Isotopes of plutonium

Main isotopes of Chemistry:plutonium (94Pu) | Iso­tope | Decay

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| 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  | –
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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.[note 1] Like all artificial elements, it has no stable isotopes. It was synthesized before being found in nature, with the first isotope synthesized being 238Pu in 1940. Twenty-two plutonium radioisotopes have been characterized. The most stable are 244Pu with a half-life of 81.3 million years, 242Pu with a half-life of 375,000 years, 239Pu with a half-life of 24,110 years, and 240Pu with a half-life of 6,561 years. This element also has eight meta states; all have half-lives of less than one second.

The known isotopes of plutonium range from 226Pu to 247Pu. The primary decay modes before the most stable isotope, 244Pu, are spontaneous fission and alpha decay; 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.

## Contents

* 1 List of isotopes
* 2 Actinides vs fission products
* 3 Notable isotopes
* 4 Production and uses
* 5 Manufacture
* 5.1 Plutonium-240, -241 and -242
* 5.2 Plutonium-239
* 5.3 Plutonium-238
* 6 Plutonium-240 as an obstacle to nuclear weapons
* 7 Notes
* 8 References
* 9 Sources

## List of isotopes

Nuclide
[n 1] | Z | N | Isotopic mass (u)
[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

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Excitation energy
226Pu[2] | 94  | 132  | 226.03825(22)#  | (2024)[n 9] | ≥1 ms  | α  | 222U  | 0+  |
227Pu[3] | 94  | 133  | 227.03947(11)#  | 2024 | 0.78+0.39
−0.19 s | α  | 223U  | 5/2+#  |
228Pu  | 94  | 134  | 228.038763(25)  | 1994 | 2.1(13) s  | α  | 224U  | 0+  |
229Pu  | 94  | 135  | 229.040145(65)  | 1994 | 91(26) s  | α (~50%)  | 225U  | 3/2+#  |
β+ (~50%)  | 229Np
SF (<7%)  | (various)
230Pu  | 94  | 136  | 230.039648(16)  | 1990 | 105(10) s  | α (>73%)[4] | 226U  | 0+  |
β+ (<27%)  | 230Np
231Pu  | 94  | 137  | 231.041126(24)  | 1999 | 8.6(5) min  | β+ (87%)  | 231Np  | (3/2+)  |
α (13%)  | 227U
232Pu  | 94  | 138  | 232.041182(18)  | 1973 | 33.7(5) min  | EC (77%) [5] | 232Np  | 0+  |
α (23%)  | 228U
233Pu  | 94  | 139  | 233.042997(58)  | 1957 | 20.9(4) min  | β+ (99.88%)  | 233Np  | 5/2+#  |
α (0.12%)  | 229U
234Pu  | 94  | 140  | 234.0433175(73)  | 1949 | 8.8(1) h  | EC (94%)  | 234Np  | 0+  |
α (6%)  | 230U
235Pu  | 94  | 141  | 235.045285(22)  | 1957 | 25.3(5) min  | β+ | 235Np  | (5/2+)  |
α (0.0028%)  | 231U
236Pu  | 94  | 142  | 236.0460567(19)  | 1949 | 2.858(8) y  | α[n 10] | 232U  | 0+  |
SF (1.9 × 10−7%)  | (various)
CD (2 × 10−12%)  | 208Pb
28Mg
236mPu  | 1185.45(15) keV  | 2005 | 1.2(3) μs  | IT  | 236Pu  | 5−  |
237Pu  | 94  | 143  | 237.0484079(18)  | 1949 | 45.64(4) d  | EC  | 237Np  | 7/2−  |
α (0.0042%)  | 233U
237m1Pu  | 145.543(8) keV  | 1957 | 180(20) ms  | IT  | 237Pu  | 1/2+  |
237m2Pu  | 2600(20) keV  | 1969 | 103(7) ns  | SF  | (various)  |  |
237m3Pu  | 2900(250) keV  | 1970 | 1.1(1) μs  | SF  | (various)  |  |
238Pu | 94  | 144  | 238.0495582(12)  | 1949 | 87.7(1) y  | α  | 234U  | 0+  | Trace[n 11]
SF (1.9 × 10−7%)  | (various)
CD (1.4 × 10−14%)  | 206Hg
32Si
CD (6 × 10−15%)  | 210,208Pb
28,30Mg
239Pu[n 12][n 13] | 94  | 145  | 239.0521616(12)  | 1946 | 2.411(3)×104 y  | α  | 235U | 1/2+  | Trace[n 14]
SF (3.1 × 10−10%)  | (various)
239m1Pu  | 391.584(3) keV  | 1955 | 193(4) ns  | IT  | 239Pu  | 7/2−  |
239m2Pu  | 3100(200) keV  | 1970 | 7.5(10) μs  | SF  | (various)  | (5/2+)  |
240Pu | 94  | 146  | 240.0538117(12)  | 1949 | 6.561(7)×103 y  | α  | 236U  | 0+  | Trace[n 15]
SF (5.796 × 10−6%)  | (various)
CD (<1.3 × 10−11%)  | 206Hg
34Si
240mPu  | 1308.74(5) keV  | 1967 | 165(10) ns  | IT  | 240Pu  | 5−  |
241Pu[n 12] | 94  | 147  | 241.0568497(12)  | 1949 | 14.329(29) y[n 16] | β− | 241Am  | 5/2+  |
α (0.00245%)  | 237U
SF (<2.4 × 10−14%)  | (various)
241m1Pu  | 161.6853(9) keV  | 1975 | 0.88(5) μs  | IT  | 241Pu  | 1/2+  |
241m2Pu  | 2200(200) keV  | 1970 | 20.5(22) μs  | SF  | (various)  |  |
242Pu | 94  | 148  | 242.0587410(13)  | 1950 | 3.75(2)×105 y  | α  | 238U | 0+  |
SF (5.510 × 10−4%)  | (various)
243Pu[n 12] | 94  | 149  | 243.0620021(27)  | 1951 | 4.9553(25) h  | β− | 243Am  | 7/2+  |
243mPu  | 383.64(25) keV  | 1975 | 330(30) ns  | IT  | 243Pu  | (1/2+)  |
244Pu | 94  | 150  | 244.0642044(25)  | 1954 | 8.13(3)×107 y  | α (99.88%)  | 240U  | 0+  | Trace[n 17]
SF (0.123%)  | (various)
β−β− (<7.3 × 10−9%)  | 244Cm
244mPu  | 1216.0(5) keV  | 2016 | 1.75(12) s  | IT  | 244Pu  | 8−  |
245Pu  | 94  | 151  | 245.067825(15)  | 1955 | 10.5(1) h  | β− | 245Am  | (9/2−)  |
245m1Pu  | 264.5(3) keV  | 2007 | 330(20) ns  | IT  | 245Pu  | (5/2+)  |
245m2Pu  | 2000(400) keV  | (1980)[n 18] | 90(30) ns  | SF  | (various)  |  |
246Pu  | 94  | 152  | 246.070204(16)  | 1955 | 10.84(2) d  | β− | 246Am  | 0+  |
247Pu  | 94  | 153  | 247.07430(22)#  | 1983 | 2.27(23) d  | β− | 247Am  | 1/2+#  |

