Caesium-135 is a mildly radioactive isotope of caesium with a half-life of 2.3 million years. It decays via emission of a low-energy beta particle into the stable isotope barium-135. Caesium-135 is one of the 7 long-lived fission products and the only alkaline one. In nuclear reprocessing, it stays with 137Cs and other medium-lived fission products rather than with other long-lived fission products. The low decay energy, lack of gamma radiation, and long half-life of 135Cs make this isotope much less hazardous than 137Cs or 134Cs.