A radioactive element of the lanthanide series that is prepared by fission of uranium or by neutron bombardment of neodymium and has over 30 isotopes, the most stable of which have half-lives of 2.62 years (Pm-147), 5.53 years (Pm-146), and 17.7 years (Pm-145). Promethium-147, the most easily obtained isotope, is used as a source of beta rays. Atomic number 61; melting point 1,042°C; boiling point 3000°C; specific gravity 7.264 (25°C); valence 3. See Periodic Table.