The $\alpha$ decay of mass-separated Bi isotopes is studied to characterize the shell-model intruder states in the odd-mass Tl and Bi isotopes. Allowed $\alpha$ decay is observed between the ${}_{}{}^{189-195}{}_{}{}^{}\mathrm{Bi}$ $\pi h_{\frac{9}{2}}$ ground states and the ${}_{}{}^{185-191}{}_{}{}^{}\mathrm{Tl}$ $\pi h_{\frac{9}{2}}$ intruder states and between the ${}_{}{}^{191-197}{}_{}{}^{}\mathrm{Bi}$ $\pi s_{\frac{1}{2}}$ intruder states and the ${}_{}{}^{187-193}{}_{}{}^{}\mathrm{Tl}$ $\pi s_{\frac{1}{2}}$ ground states. The observation of forbidden $\alpha$ branches provides excitation energies for the intruder states in ${}_{}{}^{189,191}{}_{}{}^{}\mathrm{Tl}$ and ${}_{}{}^{189,191,193,195}{}_{}{}^{}\mathrm{Bi}$ and confirms the intruder-state excitation energies in ${}_{}{}^{185,187}{}_{}{}^{}\mathrm{Tl}$.

• Received 8 February 1985

DOI:https://doi.org/10.1103/PhysRevLett.54.1783