Radionuclides Half life (T ) 1/2 90 Strontium-90 ( Sr) 28.5 yrs Caesium-137 ( 137 Cs) 30.1 yrs Radium-226 ( 226 Ra) 1600 yrs 14 Carbon-14 ( C) 5736 yrs 8 40 Potassium-40 ( K) 1.28 x 10 yrs 38