Xenon-129

Main isotopes of xenon

iso NA half-life DM DE (MeV) DP 124Xe 0.095% is stable with 70 neutrons 125Xe syn 16.9 h ε 1.652 125I 126Xe 0.089% is stable with 72 neutrons 127Xe syn 36.345 d ε 0.662 127I 128Xe 1.910% is stable with 74 neutrons 129Xe 26.401% is stable with 75 neutrons 130Xe 4.071% is stable with 76 neutrons 131Xe 21.232% is stable with 77 neutrons 132Xe 26.909% is stable with 78 neutrons 133Xe syn 5.247 d β− 0.427 133Cs 134Xe 10.436% is stable with 80 neutrons 135Xe syn 9.14 h β− 1.16 135Cs 136Xe 8.857% 2.165×1021 y β−β− 2.45783 136Ba
Standard atomic weight (Ar)	131.293(6)

Naturally occurring xenon (₅₄Xe) is made of eight stable isotopes and one very long-lived isotope. (¹²⁴Xe, ¹²⁶Xe, and ¹³⁴Xe are predicted to undergo double beta decay, but this has never been observed in these isotopes, so they are considered to be stable.) Xenon has the second highest number of stable isotopes. Only tin, with 10 stable isotopes, has more. Beyond these stable forms, there are over 30 unstable isotopes and isomers that have been studied, the longest-lived of which is ¹³⁶Xe, which undergoes double beta decay with a half-life of 2.165 ± 0.016(stat) ± 0.059(sys) ×10²¹ years with the next longest lived being ¹²⁷Xe with a half-life of 36.345 days. 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, and therefore 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, 2.65×10⁶barns, 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. Fortunately the designers had made provisions in the design to increase the reactor's reactivity (the number of neutrons per fission that go on to fission other atoms of nuclear fuel).

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