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Xenon-123

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 ββ 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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