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Isotopes of plutonium



Plutonium (Pu) has no stable isotopes. A standard atomic mass cannot be given.

Additional recommended knowledge

Contents

Older names

Older names for plutonium:

Decay modes

Twenty plutonium radioisotopes have been characterized. The most stable are Pu-244, with a half-life of 80.8 million years, Pu-242, with a half-life of 373,300 years, and Pu-239, with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states, though none are very stable (all have half-lives less than one second).

The isotopes of plutonium range in atomic weight from 228.0387 u (Pu-228) to 247.074 u (Pu-247). The primary decay modes before the most stable isotope, Pu-244, are spontaneous fission and alpha emission; the primary mode after is beta emission. The primary decay products before Pu-244 are uranium and neptunium isotopes (neglecting the wide range of daughter nuclei created by fission processes), and the primary products after are americium isotopes.

Production and uses

 

Pu-239, a fissile isotope which is the second most used nuclear fuel in nuclear reactors after U-235, and the most used fuel in the fission portion of nuclear weapons, is produced from U-238 by neutron capture followed by two beta decays.

Pu-240, Pu-241, Pu-242 are produced by further neutron capture. The odd-mass isotopes Pu-239 and Pu-241 have about a 3/4 chance of undergoing fission on capture of a thermal neutron and about a 1/4 chance of retaining the neutron and becoming the following isotope. The even-mass isotopes are not fissile and also have a lower overall probability (cross section) of neutron capture; therefore, they tend to accumulate in nuclear fuel used in a thermal reactor, the design of all nuclear power plants today. In plutonium that has been used a second time in thermal reactors in MOX fuel, Pu-240 may even be the most common isotope. All plutonium isotopes and other actinides, however, are fissionable with fast neutrons.

Pu-241 has a halflife of 14 years. While nuclear fuel is being used in a reactor, a Pu-241 nucleus is much more likely to fission or to capture a neutron than to decay. However, in spent nuclear fuel that is cooled for decades after use, much or most of the Pu-241 will beta decay to americium-241, one of the minor actinides and less fissionable.

Pu-243 has a halflife of only 5 hours, beta decaying to americium-243. Because Pu-243 has little opportunity to capture an additional neutron before decay, the nuclear fuel cycle does not produce long-lived Pu-244 in significant quantity.

Pu-238 is not normally produced in as large quantity by the nuclear fuel cycle, but some is produced from neptunium-237 by neutron capture (this reaction can also be used with purified neptunium to produce Pu-238 relatively free of other plutonium isotopes for use in radioisotope thermoelectric generators), by the (n,2n) reaction on Pu-239, or by alpha decay of curium-242 which is produced by neutron capture from Am-241.

Pu-240 as obstacle to nuclear weapons

Pu-240 undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of Pu-240 limits the plutonium's nuclear bomb potential because the neutron flux from spontaneous fission, initiates the chain reaction prematurely and reduces the bomb's power by exploding the core before full implosion is reached. Plutonium consisting of more than about 90% Pu-239 is called weapons-grade plutonium; plutonium from spent nuclear fuel from commercial power reactors generally contains at least 20% Pu-240 and is called reactor-grade plutonium. However, modern nuclear weapons use fusion boosting which mitigates the predetonation problem; if the pit can generate a nuclear weapon yield of even a fraction of a kiloton, which is enough to start deuterium-tritium fusion, the resulting burst of neutrons will fission enough plutonium to ensure a yield of tens of kilotons.

Pu-240 contamination is the reason plutonium weapons must use the implosion method. Theoretically, pure Pu-239 could be used in a gun-type nuclear weapon, but achieving this level of purity is prohibitively difficult. Pu-240 contamination has proven a mixed blessing to nuclear weapons design. While it created delays and headaches during the Manhattan Project because of the need to develop implosion technology, those very same difficulties are currently a barrier to nuclear proliferation. Implosion devices are also inherently more efficient and less prone toward accidental detonation than are gun-type weapons.

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)
excitation energy
228Pu 94 134 228.03874(3) 1.1(+20-5) s 0+
229Pu 94 135 229.04015(6) 120(50) s 3/2+#
230Pu 94 136 230.039650(16) 1.70(17) min 0+
231Pu 94 137 231.041101(28) 8.6(5) min 3/2+#
232Pu 94 138 232.041187(19) 33.7(5) min 0+
233Pu 94 139 233.04300(5) 20.9(4) min 5/2+#
234Pu 94 140 234.043317(7) 8.8(1) h 0+
235Pu 94 141 235.045286(22) 25.3(5) min (5/2+)
236Pu 94 142 236.0460580(24) 2.858(8) a 0+
237Pu 94 143 237.0484097(24) 45.2(1) d 7/2-
237m1Pu 145.544(10) keV 180(20) ms 1/2+
237m2Pu 2900(250) keV 1.1(1) µs
238Pu 94 144 238.0495599(20) 87.7(1) a 0+
239Pu 94 145 239.0521634(20) 24.11(3)E+3 a 1/2+
239m1Pu 391.584(3) keV 193(4) ns 7/2-
239m2Pu 3100(200) keV 7.5(10) µs (5/2+)
240Pu 94 146 240.0538135(20) 6561(7) a 0+
241Pu 94 147 241.0568515(20) 14.290(6) a 5/2+
241m1Pu 161.6(1) keV 0.88(5) µs 1/2+
241m2Pu 2200(200) keV 21(3) µs
242Pu 94 148 242.0587426(20) 3.75(2)E+5 a 0+
243Pu 94 149 243.062003(3) 4.956(3) h 7/2+
243mPu 383.6(4) keV 330(30) ns (1/2+)
244Pu 94 150 244.064204(5) 8.00(9)E+7 a 0+
245Pu 94 151 245.067747(15) 10.5(1) h (9/2-)
246Pu 94 152 246.070205(16) 10.84(2) d 0+
247Pu 94 153 247.07407(32)# 2.27(23) d 1/2+#

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.


Isotopes of neptunium Isotopes of plutonium Isotopes of americium
Index to isotope pages
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Isotopes_of_plutonium". A list of authors is available in Wikipedia.
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