What is "caesium-137"

n. A radioactive isotope of caesium, (nuclide 137 55 Cr), having 55 protons and 82 neutrons and which is formed mainly as a fission product by nuclear fission.

Caesium-137 (, Cs-137), cesium-137, or radiocaesium, is a radioactive isotope of caesium which is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons. It is among the most problematic of the short-to-medium-lifetime fission products because it easily moves and spreads in nature due to the high water solubility of caesium's most common chemical compounds, which are salts.