# Isotopes of plutonium

Main isotopes of plutonium (94Pu)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
238Pu trace 87.74 y SF
α 234U
239Pu trace 2.41×104 y SF
α 235U
240Pu trace 6500 y SF
α 236U
241Pu syn 14 y β 241Am
SF
242Pu syn 3.73×105 y SF
α 238U
244Pu trace 8.08×107 y α 240U
SF

Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being 238Pu in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years, plutonium-242 with a half-life of 373,300 years, and plutonium-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; all have half-lives of less than one second.

The isotopes of plutonium range in atomic weight from 228.0387 u (228Pu) to 247.074 u (247Pu). The primary decay modes before the most stable isotope, 244Pu, are spontaneous fission and alpha emission; the primary mode after is beta emission. The primary decay products before 244Pu are isotopes of uranium and neptunium (not considering fission products), and the primary decay products after are isotopes of americium.

## List of isotopes

Nuclide
[n 1]
Z N Isotopic mass (Da)
[n 2][n 3]
Half-life
Decay
mode

[n 4]
Daughter
isotope

[n 5][n 6]
Spin and
parity
[n 7][n 8]
Isotopic
abundance
Excitation energy
228Pu 94 134 228.03874(3) 1.1(+20−5) s α (99.9%) 224U 0+
β+ (.1%) 228Np
229Pu 94 135 229.04015(6) 120(50) s α 225U 3/2+#
230Pu 94 136 230.039650(16) 1.70(17) min α 226U 0+
β+ (rare) 230Np
231Pu 94 137 231.041101(28) 8.6(5) min β+ 231Np 3/2+#
α (rare) 227U
232Pu 94 138 232.041187(19) 33.7(5) min EC (89%) 232Np 0+
α (11%) 228U
233Pu 94 139 233.04300(5) 20.9(4) min β+ (99.88%) 233Np 5/2+#
α (.12%) 229U
234Pu 94 140 234.043317(7) 8.8(1) h EC (94%) 234Np 0+
α (6%) 230U
235Pu 94 141 235.045286(22) 25.3(5) min β+ (99.99%) 235Np (5/2+)
α (.0027%) 231U
236Pu 94 142 236.0460580(24) 2.858(8) y α 232U 0+
SF (1.37×10−7%) (various)
CD (2×10−12%) 208Pb
28Mg
β+β+ (rare) 236U
237Pu 94 143 237.0484097(24) 45.2(1) d EC 237Np 7/2−
α (.0042%) 233U
237m1Pu 145.544(10)2 keV 180(20) ms IT 237Pu 1/2+
237m2Pu 2900(250) keV 1.1(1) μs
238Pu 94 144 238.0495599(20) 87.7(1) y α 234U 0+ Trace[n 9]
SF (1.9×10−7%) (various)
CD (1.4×10−14%) 206Hg
32Si
CD (6×10−15%) 180Yb
30Mg
28Mg
239Pu[n 10][n 11] 94 145 239.0521634(20) 2.411(3)×104 y α 235U 1/2+ Trace[n 12]
SF (3.1×10−10%) (various)
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) 6.561(7)×103 y α 236U 0+ Trace[n 13]
SF (5.7×10−6%) (various)
CD (1.3×10−13%) 206Hg
34Si
241Pu[n 10] 94 147 241.0568515(20) 14.290(6) y β (99.99%) 241Am 5/2+
α (.00245%) 237U
SF (2.4×10−14%) (various)
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)×105 y α 238U 0+
SF (5.5×10−4%) (various)
243Pu[n 10] 94 149 243.062003(3) 4.956(3) h β 243Am 7/2+
243mPu 383.6(4) keV 330(30) ns (1/2+)
244Pu 94 150 244.064204(5) 8.00(9)×107 y α (99.88%) 240U 0+ Trace[n 14]
SF (.123%) (various)
ββ (7.3×10−9%) 244Cm
245Pu 94 151 245.067747(15) 10.5(1) h β 245Am (9/2−)
246Pu 94 152 246.070205(16) 10.84(2) d β 246mAm 0+
247Pu 94 153 247.07407(32)# 2.27(23) d β 247Am 1/2+#
This table header & footer:
1. ^ mPu – Excited nuclear isomer.
2. ^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. ^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. ^ Modes of decay:
 CD: Cluster decay EC: Electron capture IT: Isomeric transition SF: Spontaneous fission
5. ^ Bold italics symbol as daughter – Daughter product is nearly stable.
6. ^ Bold symbol as daughter – Daughter product is stable.
7. ^ ( ) spin value – Indicates spin with weak assignment arguments.
8. ^ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
9. ^ Double beta decay product of 238U
10. ^ a b c fissile nuclide
11. ^ Most useful isotope for nuclear weapons
12. ^ Neutron capture product of 238U
13. ^ Intermediate decay product of 244Pu
14. ^ Interstellar, some may also be primordial but such claims are disputed

