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dubnium (symbol Db) Synthetic, radioactive transactinide element. Six isotopes have been synthesized. It was first reported by a Soviet team at Dubna in 1967. They claimed the isotopes of mass numbers 260 and 261, as a result of bombarding americium with neon ions. In 1970, a team at the University of California claimed the isotope 260 (half-life 1.6 seconds) obtained by bombarding californium with nitrogen nuclei. It was previously named hahnium. Properties: at.no. 105; r.a.m. 262.