OBSERVATION OF THE NEW NEUTRON-RICH NUCLIDE HG-208

The new neutron-rich isotope 208Hg was observed for the first time in the reaction products from a thick (nat)Pb target bombarded by a 30 Me V/nucleon C-12 beam. A high efficiency release, separation, and collec tion 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 beta- deca y daughter Tl-208 observed in the periodically extracted Tl element sa mple growing in the separated Hg element product solution. In the gamm a spectra of the TI samples a 2614.6-keV gamma activity with a half-li fe 191(+104/-50) s was observed, which could only be assigned to the d aughter Tl-208 of 208Hg beta- decay. The measured 208Hg half-life was 42(+23/-12) min and the average production cross section for the energ y of the C-12 beam ranging from 30 MeV/nucleon to 5 MeV/nucleon and th e effective target thickness of 670 mg/cm2 was deduced to be 1.1(+1.0/ -0.5) mub. Moreover, in the time-successive gamma spectra of the separ ated Hg sample, a 473.5-keV gamma activity corresponding to the gamma transition of 4+ to ground state (5+) of Tl-208 was observed and found to have the same half-life as the 208Hg beta- decay within the error range of the present work. A theoretical discussion for the obtained h alf-life of 208Hg is given.