Isotopes of neptunium

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 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_½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (225
Np) to 244.068 u (244
Np). 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.

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 - Here, the decay energy is 5.2 MeV and the decay product is Protactinium-231.
• Electron capture - Here, 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 grams/mole.

Neptunium-236

Neptunium-236 has 143 neutrons and a half-life of 154,000 years. It can decay by the following methods -

• Electron capture - here, the decay energy is 0.95 MeV and the decay product is Uranium-236.
• Beta emission - Here, the decay energy is 0.94 MeV and the decay product is Plutonium-236.
• Alpha emission - Here, the decay energy is 5.024 MeV and the decay product is Protactinium-232

This particular isotope of neptunium has a mass of 236.04657 grams/mole. It is a fissile material with a critical mass of 6.79 kg.

possible parent nuclides: alpha from Am-240

Neptunium-237

Neptunium-237 decay scheme (simplified)

Actinides				Half-life	Fission products
244Cm	241Pu f	250Cf	243Cmf	10–30 y	137Cs	90Sr	85Kr
232U  f		238Pu	f is for fissile	69–90 y			151Sm nc➔
4n	249Cf  f	242Amf		141–351	No fission product has half-life 102 to 2×105 years
	241Am		251Cf  f	431–898
240Pu	229Th	246Cm	243Am	5–7 ky
4n	245Cmf	250Cm	239Pu f	8–24 ky
	233U    f	230Th	231Pa	32–160
	4n+1	234U	4n+3	211–290	99Tc		126Sn	79Se
248Cm		242Pu		340–373	Long-lived fission products
	237Np	4n+2		1–2 My	93Zr	135Cs nc➔
236U	4n+1		247Cmf	6–23 My		107Pd	129I
244Pu				80 My	>7%	>5%	>1%	>.1%
232Th		238U	235U    f	0.7–12 Gy	fission product yield

237
Np decays via the neptunium series to thallium, unlike most other actinides which decay to isotopes of lead.

237
Np was recently shown to be capable of sustaining a chain reaction with fast neutrons, as in a nuclear weapon. However, it has a low probability of fission on bombardment with thermal neutrons, which makes it unsuitable as a fuel for nuclear power plants.

237
Np is the only neptunium isotope produced in significant quantity in the nuclear fuel cycle, both by successive neutron capture on 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 projected to be one of the most mobile nuclides at the Yucca Mountain nuclear waste repository.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)	daughter isotope(s)	nuclear spin
	excitation energy
225 Np	93	132	225.03391(8)	3# ms [>2 µs]	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	93	143	236.04657(5)	1.54(6)×105 a	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	93	144	237.0481734(20)	2.144(7)×106 a	α	233Pa	5/2+
					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+
240 Np	93	147	240.056162(16)	61.9(2) min	β-	240Pu	(5+)
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-)

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

• Isotope masses from:
  • G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (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. http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf. 
• 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 and 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. http://www.iupac.org/publications/pac/75/6/0683/pdf/. 
  • 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. http://iupac.org/publications/pac/78/11/2051/pdf/. 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 and O. Bersillon (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. http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf. 
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/. 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 978-0849304859. 

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