A rare, brilliant white, luminescent, highly radioactive metallic element found in very small amounts in uranium ores, having more than 40 isotopes and isomers with mass numbers between 201 and 234, of which Ra-226 with a half-life of 1,600 years is the most common. It is used as a neutron source for some research purposes and was formerly widely used in cancer radiotherapy and as a constituent of luminescent paints. Atomic number 88; melting point 696°C; boiling point 1,737°C; specific gravity 5; valence 2. See Periodic Table.