Berkelium (Bk) is a synthetic, radioactive, metallic element of the actinide series; also a transuranic element. It was first made in 1949 by alpha particle bombardment of americium-241 by S. G. Thompson and coworkers at the University of California at Berkeley (after which it is named). It can also be prepared by bombarding curium-244 with neutrons. Less than a gram is made each year. Nine isotopes are known the most stable of which is ²⁴⁷Bk (half-life 1.4 × 10³ years).