Half-life measurements in doubly-odd 186, 188, 190Au nuclei and the 188Hg → Au decay

Abstract A level scheme has been established for 188 Hg → Au decay which was studied with the on-line isotope separator ISOCELE. Precise conversion-electron measurements were performed with a semicircular magnetic spectrograph. The half-lives of the 16.0, 82.7 and 114.8 keV levels in 188 Au were measured with a lens electron spectrometer and reduced transition rates were deduced. Similarly the half-lives of the 36.1, 113.9, 227.7, 251.5, 288.0 and 363.6 keV levels in 186 Au and of the 28.9 and 171.5 keV levels in 190 Au were measured. Comparison of the reduced e.m. transition rates shows that a 1 + and a 2 − state have respectively the same structure in 186, 188, 190, 192 Au, the same conclusion holding for the 1 − g.s of 188, 190, 192 Au. Additional measurements in 189, 191Hg and 185, 187 Au together with data from the literature enable us to interpret them as a coupling of certain quasiparticle states of neighbouring odd- A nuclei corresponding to oblate shapes. Indications are given that some other negative- and positive-parity states in 188 Au also belong to an oblate system. The nucleus 188 Au appears as the last of a series of doubly-odd gold nuclei where no shape coexistence has, as yet, been observed.