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 ²³⁸U with neutrons to produce ²³⁹U, which then underwent beta decay to ²³⁹Np.

Main isotopes of neptunium (₉₃Np)

Iso­tope Decay abun­dance half-life (t1/2) mode pro­duct ^235Np syn 396.1 d α ^231Pa ε ^235U ^236Np syn 1.54×10^5 y ε ^236U β^− ^236Pu α ^232Pa ^237Np trace 2.144×10^6 y α ^233Pa ^239Np trace 2.356 d β^− ^239Pu

Trace quantities are found in nature from neutron capture reactions by uranium atoms, a fact not discovered until 1951.[1]

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 4 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."[2][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]	93	126	219.03162(9)	0.15(+0.72-0.07) ms	α	^215Pa	(9/2−)
^220 Np [6]	93	127	220.03254(21)#	25(+14-7) µs	α	^216Pa	1-#
^222 Np [7]	93	129		380(+260-110) ns	α	^218Pa	1-#
^223 Np [8]	93	130	223.03285(21)#	2.15(+100-52) µs	α	^219Pa	9/2−
^224 Np [9]	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+)
^235 Np	93	142	235.0440633(21)	396.1(12) d	EC	^235U	5/2+
					α (.0026%)	^231Pa
^236 Np [n 8]	93	143	236.04657(5)	1.54(6)×10^5 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 8][n 9]	93	144	237.0481734(20)	2.144(7)×10^6 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(+)
					IT (.11%)	^240Np
^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−)

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. Fissile nuclide
9. Most common nuclide
10. Produced by neutron capture in uranium ore
11. Intermediate decay product of ²⁴⁴Pu

Actinides vs fission products

Actinides and fission products by half-life
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	†		^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[12]	^249Cf^ƒ	^242mAm^ƒ		141–351 a	No fission products have a half-life in the range of 100–210 ka ...
	^241Am^ƒ		^251Cf^ƒ[13]	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[14]
^232Th^№		^238U^№	^235U^ƒ№	0.7–14.1 Ga
Legend for superscript symbols ₡  has thermal neutron capture cross section in the range of 8–50 barns ƒ  fissile m  metastable isomer №  primarily a naturally occurring radioactive material (NORM) þ  neutron poison (thermal neutron capture cross section greater than 3k barns) †  range 4–97 a: Medium-lived fission product ‡  over 200 ka: Long-lived fission product

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).[15]

²³⁶
Np
is produced in small quantities via the (n,2n) and (γ,n) capture reactions of ²³⁷
Np
,[16] however, it is nearly impossible to separate in any significant quantities from its parent ²³⁷
Np
.[17] 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.[18] 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.[18]

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.[19] 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).

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
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
Np
can capture a neutron, undergo beta decay, and become ²³⁸
Pu
, 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.[20]

References

1. 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.
2. [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]
3. Bounding Analysis of Effects of Fractionation of Radionuclides in Fallout on Estimation of Doses to Atomic Veterans DTRA-TR-07-5. 2007
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. doi:10.1016/j.physletb.2017.12.017.
6. 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.
7. 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: 032502. doi:10.1103/PhysRevLett.125.032502.
8. 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.
9. 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.
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 U-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 nearly eight quadrillion years.
15. 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.
16. Analysis of the Reuse of Uranium Recovered from the Reprocessing of Commercial LWR Spent Fuel, United States Department of Energy, Oak Ridge National Laboratory.
17. • • Jukka Lehto; Xiaolin Hou (2011). "15.15: Neptunium". Chemistry and Analysis of Radionuclides (1st ed.). John Wiley & Sons. 231. ISBN 3527633022.
18. 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.
19. P. Weiss (26 October 2002). "Neptunium Nukes? Little-studied metal goes critical". Science News. 162 (17): 259. doi:10.2307/4014034. Archived from the original on 15 December 2012. Retrieved 7 November 2013.
20. Witze, Alexandra (2014-11-27). "Nuclear power: Desperately seeking plutonium". Nature. 515 (7528): 484–486. Bibcode:2014Natur.515..484W. doi:10.1038/515484a.

• 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. Lay summary.
• 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.