Isotopes of neptunium

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.

Neptunium-237

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

²³⁷Np was recently shown to be capable of sustaining a chain reaction with fast neutrons, as in a nuclear weapon.^[1]. However, it has a low probability of fission on bombardment with thermal neutrons.

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

²³⁷Np is projected to be one of the most mobile nuclides at the Yucca Mountain nuclear waste repository.

Table

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 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.