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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.
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 (t½ 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
- 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.
- 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
236Np is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237Np, however it is nearly impossible to separate in any significant quantities from its parent 237Np. 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.
|244Cm||241Puƒ||250Cf||227Ac№||10–22 y||medium||m is
|249Cfƒ||242mAmƒ||251Cfƒ||140 y –
No fission products
|248Cm||4n+1||234U№||211–348 ky||99Tc||₡ can capture||126Sn||79Se|
|232Th№||238U№||235Uƒ№||0.7–14 Gy||fission product yield|
237Np 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 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.
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.
isotopic mass (u)
|225Np||93||132||225.03391(8)||3# ms [>2 µs]||α||221Pa||9/2-#|
|227Np||93||134||227.03496(8)||510(60) ms||α (99.95%)||223Pa||5/2-#|
|228Np||93||135||228.03618(21)#||61.4(14) s||β+ (59%)||228U|
|β+, SF (.012%)||(various)|
|229Np||93||136||229.03626(9)||4.0(2) min||α (51%)||225Pa||5/2+#|
|230Np||93||137||230.03783(6)||4.6(3) min||β+ (97%)||230U|
|231Np||93||138||231.03825(5)||48.8(2) min||β+ (98%)||231U||(5/2)(+#)|
|232Np||93||139||232.04011(11)#||14.7(3) min||β+ (99.99%)||232U||(4+)|
|233Np||93||140||233.04074(5)||36.2(1) min||β+ (99.99%)||233U||(5/2+)|
|236Np||93||143||236.04657(5)||1.54(6)×105 a||EC (87.3%)||236U||(6-)|
|236mNp||60(50) keV||22.5(4) h||EC (52%)||236U||1|
|237Np[n 2][n 3]||93||144||237.0481734(20)||2.144(7)×106 a||α||233Pa||5/2+|
|238mNp||2300(200)# keV||112(39) ns|
|240mNp||20(15) keV||7.22(2) min||β- (99.89%)||240Pu||1(+)|
|242mNp||0(50)# keV||5.5(1) min||6+#|
CD: Cluster decay
EC: Electron capture
IT: Isomeric transition
SF: Spontaneous fission
- Fissile nuclide
- Most common nuclide
- 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.
- 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.
- 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 and Xiaolin Hou (2011). "15.15: Neptunium". Chemistry and Analysis of Radionuclides (1st ed.). John Wiley & Sons. 231. ISBN 3527633022 [Amazon-US | Amazon-UK].
- Note: This is the heaviest isotope with a half-life of at least ten years before the "Sea of Instability".
- Note: Radium (element 88) is actually a sub-actinide, but 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 ten 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 1600 years, thus merits inclusion here.
- Note: specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
- 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 15 December 15012.
- Isotope masses from:
- 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.
- 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 .
- 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.
- 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 [Amazon-US | Amazon-UK].
|Isotopes of uranium||Isotopes of neptunium||Isotopes of plutonium|
|Table of nuclides|