Decay of the9/2−isomer inTl181and mass determination of low-lying states inTl181,Au177, andIr173

Abstract

A detailed spectroscopic study of the neutron-deficient isotope $^{181}\mathrm{Tl}$ and the daughter of its $\ensuremath{\alpha}$ decay, $^{177}\mathrm{Au}$, has been performed in the complete fusion reaction $^{40}\mathrm{Ca}+^{144}\mathrm{Sm}\ensuremath{\rightarrow}^{184}\mathrm{Pb}{}^{*}$ at the velocity filter SHIP (GSI). The mass excess, excitation energy, and decay scheme of the isomeric 1.40(3) ms, $9/{2}^{\ensuremath{-}}$ intruder state in $^{181}\mathrm{Tl}$ have been established for the first time. These results solve a long-standing puzzle of the unrealistically large reduced $\ensuremath{\alpha}$-decay width of this isomer. Based on this, the previously unknown masses of the long-lived isomeric states in $^{177}\mathrm{Au}$ and $^{173}\mathrm{Ir}$ have been derived. In turn, it now allows the excitation energies of previously identified bands in $^{177}\mathrm{Au}$ and $^{173}\mathrm{Ir}$ to be deduced and compared with theoretical predictions. First measurements of $\ensuremath{\alpha}$-decay branching ratios for $^{181}\mathrm{Tl}{}^{m}$ and $^{177}\mathrm{Au}{}^{m,g}$ are also reported.