Promethium

Promethium is a radioactive lanthanide metal, so every form of it eventually decays into another element. Only tiny traces occur naturally, produced by rare nuclear processes such as uranium fission and the decay of europium. Nearly all useful promethium is made in nuclear reactors, especially as a fission product of uranium, and then separated from other materials. Its best-known uses depend on the isotope promethium-147: its beta radiation can make special luminous materials glow and can power compact nuclear batteries or instruments that measure material thickness. Promethium-145 lasts longer than the other isotopes, but promethium-147 is the isotope used in these applications.