Isotopes of xenon

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Main isotopes of xenon (54Xe)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
124Xe 0.095% 1.8×1022 y[1] εε 124Te
125Xe syn 16.9 h ε 125I
126Xe 0.089% stable
127Xe syn 36.345 d ε 127I
128Xe 1.910% stable
129Xe 26.401% stable
130Xe 4.071% stable
131Xe 21.232% stable
132Xe 26.909% stable
133Xe syn 5.247 d β 133Cs
134Xe 10.436% stable
135Xe syn 9.14 h β 135Cs
136Xe 8.857% 2.165×1021 y[2] ββ 136Ba
Standard atomic weight Ar, standard(Xe)

Naturally occurring xenon (54Xe) consists of seven stable isotopes and two very long-lived isotopes. Double electron capture has been observed in 124Xe (half-life 1.8 ± 0.5(stat) ± 0.1(sys) ×1022 years)[1] and double beta decay in 136Xe (half-life 2.165 ± 0.016(stat) ± 0.059(sys) ×1021 years),[2] which are among the longest measured half-lives of all nuclides. The isotopes 126Xe and 134Xe are also predicted to undergo double beta decay,[4] but this has never been observed in these isotopes, so they are considered to be stable.[5][6] Xenon has the second-highest number of stable isotopes. Only tin, with 10 stable isotopes, has more.[7] Beyond these stable forms, 32 artificial unstable isotopes and various isomers have been studied, the longest-lived of which is 127Xe with a half-life of 36.345 days. All other isotopes have half-lives less than 12 days, most less than 20 hours. The shortest-lived isotope, 108Xe,[8] has a half-life of 58 µs, and is the heaviest known nuclide with equal numbers of protons and neutrons. Of known isomers, the longest-lived is 131mXe with a half-life of 11.934 days. 129Xe is produced by beta decay of 129I (half-life: 16 million years); 131mXe, 133Xe, 133mXe, and 135Xe are some of the fission products of both 235U and 239Pu, so are used as indicators of nuclear explosions.

The artificial isotope 135Xe is of considerable significance in the operation of nuclear fission reactors. 135Xe has a huge cross section for thermal neutrons, 2.65×106 barns, so it acts as a neutron absorber or "poison" that can slow or stop the chain reaction after a period of operation. This was discovered in the earliest nuclear reactors built by the American Manhattan Project for plutonium production. Because of this effect, designers must make provisions to increase the reactor's reactivity (the number of neutrons per fission that go on to fission other atoms of nuclear fuel) over the initial value needed to start the chain reaction.

Relatively high concentrations of radioactive xenon isotopes are also found emanating from nuclear reactors due to the release of this fission gas from cracked fuel rods or fissioning of uranium in cooling water.[citation needed] The concentrations of these isotopes are still usually low compared to the naturally occurring radioactive noble gas 222Rn.

Because xenon is a tracer for two parent isotopes, Xe isotope ratios in meteorites are a powerful tool for studying the formation of the solar system. The I-Xe method of dating gives the time elapsed between nucleosynthesis and the condensation of a solid object from the solar nebula (xenon being a gas, only that part of it that formed after condensation will be present inside the object). Xenon isotopes are also a powerful tool for understanding terrestrial differentiation. Excess 129Xe found in carbon dioxide well gases from New Mexico was believed to be from the decay of mantle-derived gases soon after Earth's formation.[9]


Xenon-124 is an isotope of xenon that undergoes double electron capture to tellurium-124 with a very long half life of 1.8×1022 years, more than 12 orders of magnitude longer than the age of the universe ((13.799±0.021)×109 years). Such decays have been observed in the XENON1T detector in 2019, and are the rarest processes ever directly observed.[10] (Even slower decays of other nuclei have been measured, but by detecting decay products that have accumulated over billions of years rather than observing them directly.[11])


Isotopes of xenon,  133Xe
Name, symbolIsotopes of xenon,133Xe
Nuclide data
Natural abundancesyn
Half-life5.243 d (1)
Decay products133Cs
Isotope mass132.9059107 u
Decay modes
Decay modeDecay energy (MeV)
Isotopes of xenon
Complete table of nuclides

