The longest half-life is that of -(AIIMS 90)
The half-life of uranium-238 is about 4.5 billion years, uranium-235 about 700 million years, and uranium-234 about 25 thousand years. Uranium atoms decay into other atoms, or radionuclides, that are also radioactive and commonly called "decay products." Radium half life 1600 years,radon (Rn222) half life 3.8 days,Caesium-137 has a half-life of about 30.17 years
Radon
Radium
Uranium
Cesium
Physics of Radiotherapy