Caesium-137 (also Cs-137) has a half-life of approximately 30 years, which means that after a period of 30 years, the amount of Cs-137 will have halved due to radioactive decay. Due to its long half-life, caesium-137 is the most important nuclide in respect of long-term contamination and, without countering actions, the most significant nuclide for the radiation exposure of the population after a nuclear accident.