Xenon-133 (sold as a drug under the brand name Xeneisol, ATC code V09EX03 (WHO)) is an isotope of xenon. It is a radionuclide that is inhaled to assess pulmonary function, and to image the lungs.[12] It is also used to image blood flow, particularly in the brain.[13] 133Xe is also an important fission product.[citation needed] It is discharged to the atmosphere in small quantities by some nuclear power plants.[14]


Xenon-135 is a radioactive isotope of xenon, produced as a fission product of uranium. It has a half-life of about 9.2 hours and is the most powerful known neutron-absorbing nuclear poison (having a neutron absorption cross-section of 2 million barns[15]). The overall yield of xenon-135 from fission is 6.3%, though most of this results from the radioactive decay of fission-produced tellurium-135 and iodine-135. Xe-135 exerts a significant effect on nuclear reactor operation (xenon pit). It is discharged to the atmosphere in small quantities by some nuclear power plants.[14]


Xenon-136 is an isotope of xenon that undergoes double beta decay to barium-136 with a very long half life of 2.11×1021 years, more than 10 orders of magnitude longer than the age of the universe ((13.799±0.021)×109 years).

List of isotopes[edit]

Z(p) N(n)  
isotopic mass (u)
half-life decay
mode(s)[16][n 1]
isotope(s)[n 2]
spin and
(mole fraction)
range of natural
(mole fraction)
excitation energy
108Xe[8] 54 54 58(+106-23) µs α 104Te 0+
109Xe 54 55 13(2) ms α 105Te
110Xe 54 56 109.94428(14) 310(190) ms
[105(+35−25) ms]
β+ 110I 0+
α 106Te
111Xe 54 57 110.94160(33)# 740(200) ms β+ (90%) 111I 5/2+#
α (10%) 107Te
112Xe 54 58 111.93562(11) 2.7(8) s β+ (99.1%) 112I 0+
α (.9%) 108Te
113Xe 54 59 112.93334(9) 2.74(8) s β+ (92.98%) 113I (5/2+)#
β+, p (7%) 112Te
α (.011%) 109Te
β+, α (.007%) 109Sb
114Xe 54 60 113.927980(12) 10.0(4) s β+ 114I 0+
115Xe 54 61 114.926294(13) 18(4) s β+ (99.65%) 115I (5/2+)
β+, p (.34%) 114Te
β+, α (3×10−4%) 111Sb
116Xe 54 62 115.921581(14) 59(2) s β+ 116I 0+
117Xe 54 63 116.920359(11) 61(2) s β+ (99.99%) 117I 5/2(+)
β+, p (.0029%) 116Te
118Xe 54 64 117.916179(11) 3.8(9) min β+ 118I 0+
119Xe 54 65 118.915411(11) 5.8(3) min β+ 119I 5/2(+)
120Xe 54 66 119.911784(13) 40(1) min β+ 120I 0+
121Xe 54 67 120.911462(12) 40.1(20) min β+ 121I (5/2+)
122Xe 54 68 121.908368(12) 20.1(1) h β+ 122I 0+
123Xe 54 69 122.908482(10) 2.08(2) h EC 123I 1/2+
123mXe 185.18(22) keV 5.49(26) µs 7/2(−)
124Xe 54 70 123.905893(2) 1.8(0.5 (stat), 0.1 (sys))×1022 y[1] Double EC 124Te 0+ 9.52(3)×10−4
125Xe 54 71 124.9063955(20) 16.9(2) h β+ 125I 1/2(+)
125m1Xe 252.60(14) keV 56.9(9) s IT 125Xe 9/2(−)
125m2Xe 295.86(15) keV 0.14(3) µs 7/2(+)
126Xe 54 72 125.904274(7) Observationally Stable[n 3] 0+ 8.90(2)×10−4
127Xe 54 73 126.905184(4) 36.345(3) d EC 127I 1/2+
127mXe 297.10(8) keV 69.2(9) s IT 127Xe 9/2−
