Isotopes of neptunium

Isotopes of neptunium (₉₃Np)
ε	235U
β−	236Pu
α	232Pa

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
.

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. 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."[3][4]

List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da)[5] [n 2][n 3]	Half-life	Decay mode [n 4]	Daughter isotope [n 5]	Spin and parity [n 6][n 7]	Isotopic abundance
Nuclide [n 1]	Excitation energy[n 7]			Half-life	Decay mode [n 4]	Daughter isotope [n 5]	Spin and parity [n 6][n 7]	Isotopic abundance
²¹⁹ Np [6]	93	126	219.03162(9)	0.15+0.72 −0.07 ms	α	²¹⁵Pa	(9/2−)
²²⁰ Np [7]	93	127	220.03254(21)#	25+14 −7 μs	α	²¹⁶Pa	1-#
²²² Np [8]	93	129		380+260 −110 ns	α	²¹⁸Pa	1-#
²²³ Np [9]	93	130	223.03285(21)#	2.15+100 −52 μs	α	²¹⁹Pa	9/2−
²²⁴ Np [10]	93	131	224.03422(21)#	38+26 −11 μs	α (83%)	^220m1Pa	1-#
²²⁴ Np [10]	93	131	224.03422(21)#	38+26 −11 μs	α (17%)	^220m2Pa	1-#
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−#
²²⁷ Np	93	134	227.03496(8)	510(60) ms	β⁺ (.05%)	²²⁷U	5/2−#
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+#
²²⁹ Np	93	136	229.03626(9)	4.0(2) min	β⁺ (49%)	²²⁹U	5/2+#
²³⁰ Np	93	137	230.03783(6)	4.6(3) min	β⁺ (97%)	²³⁰U
²³⁰ Np	93	137	230.03783(6)	4.6(3) min	α (3%)	²²⁶Pa
²³¹ Np	93	138	231.03825(5)	48.8(2) min	β⁺ (98%)	²³¹U	(5/2)(+#)
²³¹ Np	93	138	231.03825(5)	48.8(2) min	α (2%)	²²⁷Pa	(5/2)(+#)
²³² Np	93	139	232.04011(11)#	14.7(3) min	β⁺ (99.99%)	²³²U	(4+)
²³² Np	93	139	232.04011(11)#	14.7(3) min	α (.003%)	²²⁸Pa	(4+)
²³³ Np	93	140	233.04074(5)	36.2(1) min	β⁺ (99.99%)	²³³U	(5/2+)
²³³ Np	93	140	233.04074(5)	36.2(1) min	α (.001%)	²²⁹Pa	(5/2+)
²³⁴ Np	93	141	234.042895(9)	4.4(1) d	β⁺	²³⁴U	(0+)
^234m Np				~9 min[11]	IT	²³⁴Np	5+
^234m Np				~9 min[11]	EC	²³⁴U	5+
²³⁵ Np	93	142	235.0440633(21)	396.1(12) d	EC	²³⁵U	5/2+
²³⁵ Np	93	142	235.0440633(21)	396.1(12) d	α (.0026%)	²³¹Pa	5/2+
²³⁶ Np [n 8]	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
^236m Np	60(50) keV			22.5(4) h	β⁻ (48%)	²³⁶Pu	1
²³⁷ Np [n 8][n 9]	93	144	237.0481734(20)	2.144(7)×10⁶ y	α	²³³Pa	5/2+	Trace[n 10]
					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 10]
²⁴⁰ Np	93	147	240.056162(16)	61.9(2) min	β⁻	²⁴⁰Pu	(5+)	Trace[n 11]
^240m Np	20(15) keV			7.22(2) min	β⁻ (99.89%)	²⁴⁰Pu	1(+)
^240m Np	20(15) keV			7.22(2) min	IT (.11%)	²⁴⁰Np	1(+)
²⁴¹ Np	93	148	241.05825(8)	13.9(2) min	β⁻	²⁴¹Pu	(5/2+)
²⁴² Np	93	149	242.06164(21)	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:

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

CD: Cluster decay

EC: Electron capture

IT: Isomeric transition

SF: Spontaneous fission
Bold italics symbol as daughter – Daughter product is nearly stable.
( ) spin value – Indicates spin with weak assignment arguments.
# – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
Fissile nuclide
Most common nuclide
Produced by neutron capture in uranium ore
Intermediate decay product of ²⁴⁴Pu