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

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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. ↑ Only published as a preprint and not a refereed journal
10. ↑ Theorized to also undergo β+β+ decay to 236U
11. ↑ Double beta decay product of 238U
12. ↑ 12.0 12.1 12.2 fissile nuclide
13. ↑ Most useful isotope for nuclear weapons
14. ↑ Neutron capture product of 238U
15. ↑ Intermediate decay product of 244Pu
16. ↑ Can undergo Bound-state β− decay with a half-life of 4.2 days when fully ionized[6]
17. ↑ Interstellar, some may also be primordial but such claims are disputed
18. ↑ No evidence of this isomeric state in the refereed literature

## Actinides vs fission products

Actinides and fission products by half-life v * d * e

Actinides[7] by decay chain | Half-life
range (y)  | Fission products of 235U by yield<ref>Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.</ref>
4n | 4n+1 | 4n+2 | 4n+3
| 4.5–7%  | 0.04–1.25%  | <0.001%
228Ra№ |  |  |  | 4–6  | †  |  | 155Euþ |
244Cmƒ | 241Puƒ | 250Cf  | 227Ac№ | 10–29  | 90Sr  | 85Kr  | 113mCdþ
232Uƒ |  | 238Puƒ | 243Cmƒ | 29–97  | 137Cs  | 151Smþ | 121mSn
248Bk[8] | 249Cfƒ | 242mAmƒ |  | 141–351  |

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

| 241Amƒ |  | 251Cfƒ[9] | 430–900
|  | 226Ra№ | 247Bk  | 1.3 k – 1.6 k
240Pu  | 229Th  | 246Cmƒ | 243Amƒ | 4.7 k – 7.4 k
| 245Cmƒ | 250Cm  |  | 8.3 k – 8.5 k
|  |  | 239Puƒ | 24.1 k
|  | 230Th№ | 231Pa№ | 32 k – 76 k
236Npƒ | 233Uƒ | 234U№ |  | 150 k – 250 k  | ‡  | 99Tc₡ | 126Sn
248Cm  |  | 242Pu  |  | 327 k – 375 k  |  | 79Se₡
|  |  |  | 1.53 M  | 93Zr
| 237Npƒ |  |  | 2.1 M – 6.5 M  | 135Cs₡ | 107Pd
236U  |  |  | 247Cmƒ | 15 M – 24 M  |  | 129I₡
244Pu  |  |  |  | 80 M  |