128Xe 54 74 127.9035313(15) Stable[n 4] 0+ 0.019102(8)
129Xe[n 5] 54 75 128.9047794(8) Stable[n 4] 1/2+ 0.264006(82)
129mXe 236.14(3) keV 8.88(2) d IT 129Xe 11/2−
130Xe 54 76 129.9035080(8) Stable[n 4] 0+ 0.040710(13)
131Xe[n 6] 54 77 130.9050824(10) Stable[n 4] 3/2+ 0.212324(30)
131mXe 163.930(8) keV 11.934(21) d IT 131Xe 11/2−
132Xe[n 6] 54 78 131.9041535(10) Stable[n 4] 0+ 0.269086(33)
132mXe 2752.27(17) keV 8.39(11) ms IT 132Xe (10+)
133Xe[n 6][n 7] 54 79 132.9059107(26) 5.2475(5) d β 133Cs 3/2+
133mXe 233.221(18) keV 2.19(1) d IT 133Xe 11/2−
134Xe[n 6] 54 80 133.9053945(9) Observationally Stable[n 8] 0+ 0.104357(21)
134m1Xe 1965.5(5) keV 290(17) ms IT 134Xe 7−
134m2Xe 3025.2(15) keV 5(1) µs (10+)
135Xe[n 9] 54 81 134.907227(5) 9.14(2) h β 135Cs 3/2+
135mXe 526.551(13) keV 15.29(5) min IT (99.99%) 135Xe 11/2−
β (.004%) 135Cs
136Xe[n 10] 54 82 135.907219(8) 2.165(0.016 (stat), 0.059 (sys))×1021 y[2] ββ 136Ba 0+ 0.088573(44)
136mXe 1891.703(14) keV 2.95(9) µs 6+
137Xe 54 83 136.911562(8) 3.818(13) min β 137Cs 7/2−
138Xe 54 84 137.91395(5) 14.08(8) min β 138Cs 0+
139Xe 54 85 138.918793(22) 39.68(14) s β 139Cs 3/2−
140Xe 54 86 139.92164(7) 13.60(10) s β 140Cs 0+
141Xe 54 87 140.92665(10) 1.73(1) s β (99.45%) 141Cs 5/2(−#)
β, n (.043%) 140Cs
142Xe 54 88 141.92971(11) 1.22(2) s β (99.59%) 142Cs 0+
β, n (.41%) 141Cs
143Xe 54 89 142.93511(21)# 0.511(6) s β 143Cs 5/2−
144Xe 54 90 143.93851(32)# 0.388(7) s β 144Cs 0+
β, n 143Cs
145Xe 54 91 144.94407(32)# 188(4) ms β 145Cs (3/2−)#
146Xe 54 92 145.94775(43)# 146(6) ms β 146Cs 0+
147Xe 54 93 146.95356(43)# 130(80) ms
[0.10(+10−5) s]
β 147Cs 3/2−#
β, n 146Cs
  1. ^ Abbreviations:
    EC: Electron capture
    IT: Isomeric transition
  2. ^ Bold for stable isotopes
  3. ^ Suspected of undergoing β+β+ decay to 126Te
  4. ^ a b c d e Theoretically capable of spontaneous fission
  5. ^ Used in a method of radiodating groundwater and to infer certain events in the Solar System's history
  6. ^ a b c d Fission product
  7. ^ Has medical uses
  8. ^ Suspected of undergoing ββ decay to 134Ba with a half-life over 11×1015 years
  9. ^ Most powerful known neutron absorber, produced in nuclear power plants as a decay product of 135I, itself a decay product of 135Te, a fission product. Normally absorbs neutrons in the high neutron flux environments to become 136Xe; see iodine pit for more information
  10. ^ Primordial radionuclide


  • The isotopic composition refers to that in air.
  • Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
  • Commercially available materials may have been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations from the given mass and composition can occur.
  • 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.


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