The new neutron-rich isotope ${}_{}{}^{208}{}_{}{}^{}\mathrm{Hg}$ was observed for the first time in the reaction products from a thick ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Pb}$ target bombarded by a 30 MeV/nucleon ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ beam. A high efficiency release, separation, and collection of Hg products were of success with a good selectivity by using a special off-line gas-phase thermochromatographic process followed by a liquid-liquid procedure which was developed in the present work. The assignment of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Hg}$ was based on the identification of its ${\mathrm{\beta }}^{\mathrm{-}}$ decay daughter ${}_{}{}^{208}{}_{}{}^{}\mathrm{Tl}$ observed in the periodically extracted Tl element sample growing in the separated Hg element product solution. In the γ spectra of the Tl samples a 2614.6-keV γ activity with a half-life ${191}_{\mathrm{-}50}^{+104\mathrm{}}$ s was observed, which could only be assigned to the daughter ${}_{}{}^{208}{}_{}{}^{}\mathrm{Tl}$ of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Hg}$ ${\mathrm{\beta }}^{\mathrm{-}}$ decay. The measured ${}_{}{}^{208}{}_{}{}^{}\mathrm{Hg}$ half-life was ${42}_{\mathrm{-}12}^{+23\mathrm{}}$ min and the average production cross section for the energy of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ beam ranging from 30 MeV/nucleon to 5 MeV/nucleon and the effective target thickness of 670 mg/${\mathrm{cm}}^2$ was deduced to be 1.$1_{\mathrm{-}0.5}^{+1.0\mathrm{}}$ μb. Moreover, in the time-successive γ spectra of the separated Hg sample, a 473.5-keV γ activity corresponding to the γ transition of $4^{+}$ to ground state ($5^{+}$) of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Tl}$ was observed and found to have the same half-life as the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Hg}$ ${\mathrm{\beta }}^{\mathrm{-}}$ decay within the error range of the present work. A theoretical discussion for the obtained half-life of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Hg}$ is given.

• Received 1 July 1993

DOI:https://doi.org/10.1103/PhysRevC.49.R592