Caesium-135

Caesium-135 is a caesium radioisotope with a half-life of 2.3 million years, undergoing low-energy beta decay to barium-135. It is one of only 7 long-lived fission products, and one of the 3 abundant ones. ¹³⁵Cs's low decay energy, lack of gamma radiation, and long half-life, make this isotope less hazardous than Cs-137 or Cs-134.

Its precursor Xenon-135 has a high fission yield of 6.3333%, but also has the highest known neutron capture cross section of any nuclide, so some of the Xe-135 produced in a nuclear reactor (as much as >90% at steady-state full power [http://canteach.candu.org/library/20041204.pdf] ) will be converted to stable Xenon-136 before it can decay to Cs-135. A much smaller amount of ¹³⁵Cs will also be produced from nonradioactive fission product Caesium-133 by successive neutron capture to Cs-134 and then Cs-135.

¹³⁵Cs's thermal neutron capture cross section and resonance integral are 8.3±0.3 and 38.1±2.6 barns respectively. [http://www.springerlink.com/content/t155749703tr4244/] Disposal of Cs-135 by nuclear transmutation is difficult, because of the low cross section, because neutron irradiation of mixed-isotope fission caesium produces more Cs-135 from stable Cs-133, and because the intense medium-term radioactivity of Cs-137 makes handling difficult. [http://www.nea.fr/html/pt/docs/iem/jeju02/session6/SessionVI-08.pdf]

ee also

*Isotopes of caesium
*Isotopes of xenon
* [http://www.ead.anl.gov/pub/doc/cesium.pdf ANL factsheet]