Isotopes of neptunium

Actinides and fission products by half-life
Actinides[1] by decay chain				Half-life range (y)	Fission products of ^235U by yield[2]
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
^228Ra^№				4–6	†		^155Eu^þ
^244Cm	^241Pu^ƒ	^250Cf	^227Ac^№	10–29		^90Sr	^85Kr	^113mCd^þ
^232U^ƒ		^238Pu^№	^243Cm^ƒ	29–97		^137Cs	^151Sm^þ	^121mSn
^248Bk[3]	^249Cf^ƒ	^242mAm^ƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	^241Am^ƒ		^251Cf^ƒ[4]	430–900
		^226Ra^№	^247Bk	1.3 k – 1.6 k
^240Pu	^229Th^№	^246Cm	^243Am^ƒ	4.7 k – 7.4 k
	^245Cm^ƒ	^250Cm		8.3 k – 8.5 k
			^239Pu^ƒ№	24.1 k
		^230Th^№	^231Pa^№	32 k – 76 k
^236Np^ƒ	^233U^ƒ№	^234U^№		150 k – 250 k	‡	^99Tc^₡	^126Sn
^248Cm		^242Pu		327 k – 375 k			^79Se^₡
				1.53 M		^93Zr
	^237Np^№			2.1 M – 6.5 M		^135Cs^₡	^107Pd
^236U^№			^247Cm^ƒ	15 M – 24 M			^129I^₡
^244Pu^№				80 M	... nor beyond 15.7 M years[5]
^232Th^№		^238U^№	^235U^ƒ№	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 m  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

Neptunium (Np) is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized 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 by uranium atoms.

Twenty neptunium radioisotopes have been characterized, with the most stable being 237Np with a half-life of 2.14 million years, 236Np with a half-life of 154,000 years, and 235Np 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 236mNp (t_1/2 22.5 hours).

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

Some notable isotopes

Neptunium-235

Neptunium-235 has 142 neutrons and a half-life of 400 days. This isotope of Neptunium either decays by:

• Emitting an alpha particle: 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 particular isotope of neptunium has a weight of 235.0440633 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.
• 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.^[6]

236Np is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237Np,^[7] however it is nearly impossible to separate in any significant quantities from its parent 237Np.^[8] 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

Neptunium-237 decay scheme (simplified)

237Np 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, 237Np 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.^[9] 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.).

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

237Np 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 237Np can capture a neutron and become 238Pu, this product being useful as an thermal energy source for the production of electricity in deep space probes and, of recent note, the Mars Science Laboratory (Curiosity rover). These applications are economically practical where photovoltaic power sources are weak or inconsistent.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)[10][n 1]	daughter isotope(s)	nuclear spin
	excitation energy
225Np	93	132	225.03391(8)	3# ms [>2 µs]	α	^221Pa	9/2−#
226Np	93	133	226.03515(10)#	35(10) ms	α	^222Pa
227Np	93	134	227.03496(8)	510(60) ms	α (99.95%)	^223Pa	5/2−#
					β^+ (.05%)	^227U
228Np	93	135	228.03618(21)#	61.4(14) s	β^+ (59%)	^228U
					α (41%)	^224Pa
					β^+, SF (.012%)	(various)
229Np	93	136	229.03626(9)	4.0(2) min	α (51%)	^225Pa	5/2+#
					β^+ (49%)	^229U
230Np	93	137	230.03783(6)	4.6(3) min	β^+ (97%)	^230U
					α (3%)	^226Pa
231Np	93	138	231.03825(5)	48.8(2) min	β^+ (98%)	^231U	(5/2)(+#)
					α (2%)	^227Pa
232Np	93	139	232.04011(11)#	14.7(3) min	β^+ (99.99%)	^232U	(4+)
					α (.003%)	^228Pa
233Np	93	140	233.04074(5)	36.2(1) min	β^+ (99.99%)	^233U	(5/2+)
					α (.001%)	^229Pa
234Np	93	141	234.042895(9)	4.4(1) d	β^+	^234U	(0+)
235Np	93	142	235.0440633(21)	396.1(12) d	EC	^235U	5/2+
					α (.0026%)	^231Pa
236Np[n 2]	93	143	236.04657(5)	1.54(6)×10^5 y	EC (87.3%)	^236U	(6−)
					β^− (12.5%)	^236Pu
					α (.16%)	^232Pa
236mNp	60(50) keV			22.5(4) h	EC (52%)	^236U	1
					β^− (48%)	^236Pu
237Np[n 2][n 3]	93	144	237.0481734(20)	2.144(7)×10^6 y	α	^233Pa	5/2+
					SF (2×10^−10%)	(various)
					CD (4×10^−12%)	^207Tl ^30Mg
238Np	93	145	238.0509464(20)	2.117(2) d	β^−	^238Pu	2+
238mNp	2300(200)# keV			112(39) ns
239Np	93	146	239.0529390(22)	2.356(3) d	β^−	^239Pu	5/2+
240Np	93	147	240.056162(16)	61.9(2) min	β^−	^240Pu	(5+)
240mNp	20(15) keV			7.22(2) min	β^− (99.89%)	^240Pu	1(+)
					IT (.11%)	^240Np
241Np	93	148	241.05825(8)	13.9(2) min	β^−	^241Pu	(5/2+)
242Np	93	149	242.06164(21)	2.2(2) min	β^−	^242Pu	(1+)
242mNp	0(50)# keV			5.5(1) min			6+#
243Np	93	150	243.06428(3)#	1.85(15) min	β^−	^243Pu	(5/2−)
244Np	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. Fissile nuclide
3. Most common nuclide

Notes

• 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.

References

1. 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 isotopes have half-lives of at least four years (the longest-lived isotope 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.
2. Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
3. 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. 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."
4. This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
5. Excluding those "classically stable" isotopes 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.
6. Final Report, Evaluation of nuclear criticality safety data and limits for actinides in transport, Republic of France, Institut de Radioprotection et de Sûreté Nucléaire, Département de Prévention et d'étude des Accidents.
7. Analysis of the Reuse of Uranium Recovered from the Reprocessing of Commercial LWR Spent Fuel, United States Department of Energy, Oak Ridge National Laboratory.
8. • • Jukka Lehto; Xiaolin Hou (2011). "15.15: Neptunium". Chemistry and Analysis of Radionuclides (1st ed.). John Wiley & Sons. 231. ISBN 3527633022.
9. P. Weiss (26 October 2002). "Neptunium Nukes? Little-studied metal goes critical". Science News 162 (17): 259. Archived from the original on 2012-12-15. Retrieved 7 November 2013.
10. "Universal Nuclide Chart". nucleonica. (registration required (help)).

• Isotope masses from:
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). 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:
  • J. R. de Laeter; J. K. Böhlke; P. De Bièvre; H. Hidaka; H. S. Peiser; K. J. R. Rosman; P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry 75 (6): 683–800. doi:10.1351/pac200375060683.
  • M. E. Wieser (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. See editing notes on this article's talk page.
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005.
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9.