.04074(5)	36.2(1) min	β+ (99.99%)	233U	(5/2+)
					α (.001%)	229Pa
234 Np	93	141	234.042895(9)	4.4(1) d	β+	234U	(0+)
234m Np				~9 min[9]	IT	234Np	5+
					EC	234U
235 Np	93	142	235.0440633(21)	396.1(12) d	EC	235U	5/2+
					α (.0026%)	231Pa
236 Np [n 9]	93	143	236.04657(5)	1.54(6)×105 y	EC (87.3%)	236U	(6−)
					β− (12.5%)	236Pu
					α (.16%)	232Pa
236m Np	60(50) keV			22.5(4) h	EC (52%)	236U	1
					β− (48%)	236Pu
237 Np [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 30Mg
238 Np	93	145	238.0509464(20)	2.117(2) d	β−	238Pu	2+
238m Np	2300(200)# keV			112(39) ns
239 Np	93	146	239.0529390(22)	2.356(3) d	β−	239Pu	5/2+	Trace[n 11]
240 Np	93	147	240.056162(16)	61.9(2) min	β−	240Pu	(5+)	Trace[n 12]
240m Np	20(15) keV			7.22(2) min	β− (99.89%)	240Pu	1(+)
					IT (.11%)	240Np
241 Np	93	148	241.058349(33)[10]	13.9(2) min	β−	241Pu	(5/2+)
242 Np	93	149	242.061738(87)[10]	2.2(2) min	β−	242Pu	(1+)
242m Np	0(50)# keV			5.5(1) min			6+#
243 Np	93	150	243.06428(3)#	1.85(15) min	β−	243Pu	(5/2−)
244 Np	93	151	244.06785(32)#	2.29(16) min	β−	244Pu	(7−)
This table header & footer: view

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. # – 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^[update], 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 ²⁴⁴Pu

Actinides vs fission products

Actinides and fission products by half-life v t e
Actinides[11] by decay chain				Half-life range (a)	Fission products of 235U by yield[12]
4n	4n + 1	4n + 2	4n + 3		4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6 a		155Euþ
248Bk[13]				> 9 a
244Cmƒ	241Puƒ	250Cf	227Ac№	10–29 a	90Sr	85Kr	113mCdþ
232Uƒ		238Puƒ	243Cmƒ	29–97 a	137Cs	151Smþ	121mSn
	249Cfƒ	242mAmƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	241Amƒ		251Cfƒ[14]	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₡
				1.33 Ma	135Cs₡
	237Npƒ			1.61–6.5 Ma	93Zr	107Pd
236U			247Cmƒ	15–24 Ma		129I₡
244Pu				80 Ma	... nor beyond 15.7 Ma[15]
232Th№		238U№	235Uƒ№	0.7–14.1 Ga
₡,  has thermal neutron capture cross section in the range of 8–50 barns ƒ,  fissile №,  primarily a naturally occurring radioactive material (NORM) þ,  neutron poison (thermal neutron capture cross section greater than 3k barns)

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),^[16] though precise experimental data is not available.^[17]

²³⁶
Np
is produced in small quantities via the (n,2n) and (γ,n) capture reactions of ²³⁷
Np
,^[18] however, it is nearly impossible to separate in any significant quantities from its parent ²³⁷
Np
.^[19] 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.^[17] Nevertheless, ²³⁶
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.^[20] Several alternative production routes for this isotope have been investigated, namely those that reduce isotopic separation from ²³⁷
Np
or the isomer ^236m
Np
. The most favorable reactions to accumulate ²³⁶
Np
were shown to be proton and deuteron irradiation of uranium-238.^[20]

Neptunium-237

Neptunium-237 decay scheme (simplified)

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, ²³⁷
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.^[21] 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

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

²³⁷
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 ²³⁸
Pu production

Main articles: Plutonium-238 and Radioisotope thermoelectric generator

When exposed to neutron bombardment ²³⁷
Np
can capture a neutron, undergo beta decay, and become ²³⁸
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.^[22]

Shortage of ²³⁷
Np stockpiles

The long half-life (T_½ ~ 88 years) of ²³⁸
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.

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

Stockpiles of ²³⁸
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 ²³⁷
Np
from irradiated nuclear fuels is therefore necessary for the resumption of ²³⁸
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 ²³⁹U can be made by neutron capture in uranium-238.^[23]

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."^[24][25]

References

1. 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.
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 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.
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 ²¹⁹Np". 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 ²²²Np 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 ²²³Np 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 ²²⁴Np" (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 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.
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 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]."
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 ²³²Th; e.g., while ^113mCd has a half-life of only fourteen years, that of ¹¹³Cd 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 ²³⁵U and ²³⁹Pu". 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.
20. 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.
21. 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.
22. 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.
23. "Periodic Table Of Elements: LANL - Neptunium". Los Alamos National Laboratory. Retrieved 2013-10-13.
24. [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]
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.