... nor beyond 15.7 M years[10]

232Th№ |  | 238U№ | 235Uƒ№ | 0.7 G – 14.1 G

Legend for superscript symbols
₡ has thermal neutron capture cross section in the range of 8–50 barns
ƒ fissile
m metastable isomer
№ primarily a naturally occurring radioactive material (NORM)
þ neutron poison (thermal neutron capture cross section greater than 3k barns)
† range 4–97 y: Medium-lived fission product
‡ over 200,000 y: Long-lived fission product

## Notable isotopes

* Plutonium-238 has a half-life of 87.74 years[11] and emits alpha particles. Pure 238Pu for radioisotope thermoelectric generators that power some spacecraft is produced by neutron capture on neptunium-237 but plutonium from spent nuclear fuel can contain as much as a few percent 238Pu, originating from 237Np, alpha decay of 242Cm, or (n,2n) reactions.
* Plutonium-239 has half-life 24,100 years. 239Pu and 241Pu are fissile; meaning their nuclei can split by being bombarded by slow thermal neutrons, releasing energy, gamma radiation and more neutrons. It can therefore sustain a nuclear chain reaction, leading to applications in nuclear weapons and nuclear reactors. 239Pu is synthesized by irradiating uranium-238 with neutrons in a nuclear reactor, then recovered via nuclear reprocessing of the fuel. Further neutron capture produces successively heavier isotopes.
* Plutonium-240 has a high rate of spontaneous fission, raising the background neutron radiation of plutonium. Plutonium is graded by proportion of 240Pu: weapons grade (<7%), fuel grade (7–19%) and reactor grade (>19%). Lower grades are less suited for bombs and thermal reactors but can fuel fast reactors.
* Plutonium-241 is fissile, but beta decays with a half-life of 14 years to americium-241.
* Plutonium-242 is not fissile, nor very fertile (requiring 3 more neutron captures to become fissile); and has a low neutron capture cross section, and a longer half-life than any of the lighter isotopes.
* Plutonium-244 is the most stable isotope of plutonium, with a half-life of about 80 million years. It is not significantly produced in nuclear reactors because 243Pu has a short half-life, but some is produced in nuclear explosions. 244Pu has been found in interstellar space[12] and has the second longest half-life of any non-primordial radioisotope.

## Production and uses

A pellet of 238Pu, glowing from its own heat, used for radioisotope thermoelectric generators. Transmutation flow between 238Pu and 244Cm in LWR.[13]
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 but not fissile and also have a lower 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 portion 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.[14]

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 of 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[15]
(barns) | Decay
Mode | Half-life

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Capture | Fission
238U | 2.683 | 0.000 | α | 4.468 × 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

Main article: Physics:Plutonium-239

A 5.3 kg ring of weapons-grade electrorefined plutonium, 99.96% purity. This is enough plutonium for an efficient nuclear weapon. The ring shape is needed to depart from a spherical shape and avoid criticality.

239Pu 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. 239Pu is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons. Uranium-238 is present in quantity in most reactor fuel; hence 239Pu is continuously made in these reactors. Since 239Pu can itself be split by neutrons to release energy, 239Pu provides a portion of the energy generation in a nuclear reactor.

The formation of 239Pu from 238U[16] Element | Isotope | Thermal neutron capture
cross section (barn) | Thermal neutron fission
Cross section (barn) | decay mode | Half-life

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U | 238 | 2.68 | 5·10−6 | α | 4.47 × 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

Main article: Physics:Plutonium-238

There are small amounts of 238Pu in the plutonium from usual reactors. However, isotopic separation would be quite expensive compared to another method: when 235U 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, with a half-life of 7 days, decays to 237Np. Since nearly all neptunium is produced in this way or consists of isotopes that decay quickly, one gets nearly pure 237Np. After chemical separation of neptunium, 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

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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

## Plutonium-240 as an obstacle to nuclear weapons

240Pu undergoes spontaneous fission at a small but significant rate (5.8×10−6%).[17] The presence of 240Pu limits the plutonium's use in a nuclear bomb, because a neutron from spontaneous fission starts the chain reaction prematurely, causing an early release of energy that 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 yield.

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.

