Isotopes of plutonium

Isotopes of plutonium (₉₄Pu)

Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 238Pu trace 87.7 y α 234U SF – 239Pu trace 2.411×104 y α 235U SF – 240Pu trace 6.561×103 y α 236U SF – 241Pu synth 14.329 y β− 241Am α 237U SF – 242Pu synth 3.75×105 y α 238U SF – 244Pu trace 8.00×107 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	Z	N	Isotopic mass (Da)	Half-life	Decay mode	Daughter isotope	Spin and parity	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
					SF (1.9×10−7%)	(various)
					CD (1.4×10−14%)	206Hg 32Si
					CD (6×10−15%)	180Yb 30Mg 28Mg
239Pu	94	145	239.0521634(20)	2.411(3)×104 y	α	235U	1/2+	Trace
					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
					SF (5.7×10−6%)	(various)
					CD (1.3×10−13%)	206Hg 34Si
241Pu	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	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
					SF (.123%)	(various)
					β−β− (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+#
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Actinides vs fission products

Actinides and fission products by half-life
Actinides by decay chain				Half-life range (a)	Fission products of 235U by yield
4n	4n + 1	4n + 2	4n + 3		4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6 a		155Euþ
248Bk				> 9 a
244Cmƒ	241Puƒ	250Cf	227Ac№	10–29 a	90Sr	85Kr	113mCdþ
232Uƒ		238Puƒ	243Cmƒ	29–97 a	137Cs	151Smþ	121mSn
	249Cfƒ	242mAmƒ		141–351 a	No fission products have a half-life in the range of 100 a–210 ka ...
	241Amƒ		251Cfƒ	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₡
				1.33 Ma	135Cs₡
	237Npƒ			1.61–6.5 Ma	93Zr	107Pd
236U			247Cmƒ	15–24 Ma		129I₡
244Pu				80 Ma	... nor beyond 15.7 Ma
232Th№		238U№	235Uƒ№	0.7–14.1 Ga
₡,  has thermal neutron capture cross section in the range of 8–50 barns ƒ,  fissile №,  primarily a naturally occurring radioactive material (NORM) þ,  neutron poison (thermal neutron capture cross section greater than 3k barns)

Notable isotopes

• Plutonium-238 has a half-life of 87.74 years 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 is the most important isotope of plutonium, with a half-life of 24,100 years. ²³⁹Pu and ²⁴¹Pu are fissile, meaning that the nuclei of their atoms can break apart by being bombarded by slow moving 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 containing it. Plutonium is graded by proportion of ²⁴⁰Pu: weapons grade (< 7%), fuel grade (7–19%) and reactor grade (> 19%). Lower grades are less suited for nuclear weapons and thermal reactors but can fuel fast reactors.
• Plutonium-241 is fissile, but also beta decays with a half-life of 14 years to americium-241.
• Plutonium-242 is not fissile, not very fertile (requiring 3 more neutron captures to become fissile), 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. Plutonium-244 has been found in interstellar space and is has the longest half-life of any non-primordial radioisotope.

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.
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.

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 (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

Main article: 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

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

Main article: 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

Plutonium-240 as an obstacle to nuclear weapons

Plutonium-240 undergoes spontaneous fission as a secondary decay mode at a small but significant rate (5.8×10⁻⁶%). 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.

Contamination due to ²⁴⁰Pu 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. Plutonium-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 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

• 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, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Isotopic compositions and standard atomic masses from:
  • de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
  • Wieser, Michael E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051.
• "News & Notices: Standard Atomic Weights Revised". International Union of Pure and Applied Chemistry. 19 October 2005.
• 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, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.