Isotopes of neptunium

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Neptunium (Np) has no stable isotopes. A standard atomic mass cannot be given.

19 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 lifes that are less than 50 minutes. This element also has 4 meta states, with the most stable being ^236mNp (t_½ 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 element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 94 (plutonium) isotopes.

²³⁷Np is both fissionable and fissile^[1]. ²³⁷Np eventually decays to form bismuth, unlike most other common heavy nuclei which decay to make lead.

[edit] Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
nuclide symbol	excitation energy			half-life	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
²²⁵Np	93	132	225.03391(8)	3# ms [>2 µs]	9/2-#
²²⁶Np	93	133	226.03515(10)#	35(10) ms
²²⁷Np	93	134	227.03496(8)	510(60) ms	5/2-#
²²⁸Np	93	135	228.03618(21)#	61.4(14) s
²²⁹Np	93	136	229.03626(9)	4.0(2) min	5/2+#
²³⁰Np	93	137	230.03783(6)	4.6(3) min
²³¹Np	93	138	231.03825(5)	48.8(2) min	(5/2)(+#)
²³²Np	93	139	232.04011(11)#	14.7(3) min	(4+)
²³³Np	93	140	233.04074(5)	36.2(1) min	(5/2+)
²³⁴Np	93	141	234.042895(9)	4.4(1) d	(0+)
²³⁵Np	93	142	235.0440633(21)	396.1(12) d	5/2+
²³⁶Np	93	143	236.04657(5)	154(6)E+3 a	(6-)
^236mNp	60(50) keV			22.5(4) h	1
²³⁷Np	93	144	237.0481734(20)	2.144(7)E+6 a	5/2+
²³⁸Np	93	145	238.0509464(20)	2.117(2) d	2+
^238mNp	2300(200)# keV			112(39) ns
²³⁹Np	93	146	239.0529390(22)	2.356(3) d	5/2+
²⁴⁰Np	93	147	240.056162(16)	61.9(2) min	(5+)
^240mNp	20(15) keV			7.22(2) min	1(+)
²⁴¹Np	93	148	241.05825(8)	13.9(2) min	(5/2+)
²⁴²Np	93	149	242.06164(21)	2.2(2) min	(1+)
^242mNp	0(50)# keV			5.5(1) min	6+#
²⁴³Np	93	150	243.06428(3)#	1.85(15) min	(5/2-)
²⁴⁴Np	93	151	244.06785(32)#	2.29(16) min	(7-)

[edit] 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.

[edit] References

^ Weiss, P. (October 26, 2002). Little-studied metal goes critical - Neptunium Nukes?. Science News. Retrieved on 2006-09-29.

Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003).
Isotopic compositions and standard atomic masses from Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. Vol. 75, No. 6, pp. 683-800, (2003) and Atomic Weights Revised (2005).
Half-life, spin, and isomer data selected from these sources. Editing notes on this article's talk page.
- Audi, Bersillon, Blachot, Wapstra. The Nubase2003 evaluation of nuclear and decay properties, Nuc. Phys. A 729, pp. 3-128 (2003).
- National Nuclear Data Center, Brookhaven National Laboratory. Information extracted from the NuDat 2.1 database (retrieved Sept. 2005).
- David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition, online version. CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.