A thick natural lead target has been bombarded with a 600 MeV ${}^{18}\mathrm{O}$ beam. With the use of an online gas-thermochromatographic device the mercury isotopes produced as target residues were rapidly separated and collected. A special coincident measurement technique increased the sensitivity to detecting ${\beta }^{-}$-delayed $\gamma$ rays, resulting in the observation of six neutron-rich mercury isotopes ${}^{203,205–209}\mathrm{Hg}$. Here ${}^{209}\mathrm{Hg}$ was created through an exotic $-2p+3n$ multinucleon transfer process and was identified for the first time. The half-life of ${}^{209}\mathrm{Hg}$ has been determined to be ${35}_{-6}^{+9\mathrm{}}$ s, and four $\gamma$ rays following its ${\beta }^{-}$ decays have been assigned. The measured thick-target average production cross sections of ${}^{203,205-209}\mathrm{Hg}$ were analyzed together with the known cross section data related to the $-2p\pm \mathrm{xn}$ exit channels. It has been found that the independent cross sections from various reactions induced by lighter heavy-ion projectiles below ${}^{40}\mathrm{Ar}$, display a regular $Q_{\mathrm{gg}}$ dependence, suggesting that the heavy neutron-rich target residues were produced mainly in quasielastic and deep inelastic multinucleon transfer processes. The half-life of ${}^{208}\mathrm{Hg}$ has been determined to be ${41}_{-4}^{+5\mathrm{}}$ min.

• Received 5 September 1997

DOI:https://doi.org/10.1103/PhysRevC.58.156