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

## Notes

1. ↑ Primordial plutonium-244 is predicted to occur in trace quantities in rare-earth minerals. However, as of 2021, measurements of various bastnäsite samples have placed an upper limit of its concentration on the order of 10−19 by mass.[1]

## References

* Isotope masses from:
* Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A 729: 3–128, doi:10.1016/j.nuclphysa.2003.11.001, Bibcode: 2003NuPhA.729....3A, https://hal.archives-ouvertes.fr/in2p3-00020241/document
* Half-life, spin, and isomer data selected from the following sources.
* Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A 729: 3–128, doi:10.1016/j.nuclphysa.2003.11.001, Bibcode: 2003NuPhA.729....3A, https://hal.archives-ouvertes.fr/in2p3-00020241/document
* National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/.
* Lide, David R., ed (2004). "11. Table of the Isotopes". CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.

1. ↑ Wu, Yang; Dai, Xiongxin; Christl, Marcus; Synal, Hans-Arno; Xing, Shan; Hou, Shaochun (17 June 2021). "Ultrasensitive Analytical Method for Direct Search of Primordial 244 Pu in Bastnaesite". ACS Earth and Space Chemistry 5 (6): 1316–1324. doi:10.1021/acsearthspacechem.0c00288. Bibcode: 2021ESC.....5.1316W.
2. ↑ "Свойства радиоактивного распада нового ядра 227Pu". Joint Institute for Nuclear Research. 2024. http://www1.jinr.ru/Preprints/2024/50(P7-2024-50).pdf.
3. ↑ Yang, H. B.; Gan, Z. G.; Zhang, Z. Y.; Huang, M. H.; Ma, L.; Yang, C. L.; Zhang, M. M.; Tian, Y. L. et al. (3 October 2024). "α decay of the new isotope Pu 227". Physical Review C 110 (4). doi:10.1103/PhysRevC.110.044302. Bibcode: 2024PhRvC.110d4302Y.
4. ↑ Wilson, G. L.; Takeyama, M.; Andreyev, A. N.; Andel, B.; Antalic, S.; Catford, W. N.; Ghys, L.; Haba, H. et al. (13 October 2017). "β -delayed fission of Am 230" (in en). Physical Review C 96 (4). doi:10.1103/PhysRevC.96.044315. ISSN 2469-9985.
5. ↑ Branching ratio from ENSDF at National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/.
6. ↑ Takahashi, K.; Boyd, R. N.; Mathews, G. J.; Yokoi, K. (1 October 1987). "Bound-state beta decay of highly ionized atoms". Physical Review C 36 (4): 1522–1528. doi:10.1103/PhysRevC.36.1522. Bibcode: 1987PhRvC..36.1522T. https://www.researchgate.net/publication/13335547.
7. ↑ 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.
8. ↑ 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. doi:10.1016/0029-5582(65)90719-4. Bibcode: 1965NucPh..71..299M.
"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 y. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
9. ↑ This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
10. ↑ 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.
11. ↑ Makhijani, Arjun; Seth, Anita (July 1997). "The Use of Weapons Plutonium as Reactor Fuel". Energy and Security (Takoma Park, MD: Institute for Energy and Environmental Research). http://ieer.org/wp/wp-content/uploads/1997/07/no-3.pdf. Retrieved 4 July 2016.
12. ↑ Wallner, A.; Faestermann, T.; Feige, J.; Feldstein, C.; Knie, K.; Korschinek, G.; Kutschera, W.; Ofan, A. et al. (2015). "Abundance of live 244Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis". Nature Communications 6. doi:10.1038/ncomms6956. ISSN 2041-1723. PMID 25601158. Bibcode: 2015NatCo...6.5956W.
13. ↑ 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.
14. ↑ "Plutonium Isotopic Results of Known Samples Using the Snap Gamma Spectroscopy Analysis Code and the Robwin Spectrum Fitting Routine". http://www.wmsym.org/archives/2001/21B/21B-18.pdf.
15. ↑ National Nuclear Data Center Interactive Chart of Nuclides
16. ↑ Miner 1968, p. 541
17. ↑ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties". Chinese Physics C 45 (3). doi:10.1088/1674-1137/abddae. https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf.

## Sources

* Magurno, B. A., ed (1981). Proceedings of the conference on nuclear data evaluation methods and procedures. BNL-NCS 51363.. II. Upton: Brookhaven National Laboratory. https://www-nds.iaea.org/publications/indc/indc_usa_0085_II.pdf. Retrieved August 6, 2014.
* Miner, William N.; Schonfeld, Fred W. (1968). "Plutonium". in Clifford A. Hampel. The Encyclopedia of the Chemical Elements. New York (NY): Reinhold Book Corporation. pp. 540–546. https://archive.org/details/encyclopediaofch00hamp.

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