Different radioisotopes are used for different thickness monitoring applications depending on their exact requirements - however, typically a beta emitter with a long half-life is used. Some common isotopes used are:Krypton-85. Beta emitter with a half-life of 10.8 years.Caesium-137. Beta emitter with a half-life of 30.17 years.Cobalt-60. Beta emitter with a half-life of 5.3 years, decaying into Nickle-60 which emits gamma rays.