Naturally occurringxenon (₅₄Xe) consists of nine isotopes: sevenstableisotopes and two very long-livedradioactive isotopes:double electron capture has been observed in¹²⁴Xe (half-life 1.1 ± 0.2_stat ± 0.1_sys×10²² years),^[2] anddouble beta decay in¹³⁶Xe (half-life2.18×10²¹ years), which are among the longest measured half-lives of all nuclides. The isotopes¹²⁶Xe and¹³⁴Xe are also predicted to undergo double beta decay, but they are considered to be stable until the decay processes are actually observed.^[5][6] Artificialunstable isotopes have been prepared from¹⁰⁸Xe to¹⁵⁰Xe, the longest-lived of which is¹²⁷Xe with ahalf-life of 36.342 days. All other nuclides have half-lives less than 12 days, most less than one hour. The shortest-lived isotope,¹⁰⁸Xe,^[7] 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 11.95 days, the second longest of all xenon's nuclides.

¹²⁹Xe is produced bybeta decay of natural or artificial¹²⁹I (half-life 16.1 million years);^131mXe,¹³³Xe,^133mXe, and¹³⁵Xe are some of thefission products of both²³⁵U and²³⁹Pu, so are used as indicators ofnuclear explosions.

The artificial isotope¹³⁵Xe is of considerable significance in the operation ofnuclear fission reactors.¹³⁵Xe has a hugecross section forthermal neutrons, 2.65 millionbarns, so it acts as aneutron absorber or "poison" that canslow or stop the chain reaction after a period of operation. This was discovered in theearliest nuclear reactors built by the AmericanManhattan Project forplutonium production. Because of this effect, designers must make provisions to increase the reactor'sreactivity (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. For the same reason, the xenon fission products produced in anuclear explosion and a power plant differ significantly as a large share of¹³⁵
Xe will absorb neutrons in a steady state reactor, while in a bomb it can be assumed that none of the¹³⁵
I will have had time to decay to xenon before the explosion disperses it, removing it from theneutron radiation.

Relatively high concentrations of radioactive xenon isotopes are also found emanating from nuclear reactors due to the release of this fission gas from crackedfuel rods or fissioning of uranium in cooling water.^{[citation needed]} The concentrations of these isotopes are still usually low compared to the naturally occurring radioactivenoble gas²²²Rn.

Because xenon is atracer for twoparent isotopes, Xeisotope ratios inmeteorites are a powerful tool for studying theformation of the Solar System. TheI-Xe method ofdating gives the time elapsed betweennucleosynthesis and the condensation of a solid object from thesolar 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 understandingterrestrial differentiation. Excess¹²⁹Xe found incarbon dioxide well gases fromNew Mexico was believed to be from the decay ofmantle-derived gases soon after Earth's formation.^[8] It has been suggested^{[clarification needed]} that the isotopic composition of atmospheric xenon fluctuated prior to theGOE before stabilizing, perhaps as a result of the rise in atmospheric O₂.^[9]

• The isotopic composition refers to that in air.

Xenon-124 is an isotope of xenon that undergoes double electron capture totellurium-124 with a very long half-life of1.1×10²² years, approximately 12 orders of magnitude longer than the age of the universe. This decay was observed in theXENON1T detector in 2019, and is the slowest one ever directly observed.^[12] (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.^[13])

Xenon-129 is a stable nuclide that isinhaled to assess pulmonary function, and to image thelungs by xenon NMR (see image).

Xenon-133 is a radioisotope of xenon,beta decaying to stablecaesium-133 with half-life 5.2474 days.Sold as a drug under the brand nameXeneisol, (ATC codeV09EX03 (WHO)) it isinhaled to assess pulmonary function, and to image thelungs.^[15] It is also used to image blood flow, particularly in thebrain.^[16]133Xe is afission product produced by fission ofuranium-235.^[17] It is discharged to the atmosphere in small quantities by some nuclear power plants.^[18]

Xenon-135 is aradioactive isotope ofxenon, produced as afission product of uranium. It has ahalf-life of 9.14 hours and is the most powerful knownneutron-absorbingnuclear poison (having aneutron absorption cross-section of about 2 millionbarns^[19]). The overallyield of xenon-135 from fission is 6.3%, without considering any loss by neutron capture. Xe-135 exerts a significant effect onnuclear reactor operation (xenon pit). It is discharged to the atmosphere in small quantities by some nuclear power plants.^[18]

Xenon-136 is an isotope of xenon that undergoesdouble beta decay tobarium-136 with a very long half-life of2.18×10²¹ years, approximately 11 orders of magnitude longer than the age of the universe. It is being used in theEnriched Xenon Observatory experiment to search forneutrinoless double beta decay.

• Xenon isotope geochemistry

Daughter products other than xenon

• Isotopes of caesium
• Isotopes of iodine
• Isotopes of tellurium
• Isotopes of antimony

• Isotopes of xenon
• Xenon
• Lists of isotopes by element

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