## Actinides vs fission products

Actinides[1] by decay chain Half-life
range (a)
Fission products of 235U by yield[2]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 a 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351 a

No fission products
have a half-life
in the range of
100 a–210 ka ...

241Amƒ 251Cfƒ[4] 430–900 a
226Ra 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th 231Pa 32–76 ka
236Npƒ 233Uƒ 234U 150–250 ka 99Tc 126Sn
248Cm 242Pu 327–375 ka 79Se
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs 107Pd
236U 247Cmƒ 15–24 Ma 129I
244Pu 80 Ma

... nor beyond 15.7 Ma[5]

232Th 238U 235Uƒ№ 0.7–14.1 Ga

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
metastable isomer
№  primarily a naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 a: Medium-lived fission product
‡  over 200 ka: Long-lived fission product

## Production and uses

A pellet of plutonium-238, glowing from its own heat, used for radioisotope thermoelectric generators.
Transmutation flow between 238Pu and 244Cm in LWR.[7]
Transmutation speed not shown and varies greatly by nuclide.
245Cm–248Cm are long-lived with negligible decay.

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

240Pu, 241Pu, and 242Pu are produced by further neutron capture. The odd-mass isotopes 239Pu and 241Pu 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 next heavier isotope. The even-mass isotopes are fertile material but 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 nearly all nuclear power plants today. In plutonium that has been used a second time in thermal reactors in MOX fuel, 240Pu may even be the most common isotope. All plutonium isotopes and other actinides, however, are fissionable with fast neutrons. 240Pu does have a moderate thermal neutron absorption cross section, so that 241Pu production in a thermal reactor becomes a significant fraction as large as 239Pu production.

241Pu has a half-life of 14 years, and has slightly higher thermal neutron cross sections than 239Pu for both fission and absorption. While nuclear fuel is being used in a reactor, a 241Pu nucleus is much more likely to fission or to capture a neutron than to decay. 241Pu accounts for a significant proportion of fissions in thermal reactor fuel that has been used for some time. However, in spent nuclear fuel that does not quickly undergo nuclear reprocessing but instead is cooled for years after use, much or most of the 241Pu will beta decay to americium-241, one of the minor actinides, a strong alpha emitter, and difficult to use in thermal reactors.

242Pu has a particularly low cross section for thermal neutron capture; and it takes three neutron absorptions to become another fissile isotope (either curium-245 or 241Pu) and fission. Even then, there is a chance either of those two fissile isotopes will fail to fission but instead absorb a fourth neutron, becoming curium-246 (on the way to even heavier actinides like californium, which is a neutron emitter by spontaneous fission and difficult to handle) or becoming 242Pu again; so the mean number of neutrons absorbed before fission is even higher than 3. Therefore, 242Pu is particularly unsuited to recycling in a thermal reactor and would be better used in a fast reactor where it can be fissioned directly. However, 242Pu's low cross section means that relatively little of it will be transmuted during one cycle in a thermal reactor. 242Pu's half-life is about 15 times as long as 239Pu's half-life; therefore, it is 1/15 as radioactive and not one of the larger contributors to nuclear waste radioactivity. 242Pu's gamma ray emissions are also weaker than those of the other isotopes.[8]

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

238Pu 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 238Pu relatively free of other plutonium isotopes for use in radioisotope thermoelectric generators), by the (n,2n) reaction of fast neutrons on 239Pu, or by alpha decay of curium-242, which is produced by neutron capture from 241Am. It has significant thermal neutron cross section for fission, but is more likely to capture a neutron and become 239Pu.

## Manufacture

### Plutonium-240, -241 and -242

The fission cross section for 239Pu is 747.9 barns for thermal neutrons, while the activation cross section is 270.7 barns (the ratio approximates to 11 fissions for every 4 neutron captures). The higher plutonium isotopes are created when the uranium fuel is used for a long time. For high burnup used fuel, the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel that is reprocessed to obtain weapons grade plutonium.

