Caesium (₅₅Cs) has 41 knownisotopes, ranging inmass number from 112 to 152. Only one isotope,¹³³Cs, is stable. The longest-livedradioisotopes are¹³⁵Cs with a half-life of 1.33 million years,¹³⁷
Cs with a half-life of 30.04 years and¹³⁴Cs with a half-life of 2.0650 years. All other isotopes have half-lives less than 2 weeks, most under an hour.

Caesium is an abundantfission product (135 and 137 are directly produced) and various isotopes are of concern as such, see the sections below.

Beginning in 1945 with the commencement ofnuclear testing, caesium radioisotopes were released into theatmosphere, where caesium is absorbed readily into solution and is returned to the surface of the Earth as a component ofradioactive fallout. Once caesium enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function^{[clarification needed]} of these isotopes can be estimated as a function of time.

Caesium-131 decays purely byelectron capture to the ground state of stablexenon-131 with a half-life of 9.69 days; its detectable radiation is theX-rays of xenon, with a maximum energy of 34.5 keV. It was introduced in 2004 forbrachytherapy byIsoray.^[8]

Caesium-133 is the only stableisotope of caesium. TheSI base unit of time, thesecond, is defined by aspecific caesium-133 transition. Since 1967, the official definition of a second is:

The second, symbol s, is defined by taking the fixed numerical value of the caesium frequency,Δν_Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom,^[9] to be9192631770Hz, which is equal to s⁻¹.

Caesium-134 has ahalf-life of 2.0650 years. It is produced both directly (at a very small yield because¹³⁴Xe is stable) as afission product and vianeutron capture from nonradioactive¹³³Cs (neutron capturecross section 29barns), which is a common fission product. It is not produced bynuclear weapons because¹³³Cs is created by beta decay of original fission products only long after the nuclear explosion is over.

The combined yield of¹³³Cs and¹³⁴Cs is given as 6.7896%. The proportion between the two will change with continued neutron irradiation.¹³⁴Cs also captures neutrons with a cross section of 140 barns, becoming long-lived radioactive¹³⁵Cs.

Caesium-134 undergoesbeta decay (β⁻), producing stable¹³⁴Ba after emitting on average 2.23gamma ray photons (mean energy 0.698MeV).^[10]

Caesium-135 is a mildlyradioactive isotope of caesium with a half-life of 1.33 million years. It decays via emission of a low-energy beta particle into the stable isotope barium-135. Caesium-135 is one of the sevenlong-lived fission products and the only alkaline one. In most types ofnuclear reprocessing, it stays with themedium-lived fission products (including¹³⁷
Cs which can only be separated from¹³⁵
Cs viaisotope separation) rather than with other long-lived fission products. As an exception,molten salt reactors create¹³⁵
Cs as a completely separate stream outside the fuel (after the decay of bubble-separated¹³⁵
Xe). The lowdecay energy, lack ofgamma radiation, and long half-life of¹³⁵Cs make this isotope much less hazardous than¹³⁷Cs or¹³⁴Cs.

Its precursor¹³⁵Xe has a highfission product yield (e.g., 6.3333% for²³⁵U andthermal neutrons) but also has the highest knownthermal neutron capture cross section of any nuclide. Because of this, much of the¹³⁵Xe produced in currentthermal reactors (as much as >90% at steady-state full power)^[11] will be converted to practically stable¹³⁶
Xe before it can decay to¹³⁵
Cs despite the relatively short half-life of¹³⁵
Xe. Little or no¹³⁵
Xe will be destroyed by neutron capture after a reactor shutdown, or in amolten salt reactor that continuously removes xenon from its fuel, afast neutron reactor, or a nuclear weapon. Thexenon pit is a phenomenon of excess neutron absorption through¹³⁵
Xe buildup in the reactor after a reduction in power or a shutdown and is often managed by letting the¹³⁵
Xe decay away to a level at which neutron flux can be safely controlled viacontrol rods again.

A nuclear reactor will also produce much smaller amounts of¹³⁵Cs from the nonradioactive fission product¹³³Cs by successive neutron capture to¹³⁴Cs and then¹³⁵Cs.

The thermal neutron capture cross section andresonance integral of¹³⁵Cs are8.3 ± 0.3 and38.1 ± 2.6barns respectively.^[12] Disposal of¹³⁵Cs bynuclear transmutation is difficult, because of the low cross section as well as because neutron irradiation of mixed-isotope fission caesium produces more¹³⁵Cs from stable¹³³Cs. In addition, the intense medium-term radioactivity of¹³⁷Cs makes handling of nuclear waste difficult.^[13]

• ANL factsheet

Caesium-136 has a half-life of 13.01 days. It is produced both directly (at a very small yield because¹³⁶Xe isbeta-stable) as a fission product and via neutron capture from long-lived¹³⁵Cs, though because of the lower cross-section (see above) and sort half-life, is much less abundant in spent fuel and vanishes quickly. It is also not produced by nuclear weapons because¹³⁵Cs is created by beta decay of original fission products only long after the nuclear explosion is over. Caesium-136 undergoes beta decay (β⁻
) to¹³⁶Ba.

Caesium-137, with a half-life of 30.04 years, is one of the two principalmedium-lived fission products, along with⁹⁰Sr, which are responsible for most of theradioactivity ofspent nuclear fuel from several years up to several hundred years after use. It constitutes most of the radioactivity still left from theChernobyl accident and is a major health concern for decontaminating land near theFukushima nuclear power plant.^[14]137Cs beta decays to barium-137m (a short-livednuclear isomer), which in de-excitation to its stable ground statebarium-137, usually emits agamma ray. This process is responsible for all the gamma emission from caesium-137.^[15]

¹³⁷Cs has a very low rate of neutron capture and cannot yet be feasibly disposed of in this way unless advances in neutron beam collimation (not otherwise achievable by magnetic fields), uniquely available only from withinmuon catalyzed fusion experiments (not in the other forms ofAccelerator Transmutation of Nuclear Waste) enables production of neutrons at high enough intensity to offset and overcome these low capture rates; until then, therefore,¹³⁷Cs must simply be allowed to decay.^{[citation needed]}

¹³⁷Cs has been used as atracer in hydrologic studies, analogous to the use of³H.

The heavier isotopes have half-lives of seconds or minutes. Almost all caesium produced from nuclear fission comes from beta decay of originally more neutron-rich fission products, passing throughisotopes of iodine thenisotopes of xenon. Because these elements are volatile and can diffuse through nuclear fuel or air, caesium (thus its higher-Z decay products) is often created far from the original site of fission.^{[citation needed]}

Daughter products other than caesium

• Isotopes of barium
• Isotopes of xenon
• Isotopes of iodine
• Isotopes of tellurium

• Isotopes of caesium
• Lists of isotopes by element

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