Where does radiation in the earth crust originate from ?

Natural radiation has been part of the earth since it came into existence. Primordial (i.e. since the beginning of the world) radionuclides include uranium-235, uranium-238, thorium-232, potassium-40 and neptunium-237 in rock and soil. They are unstable and undergo radioactive decay until they reach a stable state. As the decay of uranium-235, uranium-238, thorium-232 and potassium-40 is very slow, these radionuclides are still found today. Neptunium-237, on the other hand, decays fast and is no longer naturally present on earth nowadays.