Thorium-232 is the primary abundant nuclide of thorium. It is a slightly unstable radionuclide that's found in the earth's crust. It is a fertile material able to absorb a neutron and undergo transmutation into a nuclide that's able to support fission (uranium-233). In the form of Thorotrast, a thorium oxide suspension, it was used as contrast medium in early X-ray diagnostics. Thorium-232 is now classified as carcinogenic. The half-life of thorium-232 is 1.405 years.… (More on Thorium-232)