Thorium is a naturally occuring element, and can be used as fuel in a nuclear breed-reactor. The isotope Th-232 is the most common. It is found as thoriumoxide in gas lantern mantels, or in weld-electrodes, which here in Sweden can be bought at Biltema.
When a neutron, from your neutron-gun hits the Th-232, it turns into Th-233, which decays to Pa-233, and then to U-233. U-233 is fissible.
Another picture at the mantle.