Observation of the new neutron-rich nuclide 208Hg

The new neutron-rich isotope ²⁰⁸Hg was observed for the first time in the reaction products from a thick ^natPb target bombarded by a 30 MeV/nucleon ¹²C 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 ²⁰⁸Hg was based on the identification of its β^- decay daughter ²⁰⁸Tl 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⁺¹⁰⁴_-50 s was observed, which could only be assigned to the daughter ²⁰⁸Tl of ²⁰⁸Hg β^- decay. The measured ²⁰⁸Hg half-life was 42⁺²³_-12 min and the average production cross section for the energy of the ¹²C beam ranging from 30 MeV/nucleon to 5 MeV/nucleon and the effective target thickness of 670 mg/cm² was deduced to be 1.1^+1.0_-0.5 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 ²⁰⁸Tl was observed and found to have the same half-life as the ²⁰⁸Hg β^- decay within the error range of the present work. A theoretical discussion for the obtained half-life of ²⁰⁸Hg is given.