Einsteinium (Es) is a synthetic, radioactive, metallic element of the actinide series. The isotope ²⁵³Es was first identified in 1952 by G. R. Choppin and coworkers at the University of California at Berkeley; this was after it was found as a decay product of ²³⁸U produced by the first large hydrogen bomb explosion. Eleven isotopes are known, the most stable of which is ²⁵⁴Es (half-life 276 days). Only a few milligrams of einsteinium are made each year, from plutonium in nuclear reactors. Atomic number 99.