The formation of 240Pu, 241Pu, and 242Pu from 238U
Isotope Thermal neutron
cross section[9]
(barns)
Decay
Mode
Half-life
Capture Fission
238U 2.683 0.000 α 4.468 x 109 years
239U 20.57 14.11 β 23.45 minutes
239Np 77.03 β 2.356 days
239Pu 270.7 747.9 α 24,110 years
240Pu 287.5 0.064 α 6,561 years
241Pu 363.0 1012 β 14.325 years
242Pu 19.16 0.001 α 373,300 years

### Plutonium-239

Plutonium-239 is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are uranium-235 and uranium-233. Plutonium-239 is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons in a nuclear reactor. Uranium-238 is present in quantity in most reactor fuel; hence plutonium-239 is continuously made in these reactors. Since plutonium-239 can itself be split by neutrons to release energy, plutonium-239 provides a portion of the energy generation in a nuclear reactor.

A ring of weapons-grade electrorefined plutonium, with 99.96% purity. This 5.3 kg ring is enough plutonium for use in an efficient nuclear weapon. The ring shape is needed to depart from a spherical shape and avoid criticality.
The formation of 239Pu from 238U[10]
Element Isotope Thermal neutron capture
cross section (barn)
Thermal neutron fission
Cross section (barn)
decay mode Half-life
U 238 2.68 5·10−6 α 4.47 x 109 years
U 239 22 15 β 23 minutes
Np 239 30 1 β 2.36 days
Pu 239 271 750 α 24,110 years

### Plutonium-238

There are small amounts of 238Pu in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a 235U atom captures a neutron, it is converted to an excited state of 236U. Some of the excited 236U nuclei undergo fission, but some decay to the ground state of 236U by emitting gamma radiation. Further neutron capture creates 237U, which has a half-life of 7 days and thus quickly decays to 237Np. Since nearly all neptunium is produced in this way or consists of isotopes that decay quickly, one gets nearly pure 237Np by chemical separation of neptunium. After this chemical separation, 237Np is again irradiated by reactor neutrons to be converted to 238Np, which decays to 238Pu with a half-life of 2 days.

The formation of 238Pu from 235U
Element Isotope Thermal neutron
cross section
decay mode Half-life
U 235 99 α 703,800,000 years
U 236 5.3 α 23,420,000 years
U 237 β 6.75 days
Np 237 165 (capture) α 2,144,000 years
Np 238 β 2.11 days
Pu 238 α 87.7 years

## 240Pu as an obstacle to nuclear weapons

Plutonium-240 undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of 240Pu limits the plutonium's use in a nuclear bomb, because the neutron flux from spontaneous fission initiates the chain reaction prematurely, causing an early release of energy that physically disperses the core before full implosion is reached. This prevents most of the core from participation in the chain reaction and reduces the bomb's power.

Plutonium consisting of more than about 90% 239Pu is called weapons-grade plutonium; plutonium from spent nuclear fuel from commercial power reactors generally contains at least 20% 240Pu 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.

240Pu contamination is the reason plutonium weapons must use the implosion method. Theoretically, pure 239Pu could be used in a gun-type nuclear weapon, but achieving this level of purity is prohibitively difficult. 240Pu 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 same difficulties are currently a barrier to nuclear proliferation. Implosion devices are also inherently more efficient and less prone to accidental detonation than are gun-type weapons.

## References

1. ^ Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
2. ^ Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
3. ^ Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
"The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 [years]. No growth of Cf248 was detected, and a lower limit for the β half-life can be set at about 104 [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
4. ^ This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
5. ^ Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.
6. ^ Makhijani, Arjun; Seth, Anita (July 1997). "The Use of Weapons Plutonium as Reactor Fuel" (PDF). Energy and Security. Takoma Park, MD: Institute for Energy and Environmental Research. Retrieved 4 July 2016.
7. ^ Sasahara, Akihiro; Matsumura, Tetsuo; Nicolaou, Giorgos; Papaioannou, Dimitri (April 2004). "Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels". Journal of Nuclear Science and Technology. 41 (4): 448–456. doi:10.3327/jnst.41.448. Archived from the original on 2010-11-19.
8. ^
9. ^
10. ^ Miner 1968, p. 541