Caesium has at least 39 known isotopes ranging in atomic mass from 112 to 151. Only one of these, Cs, is stable. Therefore, the naturally-occurring isotope of caesium is Cs, which is not radioactive. Cs is used in atomic clocks, its vibration frequency used to define the length of the second. Another isotope, Cs is not made naturally but is made after nuclear fission has been done. It is very radioactive and used as an industrial gamma ray source. 