Dubnium was first discovered in 1968 at the Joint Institute for Nuclear Research at Dubna, Russia. Researchers there bombarded an americium-243 target with neon-22 ions.
Dubnium is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was 261Db in 1968. There are 12 known radioisotopes from 256Db to 270Db, and 1-3 isomers. The longest-lived isotope is 268Db with a half-life of 32 hours.