Actinides vs fission products

Actinides and fission products by half-life
Actinides[12] by decay chain				Half-life range (a)	Fission products of ²³⁵U by yield[13]
4n	4n + 1	4n + 2	4n + 3	Half-life range (a)	4.5–7%	0.04–1.25%	<0.001%
²²⁸Ra^№				4–6 a		¹⁵⁵Eu^þ
²⁴⁴Cm^ƒ	²⁴¹Pu^ƒ	²⁵⁰Cf	²²⁷Ac^№	10–29 a	⁹⁰Sr	⁸⁵Kr	^113mCd^þ
²³²U^ƒ		²³⁸Pu^ƒ	²⁴³Cm^ƒ	29–97 a	¹³⁷Cs	¹⁵¹Sm^þ	^121mSn
²⁴⁸Bk[14]	²⁴⁹Cf^ƒ	^242mAm^ƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	²⁴¹Am^ƒ		²⁵¹Cf^ƒ[15]	430–900 a
		²²⁶Ra^№	²⁴⁷Bk	1.3–1.6 ka
²⁴⁰Pu	²²⁹Th	²⁴⁶Cm^ƒ	²⁴³Am^ƒ	4.7–7.4 ka
	²⁴⁵Cm^ƒ	²⁵⁰Cm		8.3–8.5 ka
			²³⁹Pu^ƒ	24.1 ka
		²³⁰Th^№	²³¹Pa^№	32–76 ka
²³⁶Np^ƒ	²³³U^ƒ	²³⁴U^№		150–250 ka	⁹⁹Tc^₡	¹²⁶Sn
²⁴⁸Cm		²⁴²Pu		327–375 ka		⁷⁹Se^₡
				1.53 Ma	⁹³Zr
	²³⁷Np^ƒ			2.1–6.5 Ma	¹³⁵Cs^₡	¹⁰⁷Pd
²³⁶U			²⁴⁷Cm^ƒ	15–24 Ma		¹²⁹I^₡
²⁴⁴Pu				80 Ma	... nor beyond 15.7 Ma[16]
²³²Th^№		²³⁸U^№	²³⁵U^ƒ№	0.7–14.1 Ga	... nor beyond 15.7 Ma[16]
₡, 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 with a critical mass of 6.79 kg (15.0 lb).[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 as a nuclear fuel in weapons or reactors. 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

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.

References

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.
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.
[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]
Bounding Analysis of Effects of Fractionation of Radionuclides in Fallout on Estimation of Doses to Atomic Veterans DTRA-TR-07-5. 2007
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.
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. doi:10.1016/j.physletb.2017.12.017.
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. doi:10.1103/PhysRevLett.122.192503. PMID 31144958. S2CID 169038981.
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. doi:10.1103/PhysRevLett.125.032502. PMID 32745401. S2CID 220965400.
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.
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.
Asai, M.; Suekawa, Y.; Higashi, M.; et al. Discovery of 234 Np isomer and its decay properties (PDF) (Report) (in Japanese).
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.
Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
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]."
This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
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.
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.
Analysis of the Reuse of Uranium Recovered from the Reprocessing of Commercial LWR Spent Fuel, United States Department of Energy, Oak Ridge National Laboratory.
- - Jukka Lehto; Xiaolin Hou (2011). "15.15: Neptunium". Chemistry and Analysis of Radionuclides (1st ed.). John Wiley & Sons. 231. ISBN 978-3527633029.
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.
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 15 December 2012. Retrieved 7 November 2013.
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.

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
Isotopic compositions and standard atomic masses from:
- de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
- Wieser, Michael E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051.

"News & Notices: Standard Atomic Weights Revised". International Union of Pure and Applied Chemistry. 19 October 2005.
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.

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[5] Np – Excited nuclear isomer.

[7] ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.

[TMS-8] # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).

[9] Modes of decay:

CD: Cluster decay

EC: Electron capture

IT: Isomeric transition

SF: Spontaneous fission

[10] Bold italics symbol as daughter – Daughter product is nearly stable.

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

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

[FN-19] Fissile nuclide

[20] Most common nuclide

[uore-21] Produced by neutron capture in uranium ore

[22] Intermediate decay product of ²⁴⁴Pu

[NUBASE2020-1] 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.

[4n1-2] 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] [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]

[4] Bounding Analysis of Effects of Fractionation of Radionuclides in Fallout on Estimation of Doses to Atomic Veterans DTRA-TR-07-5. 2007

[6] 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.

[13] 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. doi:10.1016/j.physletb.2017.12.017.

[14] 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. doi:10.1103/PhysRevLett.122.192503. PMID 31144958. S2CID 169038981.

[15] 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. doi:10.1103/PhysRevLett.125.032502. PMID 32745401. S2CID 220965400.

[16] 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.

[17] 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.

[234m-18] Asai, M.; Suekawa, Y.; Higashi, M.; et al. Discovery of 234 Np isomer and its decay properties (PDF) (Report) (in Japanese).

[23] 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.

[24] Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.

[25] 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]."

[26] This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".

[27] 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.

[europa-28] 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.

[ORNL-29] Analysis of the Reuse of Uranium Recovered from the Reprocessing of Commercial LWR Spent Fuel, United States Department of Energy, Oak Ridge National Laboratory.

[Jukka-30] Jukka Lehto; Xiaolin Hou (2011). "15.15: Neptunium". Chemistry and Analysis of Radionuclides (1st ed.). John Wiley & Sons. 231. ISBN 978-3527633029.

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

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

[236Np-31] 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.

[critical-32] 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 15 December 2012. Retrieved 7 November 2013.

[33] 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.