Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Thorium
All known thorium isotopes are unstable. The most stable isotope, Th, has a half-life of 14.05 billion years, or about the age of the universe; it decays very slowly via alpha decay, starting a decay chain named the thorium series that ends at stable Pb. In the universe, thorium and uranium are the only two radioactive elements that still occur naturally in large quantities as primordial elements. It is estimated to be over three times more abundant than uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth metals.