Fission products are chemical elements produced by the fission of a fissile element. They are formed on the basis of a statistical distribution and include the isotopes of volatile and gaseous chemical elements (xenon, krypton, iodine ...) and of solid elements (caesium, strontium, ruthenium ...), which are usually radioactive. The most radioactive fission products have very short half-lives and disappear quickly. Other less radioactive fission products have lifetimes ranging from several months to several hundred years. Caesium 137 is one of the main medium-lived fission products, with a half-life of 30 years.