Isotopes of caesium

Caesium-131

Caesium-131 decays purely by electron capture to the ground state of stable xenon-131 with a half-life of 9.69 days; its detectable radiation is the X-rays of xenon, with a maximum energy of 34.5 keV. It was introduced in 2004 for brachytherapy by Isoray.

Caesium-133

Caesium-133 is the only stable isotope of caesium. The SI base unit of time, the second, is defined by a specific 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, to be 9192631770 Hz, which is equal to s⁻¹.

Caesium-134

Caesium-134 has a half-life of 2.0650 years. It is produced both directly (at a very small yield because ¹³⁴Xe is stable) as a fission product and via neutron capture from nonradioactive ¹³³Cs (neutron capture cross section 29 barns), which is a common fission product. It is not produced by nuclear weapons because ¹³³Cs is created by beta decay of original fission products long after the nuclear explosion is over.

The combined yield of ¹³³Cs and ¹³⁴Cs 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 radioactive ¹³⁵Cs.

Caesium-134 undergoes beta decay (β⁻), producing stable ¹³⁴Ba after emitting on average 2.23 gamma ray photons (mean energy 0.698 MeV).

Caesium-135

Caesium-135 is a mildly radioactive 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 seven long-lived fission products and the only alkaline one. In most types of nuclear reprocessing, it stays with the medium-lived fission products (including ¹³⁷
Cs which can only be separated from ¹³⁵
Cs via isotope separation) rather than with other long-lived fission products. As an exception, molten salt reactors create ¹³⁵
Cs as a completely separate stream outside the fuel (after the decay of bubble-separated ¹³⁵
Xe). The low decay energy, lack of gamma radiation, and long half-life of ¹³⁵Cs make this isotope much less hazardous than ¹³⁷Cs or ¹³⁴Cs.

Its precursor ¹³⁵Xe has a high fission product yield (e.g., 6.3333% for ²³⁵U and thermal neutrons) but also has the highest known thermal neutron capture cross section of any nuclide. Because of this, much of the ¹³⁵Xe produced in current thermal reactors (as much as >90% at steady-state full power) will be converted to practically stable ¹³⁶
Xe before it can decay to ¹³⁵
Cs despite the relatively short half-life of ¹³⁵
Xe. Little or no ¹³⁵
Xe will be destroyed by neutron capture after a reactor shutdown, or in a molten salt reactor that continuously removes xenon from its fuel, a fast neutron reactor, or a nuclear weapon. The xenon pit is a phenomenon of excess neutron absorption through ¹³⁵
Xe buildup in the reactor after a reduction in power or a shutdown and is often managed by letting the ¹³⁵
Xe decay away to a level at which neutron flux can be safely controlled via control rods again.

A nuclear reactor will also produce much smaller amounts of ¹³⁵Cs from the nonradioactive fission product ¹³³Cs by successive neutron capture to ¹³⁴Cs and then ¹³⁵Cs.

The thermal neutron capture cross section and resonance integral of ¹³⁵Cs are 8.3 ± 0.3 and 38.1 ± 2.6 barns respectively. Disposal of ¹³⁵Cs by nuclear transmutation is difficult, because of the low cross section as well as because neutron irradiation of mixed-isotope fission caesium produces more ¹³⁵Cs from stable ¹³³Cs. In addition, the intense medium-term radioactivity of ¹³⁷Cs makes handling of nuclear waste difficult.

• ANL factsheet

Caesium-136

Caesium-136 has a half-life of 13.01 days. It is produced both directly (at a very small yield because ¹³⁶Xe is beta-stable) as a fission product and via neutron capture from long-lived ¹³⁵Cs, 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 because ¹³⁵Cs is created by beta decay of original fission products only long after the nuclear explosion is over. Caesium-136 undergoes beta decay (β⁻
) to ¹³⁶Ba.

Caesium-137

Caesium-137, with a half-life of 30.04 years, is one of the two principal medium-lived fission products, along with ⁹⁰Sr, which are responsible for most of the radioactivity of spent nuclear fuel from several years up to several hundred years after use. It constitutes most of the radioactivity still left from the Chernobyl accident and is a major health concern for decontaminating land near the Fukushima nuclear power plant. ¹³⁷Cs beta decays to barium-137m (a short-lived nuclear isomer), which in de-excitation to its stable ground state barium-137, usually emits a gamma ray. This process is responsible for all the gamma emission from caesium-137.

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 within muon catalyzed fusion experiments (not in the other forms of Accelerator 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.

¹³⁷Cs has been used as a tracer in hydrologic studies, analogous to the use of ³H.

Fission-produced isotopes of caesium

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 through isotopes of iodine then isotopes 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.